results of 2002 virginia - chesapeake bay finfish …...all fish were collected in 2002 by the...
TRANSCRIPT
Results of 2002 Virginia - Chesapeake Bay
Finfish Ageing
by
Stephen J. Bobko, Cynthia M. Jones, and
Eric M. Robillard
VMRC/ODU Age and Growth Laboratory
Center for Quantitative Fisheries Ecology
Old Dominion University
Norfolk, VA 23529-0456
October 1, 2003
Final Report
Development and Support of Facilities to Provide
Finfish Ageing for Virginia Catches and
Application of Virtual Population Analysis to
Provide Management Advice
by
Stephen J. Bobko, Cynthia M. Jones, and
Eric M. Robillard
VMRC/ODU Age and Growth Laboratory
Center for Quantitative Fisheries Ecology
Old Dominion University
October 1, 2003
Funded by contract No. RF 02-12 from the Virginia Saltwater Recreational
Development Fund through the Virginia Marine Resources Commission
VMRC final report on finfish ageing, 2002
Center for Quantitative Fisheries Ecology Old Dominion University
Page i
Table of Contents Page
Executive Summary iii
Acknowledgements iv
Chapter 1 Atlantic croaker Introduction 1
Methods 1
Results 2
Chapter 2 black drum
Introduction 5
Methods 5
Results 6
Chapter 3 bluefish
Introduction 9
Methods 9
Results 10
Chapter 4 cobia
Introduction 14
VMRC final report on finfish ageing, 2002
Center for Quantitative Fisheries Ecology Old Dominion University
Page ii
Methods 14
Results 15
Chapter 5 red drum
Introduction 18
Methods 18
Results 19
Chapter 6 spadefish
Introduction 22
Methods 22
Results 23
Table of Contents (continued) Page
Chapter 7 Spanish mackerel Introduction 26
Methods 26
Results 29
Chapter 8 spot Introduction 34
Methods 34
VMRC final report on finfish ageing, 2002
Center for Quantitative Fisheries Ecology Old Dominion University
Page iii
Results 35
Chapter 9 spotted seatrout
Introduction 37
Methods 37
Results 38
Chapter 10 striped bass Introduction 41
Methods 41
Results 44
Chapter 11 summer flounder
Introduction 49
Methods 49
Results 52
Chapter 12 tautog Introduction 55
Methods 55
Results 56
Chapter 13 weakfish Introduction 60
Methods 60
VMRC final report on finfish ageing, 2002
Center for Quantitative Fisheries Ecology Old Dominion University
Page iv
Results 61
VMRC final report on finfish ageing, 2002
Center for Quantitative Fisheries Ecology Old Dominion University
Page v
Executive Summary
In this report we present the results of ageing finfish collected from catches made in Virginia’s
marine waters in 2002. All fish were collected in 2002 by the Virginia Marine Resources
Commission’s (VMRC) Stock Assessment Program and aged in 2003 at the Center for Quantitative
Fisheries Ecology’s Age and Growth Laboratory at Old Dominion University. This report is broken
down into chapters, one for each of the 13 species we aged. For each species, we present measures of
ageing precision and bias, graphs of year-class distributions, and age-length keys.
For three species: summer flounder, Paralichthys dentatus, (n=329); striped bass, Morone saxatilis,
(n=863); and tautog, Tautoga onitis, (n=374) multiple bony structures were used for determining fish
age. Scales and otoliths were used to age summer flounder and striped bass, and opercula and otoliths
were used to age tautog. Comparing alternative hard parts allowed us to assess their usefulness in
determining fish age as well as the relative precision of each structure. Ages were determined from
otoliths for the following species collected in Virginia waters during 2002: Atlantic croaker,
Micropogonias undulatus, (n=362); black drum, Pogonias cromis, (n=89); bluefish, Pomatomus
saltatrix, (n=716); cobia, Rachycentron canadum, (n=65); red drum, Sciaenops ocellatus, (n=65);
spadefish, Chaetodipterus faber, (n=351); spot, Leiostomus xanthurus, (n=354); spotted seatrout,
Cynoscion nebulosus, (n=405); and weakfish, Cynoscion regalis, (n=646).
In total, we made 14,406 age readings from 6,403 scales, otoliths and opercula collected during 2002.
A summary of the age ranges for all species aged is presented in Table I.
Table I. Summary of numbers aged and age ranges for the 13 marine fish species
collected for age determination in Virginia during 2002.
Number of Number of Number of Minimum Maximum
Species Fish Hard-Parts Age Readings Age Age
Atlantic croaker 362 360 820 1 12
black drum 89 88 276 0 51
bluefish 716 710 1520 0 9
cobia 65 65 230 2 15
red drum 65 64 228 1 31
spadefish 351 336 772 0 16
Spanish mackerel 895 856 1812 0 9
spot 354 353 806 0 5
spotted seatrout 405 405 910 0 5
striped bass 863 1145 2490 2 22
summer flounder 329 632 1464 1 10
tautog 374 743 1686 2 18
weakfish 646 646 1392 1 10
Totals 5514 6403 14406
VMRC final report on finfish ageing, 2002
Center for Quantitative Fisheries Ecology Old Dominion University
Page vi
As part of our continued public outreach focused at recreational anglers, we again participated in the
CCA’s Kid’s Fishing Day at Lynnhaven Fishing Pier. This was the third year our staff volunteered
their time to participate in the event. We are currently working on a spotted seatrout Species Update
report, which will be the fifth report in the series. Each report includes an overview of what is known
about a fish species’ biology, age and growth data and analyses generated in our lab, and interesting
information on fish in general.
In 2003 we upgraded our Age & Growth Laboratory website, which can be accessed at
http://web.odu.edu/fish. The website includes electronic versions of this document along with more
detailed explanations of the methods and structures we use in age determination.
Acknowledgements
We thank Roxanne Torres, Becky Taylor, and Susan McKeel for their technical expertise in preparing
otoliths, scales, and opercula for age determination. Roxanne, Becky, Susan, Serena Turner, Forrest
Crock, and Kathleen McNamee all put in long hours processing “tons” of fish in our lab. We are also
thankful for Dr. William Persons’ III hard work on our Spcies Updates and web page. A special note
of appreciation to Ron Owens, Troy Thompson, Joanie Beatley, and Myra Thompson for their many
efforts in this cooperative project. This work was funded by the Virginia Saltwater Recreational
Development Fund.
The image on the front cover is an otolith thin-section from a 315 mm (12.4 inch) total length, 5 year-
old male spot. The fifth annulus is forming at the edge of the otolith.
VMRC final report on finfish ageing, 2002 Atlantic croaker
Center for Quantitative Fisheries Ecology Old Dominion University
Page i
Chapter 1 Atlantic Croaker
Micropogonias
undulatus INTRODUCTION
Three hundred sixty two (362) Atlantic croaker,
Micropogonias undulatus, were collected by the
VMRC’s Stock Assessment Program for age
and growth analysis in 2002. We were unable
to age two fish due to the poor quality of their
otoliths. For the 360 fish we were able to age,
the average age was 5.6 years, and the standard
deviation and standard error were 2.3 and 0.1,
respectfully. Twelve age classes (1 to 12) were
represented, comprising fish from the 1990-
2001 year-classes. Fish from the 1996 through
1998 year-classes dominated the sample
METHODS
Handling of collection Otoliths were
received by the Age & Growth Laboratory in
labeled coin envelopes. In the lab they were
sorted by date of capture, their envelope labels
were verified against VMRC’s collection data,
and each fish was assigned a unique Age and
Growth Laboratory identification number. All
otoliths were stored dry in labeled cell well
plates.
Preparation Otoliths were processed
following the methods described in Barbieri et
al. (1994) with a few modifications. Briefly, the
left or right sagittal otolith was randomly
selected and attached to a glass slide with
Aremco's clear Crystalbond™ 509 adhesive. At
least two serial transverse sections were cut
through the core of each otolith with a Buehler
Isomet low-speed saw equipped with a three
inch, fine grit Norton diamond-wafering blade.
Otolith sections were placed on labeled glass
slides and covered with a thin layer of Flo-texx
mounting medium, that not only adhered the
sections to the slide, but more importantly,
provided enhanced contrast and greater
readability by increasing light transmission
through the sections.
Readings Sectioned otoliths were aged by
two different readers using a Leica MZ-12
dissecting microscope with transmitted light and
dark-field polarization at between 8 and 20
times magnification. Each reader aged all of the
otolith samples. The ageing criteria reported in
Barbieri et al. (1994) were used in age
determination, particularly regarding the
location of the first annulus (Figure 1).
All samples were aged in chronological order,
based on collection date, without knowledge of
previously estimated ages or the specimen
lengths. When the readers’ ages agreed, that age
was assigned to the fish. When the two readers
disagreed, both readers sat down together and
VMRC final report on finfish ageing, 2002 Atlantic croaker
Center for Quantitative Fisheries Ecology Old Dominion University
Page ii
re-aged the fish, again without any knowledge
of previously estimated ages or lengths, and
assigned a final age to the fish. When the
readers were unable to agree on a final age, the
fish was excluded from further analysis.
Comparison Tests Age estimates from
reader 1 were plotted against age estimates from
reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test for
symmetry was used to detect any systematic
difference between the two readers (Hoenig et al.
1995). Also, a random sub-sample of 50 fish
was selected for second readings to measure
reader precision and age reproducibility. To
detect any changes or drift in our ageing
methods, both readers re-aged the otoliths of 50
randomly selected fish previously aged in 2000.
We considered a reader to be biased if the
readings revealed consistent over or under
ageing.
RESULTS
Measurements of reader self-precision were
high, with both readers able to reproduce the
ages of previously read otoliths (100%
agreement for reader 1 and reader 2). There was
no evidence of systematic disagreement
between reader 1 and reader 2 (test of symmetry,
2 = 7.3, df = 8, P = 0.5). In Figure 2 we
present a graph of the results for between-reader
ageing precision. The average between-reader
coefficient of variation (CV) of 0.4% was not
significant and much lower than the CV of 1.7%
from 2001. There was no evidence of drift in
age determination with the CV’s from 2000
precision fish with 100% agreement for reader 1
and reader 2.
(a)
(b)
Figure 1. Otolith cross-
sections of a) a 5 year old
croaker with a small 1st
annulus, and b) a 6 year
old croaker with a large
1st annulus.
Figure 2. Between-reader comparison of otolith
age estimates for Atlantic croaker.
123456789101112
Reader 1
1
2
3
4
5
6
7
8
9
10
11
12
Reader 2
(10)
(13)(1)
(22)
(63)(1)
(1)(108)(1)
(60)
(2)(9)
(1)(20)(1)
(2)(14)
(1)(15)
(1)(6)
(8)
Otolith Ages (years)
VMRC final report on finfish ageing, 2002 Atlantic croaker
Center for Quantitative Fisheries Ecology Old Dominion University
Page iii
Of the 360 fish aged with otoliths, 12 age
classes (1 to 12) were represented (Table 1).
The average age for the sample was 5.6 years,
and the standard deviation and standard error
were 2.3 and 0.1, respectfully.
Year-class data (Figure 3) indicate that
recruitment into the fishery begins at age 1, but
large numbers are not seen until age 3, which
corresponds to the 1999 year-class for Atlantic
croaker collected in 2002. While the ratio of
males to females shows an overall higher
number of females, both sexes show trends of
high abundance for the 1996 through 1997 year-
classes.
Age-Length-Key In Table 2 we present an
age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using otolith
ages. The table is based on VMRC’s stratified
sampling of landings by total length inch
intervals.
REFERENCES
Barbieri, L.R., M.E. Chittenden, and C.M. Jones.
1994. Age, growth, and mortality of
Atlantic croaker, Micropogonias
undulatus, in the Chesapeake Bay region,
with a discussion of the apparent
geographical changes in population
dynamics. Fish. Bull. 92:1-12.
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining the
consistency of age determinations. Trans.
Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A. Brown.
1995. Analysing differences between two
age determination methods by tests of
symmetry. Can. J. Fish. Aquat. Sci.
52:364-368.
S-Plus. 1999. S-Plus 4.5 Guide to Statistics.
Data Analysis Products Division. Math
Soft, Inc. Seattle, Washington.
Figure 3. Year-class distribution for Atlantic
croaker collected for ageing in 2002.
Distributions are broken down by sex.
199019911992199319941995199619971998199920002001
Year Class
0
20
40
60
80
100
Number of Fish
Females n = 256
Males n = 99
Unknown n = 5
VMRC final report on finfish ageing, 2002 Atlantic croaker
Center for Quantitative Fisheries Ecology Old Dominion University
Page iv
VMRC final report on finfish ageing, 2002 Atlantic croaker
Center for Quantitative Fisheries Ecology Old Dominion University
Page i
Table 1. The number of Atlantic croaker assigned to each total length-at-age category for 360
fish sampled for age determination in Virginia during 2002.
Length Age (years)
1-inch 1 2 3 4 5 6 7 8 9 10 11 12
intervals Totals
7 - 7.99 7 0 0 0 0 0 0 0 0 0 0 0 7
8 - 8.99 0 5 0 0 0 0 0 0 0 0 0 0 5
9 - 9.99 2 8 7 2 1 0 0 0 0 0 0 0 20
10 - 10.99 1 0 9 19 8 2 1 0 0 0 0 0 40
11 - 11.99 0 0 6 23 17 4 1 0 1 0 0 0 52
12 - 12.99 0 0 1 15 38 13 2 2 0 1 0 1 73
13 - 13.99 0 0 0 1 25 8 4 0 0 0 0 0 38
14 - 14.99 0 0 0 2 14 19 0 3 3 2 0 1 44
15 - 15.99 0 0 0 0 7 6 2 6 4 3 3 1 32
16 - 16.99 0 0 0 1 0 5 1 3 5 2 0 0 17
17 - 17.99 0 0 0 0 0 3 1 5 0 2 0 1 12
18 - 18.99 0 0 0 0 0 1 0 1 2 4 2 2 12
19 - 19.99 0 0 0 0 0 0 0 0 1 3 0 1 5
20 - 20.99 0 0 0 0 0 0 0 0 1 0 1 0 2
21 - 21.99 0 0 0 0 0 0 0 0 0 0 0 1 1
Totals 10 13 23 63 110 61 12 20 17 17 6 8 360
Table 2. Age-Length key, as proportions-at-age in each 1 inch length-class, based on otolith
ages for Atlantic croaker sampled for age determination in Virginia during 2002.
Length Age (years)
1-inch 1 2 3 4 5 6 7 8 9 10 11 12
intervals N
7 - 7.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 7
8 - 8.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 5
9 - 9.99 0.100 0.400 0.350 0.100 0.050 0.000 0.000 0.000 0.000 0.000 0.000 0.000 20
10 - 10.99 0.025 0.000 0.225 0.475 0.200 0.050 0.025 0.000 0.000 0.000 0.000 0.000 40
11 - 11.99 0.000 0.000 0.115 0.442 0.327 0.077 0.019 0.000 0.019 0.000 0.000 0.000 52
12 - 12.99 0.000 0.000 0.014 0.205 0.521 0.178 0.027 0.027 0.000 0.014 0.000 0.014 73
13 - 13.99 0.000 0.000 0.000 0.026 0.658 0.211 0.105 0.000 0.000 0.000 0.000 0.000 38
14 - 14.99 0.000 0.000 0.000 0.045 0.318 0.432 0.000 0.068 0.068 0.045 0.000 0.023 44
15 - 15.99 0.000 0.000 0.000 0.000 0.219 0.188 0.063 0.188 0.125 0.094 0.094 0.031 32
16 - 16.99 0.000 0.000 0.000 0.059 0.000 0.294 0.059 0.176 0.294 0.118 0.000 0.000 17
17 - 17.99 0.000 0.000 0.000 0.000 0.000 0.250 0.083 0.417 0.000 0.167 0.000 0.083 12
18 - 18.99 0.000 0.000 0.000 0.000 0.000 0.083 0.000 0.083 0.167 0.333 0.167 0.167 12
19 - 19.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.200 0.600 0.000 0.200 5
20 - 20.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.500 0.000 0.500 0.000 2
21 - 21.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 1
Sample Size 360
VMRC final report on finfish ageing, 2002 Atlantic croaker
Center for Quantitative Fisheries Ecology Old Dominion University
Page i
VMRC final report on finfish ageing, 2002 black drum
Center for Quantitative Fisheries Ecology Old Dominion University
Page i
Chapter 2 Black Drum
Pogonias cromis
INTRODUCTION
A total of 88 black drum, Pogonias cromis, was
collected by the VMRC’s Stock Assessment
Program for age and growth analysis in 2002.
The average age of the sample was 18.1 years,
with a standard deviation of 14.9 and a standard
error of 1.6. The youngest fish was a young-of-
the-year (YOY) and the oldest fish was 51 years
old, representing the 2002 and 1951 year-classes,
respectively.
METHODS
Handling of collection
received by the Age & Growth Laboratory in
labeled coin envelopes. In the lab they were
sorted by date of capture, their envelope labels
were verified against VMRC’s collection data,
and each fish was assigned a unique Age and
Growth Laboratory sample number. All otoliths
were stored dry in their original VMRC coin
envelopes.
Preparation Otoliths were processed for
ageing following the methods described in
Bobko (1991) and Jones and Wells (1998).
Briefly, at least two serial transverse sections
were cut through the nucleus of each otolith
with a Buehler Isomet low-speed saw equipped
with a three inch, fine grit diamond-wafering
blade (Norton). A thin layer of Flo-texx
mounting medium was applied to the otolith
section to increase light transmission through
the translucent zones, which provided enhanced
contrast and greater readability.
Readings
two different readers using a Leica MZ-12
dissecting microscope with transmitted light at
between 8 and 20 times magnification (Figure
1). Each reader aged all of the otolith samples.
All samples were aged in chronological order,
based on collection date, without knowledge of
previously estimated ages or the specimen
lengths. When the readers’ ages agreed, that age
was assigned to the fish. When the two readers
disagreed, both readers sat down together and
re-aged the fish, again without any knowledge
of previously estimated ages or lengths, and
assigned a final age to the fish. When the
Figure 1. Otolith thin-section from a 20 year-old black drum.
VMRC final report on finfish ageing, 2002 black drum
Center for Quantitative Fisheries Ecology Old Dominion University
Page ii
readers were unable to agree on a final age, the
fish was excluded from further analysis.
Comparison Tests
reader 1 were plotted against age estimates from
reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test for
symmetry was used to detect any systematic
difference between the two readers (Hoenig et al.
1995). Also, a random sub-sample of 50 fish
was selected for second readings to measure
reader precision and age reproducibility. We
considered a reader to be biased if the readings
revealed consistent over or under ageing.
RESULTS
No bias was discovered in any of the self-
precision tests of otolith age estimates, with
both readers equally able to reproduce the ages
of previously read samples (reader 1’s CV =
0.09% and reader 2’s = 2.69%). The average
between reader coefficient of variation (CV) of
0.2% was not significant, and much lower than
the CV of 3.9% reported for black drum
collected in 2001. Figure 2 illustrates the
between readers’ precision of age estimates,
with no differences greater than one year. There
was no evidence of systematic disagreement
between reader 1 and reader 2 (test of symmetry,
2 = 7, df = 7, P = 0.43).
Of the 88 fish aged with otoliths, 35 age classes
were represented (Figure 3). The average age of
the sample was 18.1 years, with a standard
deviation of 14.9 and a standard error of 1.6.
The youngest fish was a young-of-the-year
(YOY) and the oldest fish was 51 years old,
representing the 2002 and 1951 year-classes,
respectively. Year-class data indicate that older
fish continued to dominate the fishery, although
we observed good representation from the 1995
through 2002 year-classes. The strongest year-
classes were 1967, 1998, and 1999. Black drum
collected in 2002 were predominately males, a
trend observed by Jones (1998) for the years
1990-1996.
Age-Length-Key In Table 1 we present an
age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using otolith
ages. The table is based on VMRC’s stratified
sampling of landings by total length inch
intervals.
REFERENCES
Bobko, S. J. 1991. Age, growth, and
reproduction of black drum, Pogonias
cromis, in Virginia. M.S. thesis. Old
Dominion University, Norfolk, VA.
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining the
Figure 3. Year-class distribution for black drum
collected for ageing in 2002.
Distributions are broken down by sex.
1940195019601970198019902000
Year Class
0
2
4
6
8
10
12
Number of Fish
Females n = 27
Males n = 40
Unknown n = 21
VMRC final report on finfish ageing, 2002 black drum
Center for Quantitative Fisheries Ecology Old Dominion University
Page iii
consistency of age determinations.
Trans. Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A.
Brown. 1995. Analysing differences
between two age determination
methods by tests of symmetry. Can. J.
Fish. Aquat. Sci. 52:364-368.
Jones, C.J. 1998. Report on black drum
studies for the period 1990-1996.
Study of important recreational
fishes in the Chesapeake Bay.
Federal Aid in Sport Fish
Restoration Act project F-88-R-3.
Jones, C.J. and B.K. Wells. 1998. Age,
growth, and mortality of black drum,
Pogonias cromis, in the Chesapeake
Bay region. Fish. Bull. 96:451-461.
Figure 2. Between-reader comparison of otolith
age estimates for black drum.
0 10 20 30 40 50
Reader 1
0
10
20
30
40
50Reader 2
(2)(2)(1)(10)(12)(4)(1)(2)
(1)(3)(2)(2)(1)(1)(1)
(3)(4)
(1)
(2)
(1)
(1)(2)(3)
(3)(1)(1)(1)(3)(1)(4)(1)(1)(2)(2)
(1)
(1)
(1)(1)(1)
(1)
Otolith Ages (years)
VMRC final report on finfish ageing, 2002 bluefish
Center for Quantitative Fisheries Ecology Old Dominion University
Page i
Tab
le 1
. Age
-Len
gth
key
, as
pro
po
rtio
ns-
at-a
ge in
eac
h 1
inch
len
gth
-cla
ss, b
ased
on
oto
lith
age
s fo
r b
lack
dru
m s
amp
led
fo
r ag
e
det
erm
inat
ion
in
Vir
gin
ia d
uri
ng
20
02.
Tota
l len
gth
no
t re
po
rted
fo
r tw
o f
ish
.
VMRC final report on finfish ageing, 2002 bluefish
Center for Quantitative Fisheries Ecology Old Dominion University
Page ii
Chapter 3 Bluefish
Pomatomus
saltatrix
INTRODUCTION
A total of 716 bluefish, Pomatomus saltatrix,
was collected by the VMRC’s Stock
Assessment Program for age and growth
analysis in 2002. We were unable to age six
fish due to the poor quality of their otoliths.
The average age for the 710 aged fish was 1.9
years, and the standard deviation and standard
error were 1.6 and 0.1, respectfully. The 1999
and 2000 year-classes dominated the sample.
METHODS
Handling of collection Otoliths were
received by the Age & Growth Laboratory in
labeled coin envelopes. Once in our hands, they
were sorted based on date of capture, their
envelope labels were verified against VMRC’s
collection data, and assigned unique Age and
Growth Laboratory sample numbers. All
otoliths were stored in dry in labeled cell well
plates.
Preparation We used a “bake and thin-
section” technique to process bluefish otoliths
for age determination. Otolith preparation began
by randomly selecting either the right or left
otolith. Each otolith was mounted with Crystal
Bond onto a standard microscope slide with its
distal surface orientated upwards. Once
mounted, a small mark was placed on the otolith
surface directly above the otolith focus. The
slide, with attached otolith, was then secured to
an Isomet saw equipped with two diamond
wafering blades separated by a 0.5 mm spacer,
which was slightly smaller in diameter than the
diamond blades. The otolith was positioned so
that the wafering blades straddled each side of
the otolith focus ink mark. It was crucial that
this cut be perpendicular to the long axis of the
otolith. Failure to do so resulted in
“broadening” and distortion of winter growth
zones. A proper cut resulted in annuli that were
clearly defined and delineated. Once cut, the
otolith section was placed into a ceramic
“Coors” spot plate well and baked in a
Thermolyne 1400 furnace at 400oC. Baking
time was otolith size dependent and gauged by
color, with a light caramel color desired. Once
a suitable color was reached the baked thin-
section was placed on a labeled glass slide and
covered with a thin layer of Flo-texx mounting
medium, which provided enhanced contrast and
greater readability by increasing light
transmission through the sections.
Readings Two different readers using a
LEICA MZ-12 dissecting microscope with
transmitted light and dark-field polarization at
between 8 and 100 times magnification aged all
sectioned otoliths (Figure 1). If an otolith was
VMRC final report on finfish ageing, 2002 bluefish
Center for Quantitative Fisheries Ecology Old Dominion University
Page iii
properly sectioned the sulcal groove came to a
sharp point within the middle of the focus.
Typically the first year’s annulus was found by
locating the focus of the otolith, which was
characterized as a visually distinct dark oblong
region found in the center of the otolith. The
first year’s annulus had the highest visibility
proximal to the focus along the edge of the
sulcal groove. Once located, the first year’s
annulus was followed outward from the sulcal
groove towards the dorsal perimeter of the
otolith. Often, but not always, the first year was
associated with a very distinct crenellation on
the dorsal surface and a prominent protrusion on
the ventral surface. Unfortunately both these
landmarks had a tendency to become less
prominent in older fish.
Even with the bake and thin-section technique,
interpretation of the growth zones from the
otoliths of young bluefish was difficult. Rapid
growth within the first year of life prevents a
sharp delineation between opaque and
translucent zones. When the exact location of
the first year was not clearly evident, and the
otolith had been sectioned accurately, a
combination of surface landscape (1st year
crenellation) and the position of the second
annuli were used to help determine the position
of the first annulus.
What appeared to be “double annuli” were
occasionally observed in bluefish four years of
age and older. This annulus formation typically
occurred within years 4 to 7, and was
characterized by distinct and separate annuli in
extremely close proximity to each other. We do
not know if the formation of these double annuli
were two separate annuli, or in fact only one,
but they seemed to occur during times of
reduced growth after maturation. “Double
annuli” were considered to be one annulus when
both marks joined to form a central origin. The
origins being the sulcal groove and at the outer
peripheral edge of the otolith. If these annuli
did not meet to form a central origin they were
considered two annuli, and counted as such.
All samples were aged in chronological order
based on collection date, without knowledge of
previously estimated ages or the specimen
lengths. When the readers’ ages agreed, that age
was assigned to the fish. When the two readers
disagreed, both readers sat down together and
re-aged the fish, again without any knowledge
of previously estimated ages or lengths, and
assigned a final age to the fish. When the
readers were unable to agree on a final age, the
fish was excluded from further analysis.
Comparison Tests Age estimates from
reader 1 were plotted against age estimates from
reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test for
symmetry was used to detect any systematic
difference between the two readers (Hoenig et al.
1995). Also, a random sub-sample of 50 fish
was selected for second readings to measure
reader precision and age reproducibility. We
considered a reader to be biased if the readings
revealed consistent over or under ageing.
Figure 1. Otolith thin-section from a 869mm TL
10 year-old female bluefish.
VMRC final report on finfish ageing, 2002 bluefish
Center for Quantitative Fisheries Ecology Old Dominion University
Page iv
RESULTS
The measurement of reader self-precision was
high for both readers (reader 1’s CV = 0.3% and
reader 2’s CV = 2.8%). There was evidence of
systematic disagreement between reader 1 and
reader 2 (test of symmetry, 2 = 18.9, df = 9,
P = 0.03), with reader 2 ageing fish older than
reader 1. Although this disagreement was
statistically significant, it was not considered
practically significant, with disagreements
occurring for only 35 out of 710 fish aged.
Figure 2 illustrates the between readers’
precision of age estimates, with no differences
greater than one year. The average coefficient of
variation (CV) of 1.6% was much lower than
the CV of 3.1% reported in 2001.
Of the 710 fish aged with otoliths 10 age classes
were represented (Table 1). The average age for
the sample was 1.9 years, and the standard
deviation and standard error were 1.6 and 0.1,
respectfully.
Year-class data (Figure 3) indicates that
recruitment into the fishery began at age 0,
which corresponded to the 2002 year-class for
bluefish caught in 2002. One-year-old fish were
the dominant year-class in the 2002 sample.
Age-Length-Key In Table 2 we present an
age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using otolith
ages. The table is based on VMRC’s stratified
sampling of landings by total length inch
intervals.
REFERENCES
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining the
consistency of age determinations. Trans.
Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A. Brown.
1995. Analysing differences between two
age determination methods by tests of
symmetry. Can. J. Fish. Aquat. Sci.
52:364-368.
S-Plus. 1999. S-Plus 4.5 Guide to Statistics.
Data Analysis Products Division. Math
Soft, Inc. Seattle, Washington.
Figure 2. Between-reader comparison of otolith
age estimates for bluefish.
0123456789
Reader 1
0
1
2
3
4
5
6
7
8
9
Reader 2
(6)(1)
(1)(435)(1)
(10)(145)(1)
(2)(12)(1)
(6)(6)
(36)(2)
(5)(21)(1)
(1)(9)
(2)(4)
(1)(2)
Otolith Ages (years)
VMRC final report on finfish ageing, 2002 bluefish
Center for Quantitative Fisheries Ecology Old Dominion University
Page v
Figure 3. Year-class distribution for bluefish
collected for ageing in 2002.
Distribution is broken down by sex.
1993199419951996199719981999200020012002
Year Class
0
100
200
300
400
Number of FishFemales n = 392
Males n = 278
Unknown n = 40
VMRC final report on finfish ageing, 2002 bluefish
Center for Quantitative Fisheries Ecology Old Dominion University
Page vi
Table 1. The number of bluefish assigned to each total length-at-age category for 710
fish collected for age determination in Virginia in 2002 (length not reported for 4 fish).Length Age (years)
1-inch 0 1 2 3 4 5 6 7 8 9
intervals Totals
6 - 6.99 0 3 0 0 0 0 0 0 0 0 3
7 - 7.99 0 10 0 0 0 0 0 0 0 0 10
8 - 8.99 0 1 0 0 0 0 0 0 0 0 1
9 - 9.99 0 4 0 0 0 0 0 0 0 0 4
10 - 10.99 0 8 0 0 0 0 0 0 0 0 8
11 - 11.99 1 33 0 0 0 0 0 0 0 0 34
12 - 12.99 1 68 0 0 0 0 0 0 0 0 69
13 - 13.99 5 71 0 0 0 0 0 0 0 0 76
14 - 14.99 1 84 0 0 0 0 0 0 0 0 85
15 - 15.99 0 78 12 1 0 0 0 0 0 0 91
16 - 16.99 0 32 27 2 0 0 0 0 0 0 61
17 - 17.99 0 23 33 0 0 0 0 0 0 0 56
18 - 18.99 0 18 14 1 0 0 0 0 0 0 33
19 - 19.99 0 9 6 2 0 0 0 0 0 0 17
20 - 20.99 0 3 13 0 0 0 0 0 0 0 16
21 - 21.99 0 0 5 0 0 0 0 0 0 0 5
22 - 22.99 0 0 9 0 0 0 0 0 0 0 9
23 - 23.99 0 0 6 0 0 0 0 0 0 0 6
24 - 24.99 0 0 13 1 0 0 0 0 0 0 14
25 - 25.99 0 0 7 1 0 0 0 0 0 0 8
27 - 27.99 0 0 0 0 5 7 0 0 0 0 12
28 - 28.99 0 0 0 3 0 15 2 1 0 0 21
29 - 29.99 0 0 0 0 3 10 5 1 1 0 20
30 - 30.99 0 0 0 0 0 9 9 3 0 0 21
31 - 31.99 0 0 0 0 0 4 5 4 3 1 17
32 - 32.99 0 0 0 0 1 0 1 1 1 1 5
33 - 33.99 0 0 0 0 0 0 0 2 0 0 2
34 - 34.99 0 0 0 0 0 0 1 0 0 0 1
35 - 35.99 0 0 0 0 0 0 1 0 0 0 1
Totals 8 445 145 11 9 45 24 12 5 2 706
VMRC final report on finfish ageing, 2002 bluefish
Center for Quantitative Fisheries Ecology Old Dominion University
Page vii
Table 2. Age-Length key, as proportions-at-age in each 1 inch length-class, based
on otolith ages, for bluefish collected for age determination in Virginia during 2002.Length Age (years)
1-inch 0 1 2 3 4 5 6 7 8 9
intervals N
6 - 6.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 3
7 - 7.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 10
8 - 8.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
9 - 9.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 4
10 - 10.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 8
11 - 11.99 0.029 0.971 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 34
12 - 12.99 0.014 0.986 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 69
13 - 13.99 0.066 0.934 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 76
14 - 14.99 0.012 0.988 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 85
15 - 15.99 0.000 0.857 0.132 0.011 0.000 0.000 0.000 0.000 0.000 0.000 91
16 - 16.99 0.000 0.525 0.443 0.033 0.000 0.000 0.000 0.000 0.000 0.000 61
17 - 17.99 0.000 0.411 0.589 0.000 0.000 0.000 0.000 0.000 0.000 0.000 56
18 - 18.99 0.000 0.545 0.424 0.030 0.000 0.000 0.000 0.000 0.000 0.000 33
19 - 19.99 0.000 0.529 0.353 0.118 0.000 0.000 0.000 0.000 0.000 0.000 17
20 - 20.99 0.000 0.188 0.813 0.000 0.000 0.000 0.000 0.000 0.000 0.000 16
21 - 21.99 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 5
22 - 22.99 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 9
23 - 23.99 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 6
24 - 24.99 0.000 0.000 0.929 0.071 0.000 0.000 0.000 0.000 0.000 0.000 14
25 - 25.99 0.000 0.000 0.875 0.125 0.000 0.000 0.000 0.000 0.000 0.000 8
27 - 27.99 0.000 0.000 0.000 0.000 0.417 0.583 0.000 0.000 0.000 0.000 12
28 - 28.99 0.000 0.000 0.000 0.143 0.000 0.714 0.095 0.048 0.000 0.000 21
29 - 29.99 0.000 0.000 0.000 0.000 0.150 0.500 0.250 0.050 0.050 0.000 20
30 - 30.99 0.000 0.000 0.000 0.000 0.000 0.429 0.429 0.143 0.000 0.000 21
31 - 31.99 0.000 0.000 0.000 0.000 0.000 0.235 0.294 0.235 0.176 0.059 17
32 - 32.99 0.000 0.000 0.000 0.000 0.200 0.000 0.200 0.200 0.200 0.200 5
33 - 33.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 2
34 - 34.99 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 1
35 - 35.99 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 1
Sample Size 706
VMRC final report on finfish ageing, 2002 cobia
Center for Quantitative Fisheries Ecology Old Dominion University
Page i
Chapter 4 Cobia
Rachycentron
canadum
INTRODUCTION
A total of 65 cobia, Rachycentron canadum,
was collected by the VMRC’s Stock
Assessment Program for age and growth
analysis in 2002. The average age of the sample
was 5.4 years, and the standard deviation and
standard error were 2.7 and 0.3, respectfully.
The 1995, 1997, 1998, and 1999 year-classes
dominated the sample.
METHODS
Handling of collection Otoliths were
received by the Age & Growth Laboratory in
labeled coin envelopes. Once in our hands, they
were sorted based on date of capture, their
envelope labels were verified against VMRC’s
collection data, and assigned unique Age and
Growth Laboratory sample numbers. All
otoliths were stored dry in labeled cell well
plates.
Preparation Individual otoliths were placed
into 14 mm x 5 mm x 3 mm wells (Ladd
Industries silicon rubber mold) filled with
Loctite adhesive. Each otolith was rolled
around in the Loctite to remove all trapped air
bubbles and ensure complete coverage of the
otolith surface. The otoliths were oriented
sulcal side down with the long axis of the otolith
exactly parallel with the long axis of the mold
well. Once the otoliths were properly oriented,
the mold was placed under UV light and left to
solidify overnight. Once dry, each embedded
otolith was removed from the mold and
mounted with Crystal Bond onto a standard
microscope slide. Once mounted, a small mark
was placed on the otolith surface directly above
the otolith focus. The slide, with attached otolith,
was then secured to an Isomet saw equipped
with two diamond wafering blades separated by
a 0.5 mm spacer, which was slightly smaller in
diameter than the diamond blades. The otolith
was positioned so that the wafering blades
straddled each side of the focus ink mark. The
glass slide was adjusted to ensure that the blades
were exactly perpendicular to the long axis of
the otolith. The otolith wafer section was
viewed under a dissecting microscope to
determine which side (cut surface) of the otolith
was closer to the focus. The otolith section was
mounted best-side up onto a glass side with
Crystal Bond. The section was then lightly
polished on a Buehler Ecomet 3 variable speed
grinder-polisher with Mark V Laboratory 30-
micron polishing film. After drying, a thin layer
of Flo-texx mounting medium was applied over
the polished otolith surface.
VMRC final report on finfish ageing, 2002 cobia
Center for Quantitative Fisheries Ecology Old Dominion University
Page ii
Readings Two different readers using a
LEICA MZ-12 dissecting microscope with
transmitted light and dark-field polarization at
between 8 and 100 times magnification aged all
sectioned otoliths (Figure 1). Both age readers
aged all of the otolith samples.
All samples were aged in chronological order
based on collection date, without knowledge of
previously estimated ages or the specimen
lengths. When the readers’ ages agreed, that age
was assigned to the fish. When the two readers
disagreed, both readers sat down together and
re-aged the fish, again without any knowledge
of previously estimated ages or lengths, and
assigned a final age to the fish. When the
readers were unable to agree on a final age, the
fish was excluded from further analysis.
Comparison Tests Age estimates from
reader 1 were plotted against age estimates from
reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test for
symmetry was used to detect any systematic
difference between the two readers (Hoenig et al.
1995). Also, a random sub-sample of 50 fish
was selected for second readings to measure
reader precision and age reproducibility. We
considered a reader to be biased if the readings
revealed consistent over or under ageing.
RESULTS
Measurements of reader self-precision were
high, with both readers able to reproduce the
ages of previously read otoliths (reader 1’s CV
= 0.8% and reader 2’s CV = 0.9%). There was
no evidence of systematic disagreement
between reader 1 and reader 2 (test of symmetry,
2 = 5, df = 4, P = 0.3). Figure 2 illustrates the
between
Figure 1. Otolith thin-section from a 1524mm TL
6 year old cobia.
Figure 2. Between-reader comparison of otolith
age estimates for cobia.
23456789101112131415
Reader 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Reader 2(2)
(10)
(21)(2)(1)
(5)(1)
(2)
(1)(13)
(1)
(2)
(2)
(2)
Otolith Ages (years)
Figure 3. Year-class distribution for cobia
collected for ageing in 2002.
Distribution is broken down by sex.
19871988198919901991199219931994199519961997199819992000
Year Class
0
5
10
15
20
Number of Fish
Females n = 46
Males n = 15
Unknown n = 4
VMRC final report on finfish ageing, 2002 cobia
Center for Quantitative Fisheries Ecology Old Dominion University
Page iii
readers’ precision of age estimates, with a
difference greater than one year observed for
only one fish. The average coefficient of
variation (CV) of 1.4% was considered not to be
significant, and slightly less than the CV of
1.8% reported last year.
Of the 65 fish aged, 10 age classes were
represented (Table 1). The average age of the
sample was 5.4 years, and the standard
deviation and standard error were 2.7 and 0.3,
respectfully.
Year-class data (Figure 3) indicates that
recruitment into the fishery begins at age 2,
which corresponds to the 2000 year-class for
cobia caught in 2002. The year-classes 1995,
1997, 1998, and 1999 dominated the sample.
Age-Length-Key In Table 2 we present an
age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using otolith
ages. The table is based on VMRC’s stratified
sampling of landings by total length inch
intervals.
REFERENCES
Franks, J.S., J.R. Warren, and M.V. Buchanan.
1999. Age and growth of cobia,
Rachycentron canadum, from the
northeastern Gulf of Mexico. Fish. Bull.
97:459-471.
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining the
consistency of age determinations. Trans.
Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A. Brown.
1995. Analysing differences between two
age determination methods by tests of
symmetry. Can. J. Fish. Aquat. Sci.
52:364-368.
S-Plus. 1999. S-Plus 4.5 Guide to Statistics.
Data Analysis Products Division. Math
Soft, Inc. Seattle, Washington.
VMRC final report on finfish ageing, 2002 cobia
Center for Quantitative Fisheries Ecology Old Dominion University
Page iv
Table 1. The number of cobia assigned to each total length-at-age category for 65 fish
sampled for age determination in Virginia during 2002. (lengths not reported for four fish)
Length Age (years)
1-inch 2 3 4 5 6 7 8 9 12 15
intervals Totals
29 - 29.99 1 0 0 0 0 0 0 0 0 0 1
31 - 31.99 1 0 0 0 0 0 0 0 0 0 1
36 - 36.99 0 0 1 0 0 0 0 0 0 0 1
37 - 37.99 0 1 0 0 0 0 0 0 0 0 1
38 - 38.99 0 1 1 0 0 0 0 0 0 0 2
39 - 39.99 0 1 1 1 0 0 0 0 0 0 3
40 - 40.99 0 2 0 0 0 0 0 0 0 0 2
41 - 41.99 0 2 1 0 0 0 0 0 0 0 3
42 - 42.99 0 0 2 0 0 0 0 0 0 0 2
43 - 43.99 0 1 3 0 0 0 0 0 0 0 4
44 - 44.99 0 0 0 0 0 1 0 0 0 0 1
45 - 45.99 0 0 1 0 0 1 0 0 0 0 2
46 - 46.99 0 0 3 0 0 0 0 0 0 1 4
47 - 47.99 0 0 6 0 0 0 0 0 0 0 6
48 - 48.99 0 0 2 0 0 1 0 0 0 0 3
49 - 49.99 0 0 0 1 0 0 0 0 0 0 1
50 - 50.99 0 1 2 2 1 0 0 0 0 0 6
51 - 51.99 0 0 0 0 0 1 0 0 0 0 1
52 - 52.99 0 0 0 2 1 0 0 0 0 0 3
53 - 53.99 0 0 0 1 0 0 0 0 0 0 1
54 - 54.99 0 0 0 0 0 1 0 0 1 0 2
56 - 56.99 0 0 0 0 0 2 0 0 0 0 2
57 - 57.99 0 0 0 0 0 1 1 0 0 0 2
58 - 58.99 0 0 0 0 0 0 0 1 0 0 1
60 - 60.99 0 0 0 0 0 1 0 0 0 0 1
61 - 61.99 0 0 0 0 0 1 0 0 0 1 2
62 - 62.99 0 0 0 0 0 1 0 1 0 0 2
64 - 64.99 0 0 0 0 0 0 0 0 1 0 1
Totals 2 9 23 7 2 11 1 2 2 2 61
VMRC final report on finfish ageing, 2002 cobia
Center for Quantitative Fisheries Ecology Old Dominion University
Page i
Table 2. Age-Length key, as proportions-at-age in each 1 inch length-class, based
on otolith ages for cobia sampled for age determination in Virginia during 2002.
Length Age (years)
1-inch 2 3 4 5 6 7 8 9 12 15
intervals N
29 - 29.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
31 - 31.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
36 - 36.99 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
37 - 37.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
38 - 38.99 0.000 0.500 0.500 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
39 - 39.99 0.000 0.333 0.333 0.333 0.000 0.000 0.000 0.000 0.000 0.000 3
40 - 40.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
41 - 41.99 0.000 0.667 0.333 0.000 0.000 0.000 0.000 0.000 0.000 0.000 3
42 - 42.99 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
43 - 43.99 0.000 0.250 0.750 0.000 0.000 0.000 0.000 0.000 0.000 0.000 4
44 - 44.99 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 1
45 - 45.99 0.000 0.000 0.500 0.000 0.000 0.500 0.000 0.000 0.000 0.000 2
46 - 46.99 0.000 0.000 0.750 0.000 0.000 0.000 0.000 0.000 0.000 0.250 4
47 - 47.99 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 6
48 - 48.99 0.000 0.000 0.667 0.000 0.000 0.333 0.000 0.000 0.000 0.000 3
49 - 49.99 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 1
50 - 50.99 0.000 0.167 0.333 0.333 0.167 0.000 0.000 0.000 0.000 0.000 6
51 - 51.99 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 1
52 - 52.99 0.000 0.000 0.000 0.667 0.333 0.000 0.000 0.000 0.000 0.000 3
53 - 53.99 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 1
54 - 54.99 0.000 0.000 0.000 0.000 0.000 0.500 0.000 0.000 0.500 0.000 2
56 - 56.99 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 2
57 - 57.99 0.000 0.000 0.000 0.000 0.000 0.500 0.500 0.000 0.000 0.000 2
58 - 58.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 1
60 - 60.99 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 1
61 - 61.99 0.000 0.000 0.000 0.000 0.000 0.500 0.000 0.000 0.000 0.500 2
62 - 62.99 0.000 0.000 0.000 0.000 0.000 0.500 0.000 0.500 0.000 0.000 2
64 - 64.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 1
Sample Size 61
VMRC final report on finfish ageing, 2002 red drum
Center for Quantitative Fisheries Ecology Old Dominion University
Page i
Chapter 5 Red Drum
Sciaenops
ocellatus
INTRODUCTION
Sixty-four red drum, Sciaenops ocellatus, were
collected for age and growth analysis in 2002.
Fifty-seven of the fish were five years-of-age or
less, while the remaining seven fish ranged in
age from 12 to 31 years old.
METHODS
Handling of collection Otoliths were
received by the Age & Growth Laboratory in
labeled coin envelopes. Once in our hands, they
were sorted based on date of capture, their
envelope labels were verified against VMRC’s
collection data, and assigned unique Age and
Growth Laboratory sample numbers. All
otoliths were stored dry in their original labeled
coin envelopes.
Preparation Otoliths were processed for
ageing following the methods described in
Bobko (1991) for black drum. Briefly, otoliths
were mounted on glass slides with Crystal Bond.
At least two serial transverse sections were cut
through the nucleus of each otolith with a
Buehler Isomet low-speed saw equipped with a
three inch, fine grit diamond-wafering blade
(Norton). After drying, a thin layer of Flo-texx
mounting medium was applied to the otolith
section to increase light transmission through
the translucent zones, which provided enhanced
contrast and greater readability.
Readings Two different readers aged all
sectioned otoliths using a Leica MZ-12
dissecting microscope with transmitted light and
dark-field polarization at between 8 and 20
times magnification (Figure 1).
All samples were aged in chronological order
based on collection date, without knowledge of
previously estimated ages or the specimen
lengths. When the readers’ ages agreed, that age
was assigned to the fish. When the two readers
disagreed, both readers sat down together and
re-aged the fish, again without any knowledge
Figure 1. Otolith thin-section from 26 year old
red drum.
VMRC final report on finfish ageing, 2002 red drum
Center for Quantitative Fisheries Ecology Old Dominion University
Page ii
of previously estimated ages or lengths, and
assigned a final age to the fish. When the
readers were unable to agree on a final age, the
fish was excluded from further analysis.
Red drum ages were based on a biological
birthdate of September 1, while year-class
assignment was based on a January 1 annual
birthdate. Red drum were treated in this manner
because of the timing of spawning and the fact
that the first annulus is not seen on an otolith
until a fish’s second spring. For example, a red
drum that was born in September of 1997 and
captured in March of 1999 would not have any
visible annuli on it’s otoliths, but would be aged
as a 1 year-old fish since it lived beyond one
September (September 1998). But this 1 year-
old fish caught in 1999 would be mistakenly
assigned to the 1998 year-class. In order to
properly assign the fish to it’s correct year-class,
1997, a January birthdate was used which would
make the fish 2 years-old (since the fish lived
past January 1998 and 1999) and year-class
would be assigned correctly.
Comparison Tests Age estimates from
reader 1 were plotted against age estimates from
reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test for
symmetry was used to detect any systematic
difference between the two readers (Hoenig et al.
1995). Also, both readers aged all 30 fish a
second time to measure reader precision and age
reproducibility. We considered a reader to be
biased if the readings revealed consistent over
or under ageing.
RESULTS
Measurements of reader self-precision were
high, with both readers able to reproduce 100 %
of the ages of previously read otoliths. Figure 2
illustrates the between readers’ precision of age
estimates. There was also 100% agreement
between readers.
Of the 64 fish aged with otoliths, 11 age classes
were represented (Table 1). The average age of
the sample was 4.7 years, and the standard
deviation and standard error were 7.6 and 0.9,
respectfully.
Year-class data (Figure 3) indicate that the 1996
through 2001 year-classes dominated the fishery.
Indicative of the trend in the recreational fishing,
very few older fish were collected in 2002.
Age-Length-Key In Table 2 we present an
age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using otolith
Figure 2. Between-reader comparison of otolith
age estimates for red rum.
135791113151719212325272931
Reader 1
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
Reader 2
(21)(20)(3)(10)(3)
(1)
(1)
(1)
(1)(1)
(2)
Otolith Ages (years)
Figure 3. Year-class distribution for red drum
collected for ageing in 2002.
Distribution is broken down by sex.
1971197319751977197919811983198519871989199119931995199719992001Year Class
0
5
10
15
20
Number of Fish
Females n = 7
Males n = 25
Unknown n = 32
VMRC final report on finfish ageing, 2002 red drum
Center for Quantitative Fisheries Ecology Old Dominion University
Page iii
ages. The table is based on VMRC’s stratified
sampling of landings by total length inch
intervals.
REFERENCES
Bobko, S. J. 1991. Age, growth, and
reproduction of black drum, Pogonias
cromis, in Virginia. M.S. thesis. Old
Dominion University, Norfolk, VA.
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining the
consistency of age determinations. Trans.
Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A. Brown.
1995. Analysing differences between two
age determination methods by tests of
symmetry. Can. J. Fish. Aquat. Sci.
52:364-368.
S-Plus. 1999. S-Plus 4.5 Guide to Statistics.
Data Analysis Products Division. Math
Soft, Inc. Seattle, Washington.
VMRC final report on finfish ageing, 2002 red drum
Center for Quantitative Fisheries Ecology Old Dominion University
Page iv
Table 1. The number of red drum assigned to each total length-at-age
category for 64 fish sampled for age determination in Virginia during 2002.
Length Age (years)
1-inch 1 2 3 4 5 12 20 24 28 29 31
intervals Totals
10 - 10.99 1 0 0 0 0 0 0 0 0 0 0 1
11 - 11.99 3 0 0 0 0 0 0 0 0 0 0 3
12 - 12.99 6 0 0 0 0 0 0 0 0 0 0 6
13 - 13.99 2 0 0 0 0 0 0 0 0 0 0 2
14 - 14.99 1 0 0 0 0 0 0 0 0 0 0 1
15 - 15.99 1 0 0 0 0 0 0 0 0 0 0 1
16 - 16.99 1 0 0 0 0 0 0 0 0 0 0 1
17 - 17.99 2 0 0 0 0 0 0 0 0 0 0 2
18 - 18.99 3 0 0 0 0 0 0 0 0 0 0 3
19 - 19.99 1 1 0 0 0 0 0 0 0 0 0 2
20 - 20.99 0 2 0 0 0 0 0 0 0 0 0 2
21 - 21.99 0 4 0 0 0 0 0 0 0 0 0 4
22 - 22.99 0 1 0 0 0 0 0 0 0 0 0 1
23 - 23.99 0 2 0 0 0 0 0 0 0 0 0 2
24 - 24.99 0 3 0 0 0 0 0 0 0 0 0 3
25 - 25.99 0 6 0 0 0 0 0 0 0 0 0 6
26 - 26.99 0 1 0 0 0 0 0 0 0 0 0 1
27 - 27.99 0 0 0 1 0 0 0 0 0 0 0 1
28 - 28.99 0 0 1 3 0 0 0 0 0 0 0 4
29 - 29.99 0 0 0 1 0 0 0 0 0 0 0 1
30 - 30.99 0 0 0 3 0 0 0 0 0 0 0 3
31 - 31.99 0 0 0 2 0 0 0 0 0 0 0 2
32 - 32.99 0 0 1 0 0 0 0 0 0 0 0 1
34 - 34.99 0 0 1 0 0 0 0 0 0 0 0 1
35 - 35.99 0 0 0 0 2 0 0 0 0 0 0 2
38 - 38.99 0 0 0 0 1 0 0 0 0 0 0 1
40 - 40.99 0 0 0 0 0 0 1 0 0 0 0 1
41 - 41.99 0 0 0 0 0 0 0 1 0 0 0 1
42 - 42.99 0 0 0 0 0 0 0 0 1 1 0 2
45 - 45.99 0 0 0 0 0 1 0 0 0 0 0 1
47 - 47.99 0 0 0 0 0 0 0 0 0 0 2 2
Totals 21 20 3 10 3 1 1 1 1 1 2 64
VMRC final report on finfish ageing, 2002 red drum
Center for Quantitative Fisheries Ecology Old Dominion University
Page v
Table 2. Age-Length key, as proportions-at-age in each 1 inch length-class, based
on otolith ages for red drum sampled for age determination in Virginia during 2002.
Length Age (years)
1-inch 1 2 3 4 5 12 20 24 28 29 31
intervals N
10 - 10.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
11 - 11.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 3
12 - 12.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 6
13 - 13.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
14 - 14.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
15 - 15.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
16 - 16.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
17 - 17.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
18 - 18.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 3
19 - 19.99 0.500 0.500 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
20 - 20.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
21 - 21.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 4
22 - 22.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
23 - 23.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
24 - 24.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 3
25 - 25.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 6
26 - 26.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
27 - 27.99 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
28 - 28.99 0.000 0.000 0.250 0.750 0.000 0.000 0.000 0.000 0.000 0.000 0.000 4
29 - 29.99 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
30 - 30.99 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 3
31 - 31.99 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
32 - 32.99 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
34 - 34.99 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
35 - 35.99 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 2
38 - 38.99 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 1
40 - 40.99 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 1
41 - 41.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 1
42 - 42.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.500 0.500 0.000 2
45 - 45.99 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 1
47 - 47.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 2
Sample Size 64
VMRC final report on finfish ageing, 2002 spadefish
Center for Quantitative Fisheries Ecology Old Dominion University
Page i
Chapter 6 Atlantic Spadefish
Chaetodipterus
faber
INTRODUCTION
A total of 336 spadefish, Chaetodipterus faber,
was collected for age and growth analysis in
2002. The average age of the sample was 3.1
years, and the standard deviation and standard
error were 3.1 and 0.2, respectfully.
METHODS
Handling of collection Otoliths were
received by the Age & Growth Laboratory in
labeled coin envelopes. Once in our hands, they
were sorted based on date of capture, their
envelope labels were verified against VMRC’s
collection data, and assigned unique Age and
Growth Laboratory sample numbers. All
otoliths were stored dry in labeled cell well
trays.
Preparation Otoliths were processed for
ageing using a bake and thin-section technique.
Each otolith was mounted onto a glass slide
with Aremco's clear Crystalbond™ 509
adhesive and a transverse cut was made just
outside the focus with a Buehler Isomet saw
equipped with a single three inch, high
concentration diamond wafering blade (Buehler
Series 15HC). The half of the sectioned otolith
with the focus was baked in a Thermolyne 1400
furnace at 400C for approximately one to two
minutes. The sectioned surface (through the
otolith focus) of the otolith half was attached to
a glass slide with Loctite 349 photo-active
adhesive. We then used a Hillquist Thin Section
machine to grind the embedded otolith half to a
thickness of 0.3 to 0.5mm. The otolith section
was polished on a Buehler Ecomet 3 variable
speed grinder-polisher Mark V Laboratory 30
micron polishing film. After polishing, a thin
layer of Flo-texx mounting medium was applied
to the otolith section to increase light
transmission through the translucent zones,
which provided enhanced contrast and greater
readability.
Readings Two different readers aged all
sectioned otoliths using a Leica MZ-12
dissecting microscope with transmitted light and
Figure 1. Sectioned otolith from a 3-year-old
female spadefish.
VMRC final report on finfish ageing, 2002 spadefish
Center for Quantitative Fisheries Ecology Old Dominion University
Page ii
dark-field polarization at between 8 and 100
times magnification (Figure 1).
All samples were aged in chronological order
based on collection date, without knowledge of
previously estimated ages or the specimen
lengths. When the readers’ ages agreed, that age
was assigned to the fish. When the two readers
disagreed, both readers sat down together and
re-aged the fish, again without any knowledge
of previously estimated ages or lengths, and
assigned a final age to the fish. When the
readers were unable to agree on a final age, the
fish was excluded from further analysis.
Comparison Tests Age estimates from
reader 1 were plotted against age estimates from
reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test for
symmetry was used to detect any systematic
difference between the two readers (Hoenig et al.
1995). Also, a random sub-sample of 50 fish
was selected for second readings to measure
reader precision and age reproducibility. We
considered a reader to be biased if the readings
revealed consistent over or under ageing.
RESULTS
Measurements of reader self-precision were
high, with both readers able to reproduce the
ages of previously read otoliths (reader 1’s CV
= 2.5% and reader 2’s CV = 5.4%). Figure 2
illustrates the between readers’ precision of age
estimates, with only six age difference greater
than one year. There was no evidence of
systematic disagreement between reader 1 and
reader 2 (test of symmetry, 2 = 18, df = 16, P
= 0.32). The average coefficient of variation
(CV) of 5.6% was considered not to be
significant, but a bit higher than the CV of 1.7%
reported in 2001.
Of the 336 fish aged with otoliths, 17 age
classes were represented (Table 1). The average
age of the sample was 3.1 years, and the
standard deviation and standard error were 3.1
and 0.2, respectfully. Year-class data (Figure 3)
show good representation from the 1995, 1999,
2000, and 2001 year-classes.
Figure 2. Between-reader comparison of otolith
age estimates for spadefish.
01234567891011121314151617
Reader 1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Reader 2
(1)(135)(7)
(14)(31)(6)
(2)(51)(3)
(1)(4)(14)(2)
(2)(6)(3)(1)
(3)(3)
(2)(18)
(2)
(3)
(2)(2)(1)
(2)(1)(3)
(2)
(1)(1)(1)(1)
(2)
(1)(1)
Otolith Ages (years)
Figure 3. Year-class distribution for spadefish
collected for ageing in 2002.
Distribution is broken down by sex.
198619881990199219941996199820002002
Year Class
0
50
100
150
Number of Fish
Females n = 160
Males n = 176
VMRC final report on finfish ageing, 2002 spadefish
Center for Quantitative Fisheries Ecology Old Dominion University
Page iii
Age-Length-Key In Table 2 we present an
age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using otolith
ages. The table is based on VMRC’s stratified
sampling of landings by total length inch
intervals.
REFERENCES
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining the
consistency of age determinations. Trans.
Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A. Brown.
1995. Analysing differences between two
age determination methods by tests of
symmetry. Can. J. Fish. Aquat. Sci.
52:364-368.
Kimura, D.K. 1980. Likelihood methods for
the von Bertalanffy growth curve. Fish.
Bull. 77:765-776.
S-Plus. 1999. S-Plus 4.5 Guide to Statistics.
Data Analysis Products Division. Math
Soft, Inc. Seattle, Washington.
VMRC final report on finfish ageing, 2002 spadefish
Center for Quantitative Fisheries Ecology Old Dominion University
Page iv
Table 1. The number of spadefish assigned to each total length-at-age category for 336 fish
collected for age determination in Virginia during 2002.
Length Age (years)
1-inch 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
intervals Totals
4 - 4.99 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
5 - 5.99 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4
6 - 6.99 0 79 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 83
7 - 7.99 0 59 12 3 0 0 0 0 0 0 0 0 0 0 0 0 0 74
8 - 8.99 0 13 11 6 0 0 0 0 0 0 0 0 0 0 0 0 0 30
9 - 9.99 0 1 6 8 0 0 0 0 0 0 0 0 0 0 0 0 0 15
10 - 10.99 0 0 0 8 2 0 0 0 0 0 0 0 0 0 0 0 0 10
11 - 11.99 0 0 0 7 1 0 0 0 0 0 0 0 0 0 0 0 0 8
12 - 12.99 0 0 0 10 0 0 0 0 0 0 0 0 0 0 0 0 0 10
13 - 13.99 0 0 0 11 2 1 0 0 0 0 0 0 0 0 0 0 0 14
14 - 14.99 0 0 0 7 2 0 1 0 0 0 0 0 0 0 0 0 0 10
15 - 15.99 0 0 0 5 6 1 1 0 0 0 0 0 0 0 0 0 0 13
16 - 16.99 0 0 0 0 2 3 0 0 0 0 0 0 0 0 0 0 0 5
17 - 17.99 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1
18 - 18.99 0 0 0 0 1 3 1 4 0 0 0 0 0 0 0 0 0 9
19 - 19.99 0 0 0 0 0 1 0 3 1 0 0 0 0 0 0 0 0 5
20 - 20.99 0 0 0 0 0 0 4 8 1 1 3 0 1 0 0 0 0 18
21 - 21.99 0 0 0 0 0 0 0 4 0 2 0 3 1 0 0 1 0 11
22 - 22.99 0 0 0 0 0 0 0 3 0 0 1 3 3 1 0 0 2 13
23 - 23.99 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 2
Totals 1 156 33 65 17 9 7 22 2 3 4 6 5 1 1 2 2 336
VMRC final report on finfish ageing, 2002 spadefish
Center for Quantitative Fisheries Ecology Old Dominion University
Page v
Table 2. Age-Length key, as proportions-at-age in each 1 inch length-class, based on otolith
ages for spadefish sampled for age determination in Virginia during 2002.
Length Age (years)
1-inch 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
intervals N
4 - 4.99 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1
5 - 5.99 0.00 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 4
6 - 6.99 0.00 0.95 0.05 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 83
7 - 7.99 0.00 0.80 0.16 0.04 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 74
8 - 8.99 0.00 0.43 0.37 0.20 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 30
9 - 9.99 0.00 0.07 0.40 0.53 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 15
10 - 10.99 0.00 0.00 0.00 0.80 0.20 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 10
11 - 11.99 0.00 0.00 0.00 0.88 0.13 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 8
12 - 12.99 0.00 0.00 0.00 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 10
13 - 13.99 0.00 0.00 0.00 0.79 0.14 0.07 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 14
14 - 14.99 0.00 0.00 0.00 0.70 0.20 0.00 0.10 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 10
15 - 15.99 0.00 0.00 0.00 0.38 0.46 0.08 0.08 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 13
16 - 16.99 0.00 0.00 0.00 0.00 0.40 0.60 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5
17 - 17.99 0.00 0.00 0.00 0.00 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1
18 - 18.99 0.00 0.00 0.00 0.00 0.11 0.33 0.11 0.44 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 9
19 - 19.99 0.00 0.00 0.00 0.00 0.00 0.20 0.00 0.60 0.20 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5
20 - 20.99 0.00 0.00 0.00 0.00 0.00 0.00 0.22 0.44 0.06 0.06 0.17 0.00 0.06 0.00 0.00 0.00 0.00 18
21 - 21.99 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.36 0.00 0.18 0.00 0.27 0.09 0.00 0.00 0.09 0.00 11
22 - 22.99 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.23 0.00 0.00 0.08 0.23 0.23 0.08 0.00 0.00 0.15 13
23 - 23.99 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.50 0.50 0.00 2
Sample Size 336
VMRC final report on finfish ageing, 2002 Spanish mackerel
Center for Quantitative Fisheries Ecology Old Dominion University
Page i
Chapter 7
Spanish Mackerel
Scomberomorous
maculatus
INTRODUCTION
The Spanish mackerel is a common visitor to
the Chesapeake Bay from spring to autumn,
known to appear as far north as the mouth of the
Patuxent River. It is a nearshore, schooling
species that undertakes long-distance migrations.
Spanish mackerel enter the Chesapeake Bay by
May, when the water temperatures exceed
approximately 17C (63F). Spawning occurs
off the coast of Virginia from late spring
through late summer, with fish returning to
Florida waters in autumn to overwinter (Murdy
1997).
Spanish mackerel support important recreational
and commercial fisheries along the Atlantic
coast of the United States. In Virginia (ocean to
3 miles, Chesapeake Bay and tributaries), the
five-year average catch for Spanish mackerel
(1997-2001) was 176,975 pounds per year.
Two major gears accounted for 99% of the
harvest, pound net (81% of the harvest) and gill
net (18% of the harvest) and. Virginia
Recreational anglers’ average harvest of
Spanish mackerel over the same five-year
period was 99,845 fish per year.
The Atlantic Coast Cooperative Statistical
Program (ACCPS) has identified Spanish
mackerel, Scomberomorous maculatus, as a
species with no established harvest targets in the
Northeast or Southeast. This is the first
extensive biological sampling program for
Spanish mackerel in Virginia, with the goal of
collecting data ultimately useful in making
informative management decisions on future
harvests.
A total of 2124 Spanish mackerel,
Scomberomorous maculatus, was collected by
the Virginia Marine Resource Commission
(VMRC) Stock Assessment Program and the
Center for Quantitative Fisheries Ecology
(CQFE). Age was determined for 857 Spanish
mackerel using sagittal otoliths. Fifteen of the
fish were young-of-the-year fish and ranged in
size from 230 to 407.5 mm total length. The
average total length for the remaining 842 fish
was 476.7 mm and the average age was 2.1
years old. Ten age classes were observed (0 –
9), representing fish from the 1993 through
2002 year-classes.
METHODS
Sampling – A total of 2124 Spanish mackerel
was collected from May through October 2002
by CQFE and VMRC (Table 1). Each fish was
measured for total length, fork length, standard
length, sex, maturity stage and total weight.
Sagittal otoliths were removed for age and
VMRC final report on finfish ageing, 2002 Spanish mackerel
Center for Quantitative Fisheries Ecology Old Dominion University
Page ii
growth analysis for 895 of the 2124 measured
fish.
Handling of collection All otoliths and
associated data were transferred to the Center
for Quantitative Fisheries Ecology’s Age and
Growth Laboratory as they were collected. In
the lab they were sorted by date of capture, their
envelope labels verified against VMRC’s
collection data, and each fish was assigned a
unique Age and Growth Laboratory sample
number. All otoliths were stored dry in labeled
cell well plates.
Preparation Spanish mackerel otoliths were
separated into two classes based on the total
length of the fish, (1) 500 mm and (2) 500
mm. Otoliths from fish less than 500 mm were
aged whole, with no processing required.
Otoliths from fish greater than or equal to 500
mm total length were processed using an Age
and Growth Laboratory thin section technique
modified to deal with the fragile nature of
Spanish mackerel otoliths. Briefly, an otolith
from one of these larger fish was first embedded
in a 9.5 mm x 4.5 mm x 4.5 mm silicon mold
well with Loctite 349 photo-active adhesive.
The mold was placed under ultraviolet light to
cure and harden the Loctite. Th embedded
otolith was removed from the silicon mold and
the location of the core of the otolith was then
marked with an extra fine point permanent
marker. A thin transverse section was made
using a Buelher Isomet saw equipped with two
high concentration diamond wafering blades
separated by a 0.5 mm plastic spacer. The thin-
section was then covered with a thin layer of
Flo-texx mounting medium, which not only
adhered the section to the slide but also
provided enhanced contrast and greater
readability by increasing light transmission
through the sections.
Readings By convention, a birthdate of
January 1 is assigned to all Northern
Hemisphere fish species. We use a system of
age determination that assigns age class
according to the date of sacrifice with respect to
this international accepted birthdate and the
timing of annulus formation. Although an
otolith annulus is actually the combination of an
opaque and translucent band, when ageing
otliths we actually enumerate only the opaque
bands, but still refer to them as annuli. Spanish
mackerel otolith annulus formation occurs
between the months of April and June, with
younger fish tending to lay down annuli earlier
than older fish. Fish age is written first
followed by the actual number of annuli visible
listed within parentheses (e.g., 3(3)). The
presence of a “+” after the number in the
parentheses indicates new growth, or “plus
growth” visible on the structure’s margin.
Using this method, a fish sacrificed in January
before annulus formation with three visible
annuli would be assigned the same age, 4(3+),
Table 1
Summary of the total number of Spanish mackerel sampled and aged by month.
Month October-May June July August September Total
Number of fish measured 144 589 721 509 161 2124
Number of fish aged 108 381 222 108 38 857
Percent Aged 75 64.7 30.8 21.2 23.6 40.4
VMRC final report on finfish ageing, 2002 Spanish mackerel
Center for Quantitative Fisheries Ecology Old Dominion University
Page iii
as a fish with four visible annuli sacrificed in
August after annulus formation, 4(4+). Year-
class is then assigned once the reader
determines the fish’s age and takes into account
the year of capture.
Whole otoliths were aged in a petri dish filled
with water at between 8 and 40 times
magnification using a Leica MZ-12 dissecting
microscope with polarized transmitted light.
The first annulus on whole otoliths was often
easy to identify. However, determining if plus
growth beyond the first annulus was from
otolith growth after the current year’s annulus
formation or if the fish were in fact getting
ready to deposit a second annulus was
extremely difficult to determine. Consequently
young-of-the-year and one-year old fish were
differentiated from older fish based, in part, on
the greater transparency of their whole otoliths
(Figure 1). Fish older than age two and two-
year old fish depositing their second annulus on
the edge of the otolith were difficult to age with
whole otoliths (Figures 2 and 3).
Sectioned otoliths were aged with a Leica MZ-
12 dissecting microscope with polarized
transmitted light at between 8 and 40 times
magnification. The first annulus on sectioned
otoliths was often quite distant from the core,
with subsequent annuli regularly spaced along
the sulcal groove out towards the proximal
(inner-face) edge of the otolith (Figures 2 and 3).
Without knowledge of previously estimated
ages or the specimen lengths, two different
readers aged all otoliths in chronological order
based on collection date. When the readers’
ages agreed, that age was assigned to the fish.
When the two readers disagreed, both readers
sat down together and re-aged the fish, again
without any knowledge of previously estimated
ages or lengths, and assigned a final age to the
fish. When the readers were unable to agree on
a final age, the fish was excluded from further
analysis.
Figure 1. Unbaked whole otoliths from a) a 1
year-old Spanish mackerel b) a 2 year-old
Spanish mackerel.
(a)
(b)
(a)
(b)
Figure 2. A three year old spanish mackerel
otolith from a 0.6 kg male a) thin-section b)
whole otolith with part of the tip broken off.
VMRC final report on finfish ageing, 2002 Spanish mackerel
Center for Quantitative Fisheries Ecology Old Dominion University
Page iv
Collaboration with other laboratories ageing
spanish mackerel All otoliths were sent to
the National Marine Fisheries Service (NMFS)
Panama City, Florida for second readings. At
the time of submitting this report the NMFS
data were not available for comparative analysis.
Comparison Tests Age estimates from
reader 1 were plotted against age estimates from
reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test for
symmetry was used to detect any systematic
difference between the two readers (Hoenig et al.
1995).
Growth Curves von Bertalanffy Growth
Functions (VBGF) were fit to length-at-age data
for pooled sexes, and for males and females fit
separately using the function nls of S-PLUS (S-
PLUS 1999). A likelihood ratio test (Kimura
1980) was used to determine if differences exist
between VBGF parameter estimates for mean
length at age data from males and females.
RESULTS
Sampling Two major gears, pound nets
(81%) and gill nets (18%), account for the
largest percentage of historical Spanish
mackerel harvests in Virginia. Consequently,
our sampling targeted these gears. Collections
from pound nets made up 86.4% of all measured
fish and 90.5% of all aged fish (Table 2). In
addition, 9.1% of the measured fish and 3.4% of
the aged fish were collected from gill nets.
Random sampling of Spanish mackerel
provided length data representative of Virginia’s
commercial harvest. Collected Spanish
mackerel ranged from 230 mm to 800 mm in
total length and 0.07 kg to 2.9 kg in total weight.
Sampling for age determination was
proportional to the observed number at length,
with over-representation of fish from the
extremes of the length frequency distribution
(Figure 4).
Age Determination We were only able to
age 857 of the 895 Spanish mackerel collected.
The otoliths from 27 fish were unable to be aged
because they were either broken to the point that
they could not be aged whole or could not be
thin-sectioned. Age readers could not agree on
an age for 11 fish. These 38 fish were excluded
from further age and growth analysis.
(a)
(b)
Figure 3. An eight year old spanish
mackerel otolith from a 1 kg female a)
thin-section b) whole otolith.
VMRC final report on finfish ageing, 2002 Spanish mackerel
Center for Quantitative Fisheries Ecology Old Dominion University
Page v
The average between-reader coefficient of
variation (CV) was 13.6%. Figure 5 illustrates
the between readers’ precision of age estimates.
There was evidence of systematic disagreement
between reader 1 and reader 2 (test of symmetry,
2 = 194.4, df = 9, P < 0.001), with reader 2
ageing young fish older than reader 1. The final
ageing criteria listed in the Methods – Readings
section of this document were a result of the
large numbers of disagreements. Using a Leica
MZ-8 dissection microscope with a two-head
discussion tube, both readers reaged all otoliths
for which the readers disagreed using these
finalized ageing criteria.
Of the 857 Spanish mackerel aged with otoliths,
ten age classes were represented (Table 3). The
average age was 2.0 year old, and the standard
deviation and standard error were 1.5 and 0.05,
Table 2
Percent measured by gear Percent aged by gear
Pound net 86.4 90.5
Gill Net 9.1 3.4
Haul seine 4.5 6.1
Summary of the percentage of Spanish mackerel measured and aged by gear.
230268306344382420458496534572610648686724762800
Total Length (mm)
0
50
100
150
Frequency
Measured for Length
Fish Aged
Figure 4. Length frequency distribution of Spanish mackerel collected for regular length
sampling and for age determination.
VMRC final report on finfish ageing, 2002 Spanish mackerel
Center for Quantitative Fisheries Ecology Old Dominion University
Page vi
respectfully. Year-class data (Figure 6) show
that the fishery was comprised of ten year-
classes, comprising fish from the 1993 through -
2002 year-classes, with fish primarily from the
1999 and 2001 year-classes.
Likelihood ratio tests of VBGFs fitted to mean
size at age data showed a significant difference
between the sexes ( 2 = 28.2, df = 3, P <
0.001). Based on these results, we recommend
that separate VBGF parameter estimates be used
for each sex. Length-at-age data with
corresponding VBGF curves and equations are
presented in Figure 7.
Age-Length-Key In Table 4 we present an
age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using otolith
ages. The table is based on VMRC’s stratified
sampling of landings by total length inch
intervals.
REFERENCES
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining
the consistency of age eterminations.
Trans. Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A.
Brown. 1995. Analyzing
differences between two age
determination methods by tests of
symmetry. Can. J. Fish. Aquat. Sci.
52:364-368.
Kimura, D.K. 1980. Likelihood methods for
the von Bertalanffy growth curve. Fish.
Bull. 77:765-776.
Murphy E.O., R.S. Birdsong, J.A. Musick.
1997. Fishes of the Chesapeake Bay.
Smithsonian Institute Press.
Washington and London.
1993 1994 1995 1996 1997 1998 1999 2000 2001 2002
Year Class
0
100
200
300
400
500
Nu
mb
er
of
Fis
h
Females n = 593
Males n = 254
Unknown n = 10
Figure 6. Year-class frequency distribution
for Spanish mackerel collected for ageing
in 2002. Distribution for otolith ages is
broken down by sex.
0 1 2 3 4 5 6 7 8 9 10
Reader 1
0
1
2
3
4
5
6
7
8
9
10
Read
er
2
(11)
(3) (252) (12) (1)
(168) (82) (6)
(4) (54) (134) (3)
(16) (65) (7)
(6) (21)
(4) (3)
(6)
(1) (2)
(4)
(2)
Otolith Ages (years)
Figure 5. Between-reader comparison of
otolith age estimates for Spanish mackerel.
VMRC final report on finfish ageing, 2002 Spanish mackerel
Center for Quantitative Fisheries Ecology Old Dominion University
Page vii
Figure 7. Plots of separate sex length-at-
age data for Spanish mackerel collected in
2002 with fitted von Bertalanffy growth
curves and equations.
1 3 5 7 9
Age (years)
300
400
500
600
700
800
Total Length (mm)Females (n = 592 )
Males (n = 254 )
Lt = 605.85 *(1-e(- 0.27 *(age-( -2.94 ))))
Lt = 820.98 *(1-e(- 0.22 *(age-( -2.35 ))))
VMRC final report on finfish ageing, 2002 Spanish mackerel
Center for Quantitative Fisheries Ecology Old Dominion University
Page 8
Table 3. The number of Spanish mackerel assigned to each total length-at-age category for
857 fish sampled for age determination in Virginia during 2002 (no length for 1 fish).
Length Age (years)
1-inch 0 1 2 3 4 5 6 7 8 9
intervals Totals
9 - 9.99 4 0 0 0 0 0 0 0 0 0 4
10 - 10.99 2 0 0 0 0 0 0 0 0 0 2
11 - 11.99 5 1 0 0 0 0 0 0 0 0 6
12 - 12.99 1 1 0 0 0 0 0 0 0 0 2
13 - 13.99 1 9 0 0 0 0 0 0 0 0 10
14 - 14.99 0 45 0 0 0 0 0 0 0 0 45
15 - 15.99 1 141 1 1 0 0 0 0 0 0 144
16 - 16.99 1 168 0 0 1 0 0 0 0 0 170
17 - 17.99 0 65 9 9 0 0 0 0 0 0 83
18 - 18.99 0 20 19 33 1 0 0 0 0 0 73
19 - 19.99 0 11 17 25 1 0 0 0 0 0 54
20 - 20.99 0 6 5 29 8 1 0 0 0 0 49
21 - 21.99 0 4 3 33 6 1 1 0 0 1 49
22 - 22.99 0 1 5 17 6 2 0 0 1 0 32
23 - 23.99 0 0 3 17 12 3 1 0 0 3 39
24 - 24.99 0 0 1 7 17 4 0 1 0 0 30
25 - 25.99 0 0 0 3 11 3 0 0 0 0 17
26 - 26.99 0 0 0 2 8 3 1 1 0 0 15
27 - 27.99 0 0 0 1 10 7 0 2 0 0 20
28 - 28.99 0 0 0 0 3 2 0 1 1 1 8
29 - 29.99 0 0 0 0 0 0 0 2 0 1 3
30 - 30.99 0 0 0 0 0 1 0 0 0 0 1
Totals 15 472 63 177 84 27 3 7 2 6 856
VMRC final report on finfish ageing, 2002 Spanish mackerel
Center for Quantitative Fisheries Ecology Old Dominion University
Page 9
Table 4. Age-Length key, as proportions-at-age in each 1 inch length-class, based on
otolith ages for Spanish mackerel sampled for age determination in Virginia during 2002.
Length Age (years)1-inch 0 1 2 3 4 5 6 7 8 9
intervals N
9 - 9.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 4
10 - 10.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
11 - 11.99 0.833 0.167 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 6
12 - 12.99 0.500 0.500 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
13 - 13.99 0.100 0.900 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 10
14 - 14.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 45
15 - 15.99 0.007 0.979 0.007 0.007 0.000 0.000 0.000 0.000 0.000 0.000 144
16 - 16.99 0.006 0.988 0.000 0.000 0.006 0.000 0.000 0.000 0.000 0.000 170
17 - 17.99 0.000 0.783 0.108 0.108 0.000 0.000 0.000 0.000 0.000 0.000 83
18 - 18.99 0.000 0.274 0.260 0.452 0.014 0.000 0.000 0.000 0.000 0.000 73
19 - 19.99 0.000 0.204 0.315 0.463 0.019 0.000 0.000 0.000 0.000 0.000 54
20 - 20.99 0.000 0.122 0.102 0.592 0.163 0.020 0.000 0.000 0.000 0.000 49
21 - 21.99 0.000 0.082 0.061 0.673 0.122 0.020 0.020 0.000 0.000 0.020 49
22 - 22.99 0.000 0.031 0.156 0.531 0.188 0.063 0.000 0.000 0.031 0.000 32
23 - 23.99 0.000 0.000 0.077 0.436 0.308 0.077 0.026 0.000 0.000 0.077 39
24 - 24.99 0.000 0.000 0.033 0.233 0.567 0.133 0.000 0.033 0.000 0.000 30
25 - 25.99 0.000 0.000 0.000 0.176 0.647 0.176 0.000 0.000 0.000 0.000 17
26 - 26.99 0.000 0.000 0.000 0.133 0.533 0.200 0.067 0.067 0.000 0.000 15
27 - 27.99 0.000 0.000 0.000 0.050 0.500 0.350 0.000 0.100 0.000 0.000 20
28 - 28.99 0.000 0.000 0.000 0.000 0.375 0.250 0.000 0.125 0.125 0.125 8
29 - 29.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.667 0.000 0.333 3
30 - 30.99 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 1
Sample Size 856
VMRC final report on finfish ageing, 2002 spot
Center for Quantitative Fisheries Ecology Old Dominion University
Page 10
Chapter 8 Spot
Leiostomus
xanthurus
INTRODUCTION
A total of 405 spot, Leiostomus xanthurus,
was collected by the VMRC’s Stock
Assessment Program for age and growth
analysis in 2002. The average age for the
sample was 1.5 year old, and the standard
deviation and standard error were 1.0 and
0.04, respectfully. Six age classes (0 to 5)
were represented, comprising fish from the
1997-2002 year-classes, with fish
predominantly from the 2001 year-class.
METHODS
Handling of collection Otoliths were
received by the Age & Growth Laboratory
in labeled coin envelopes. Once in our
hands, they were sorted based on date of
capture, their envelope labels were verified
against VMRC’s collection data, and
assigned unique Age and Growth
Laboratory sample numbers. All otoliths
were stored dry in labeled cell well trays.
Preparation Otoliths were processed for
ageing using a thin-sectioning technique.
The first step in otolith preparation was to
grind down the otolith in a transverse plane
to its core using a Hillquist thin section
machine’s 320-mesh diamond cup wheel. To
prevent distortion of the reading surface, the
otolith was ground exactly perpendicular to
the reading plane. The ground side of the
otolith was then placed face down in a drop
of Loctite 349 photo-active adhesive on a
labeled glass slide and placed under
ultraviolet light to allow the adhesive to
harden (approximately ten minutes). The
Hillquist thin section machine’s cup wheel
was used again to grind the otolith,
embedded in Loctite, to a thickness of 0.3 to
0.5mm. Finally, a thin layer of Flo-texx
mounting medium was applied to the otolith
section to increase light transmission
through the translucent zones, which
provided enhanced contrast and greater
readability.
Figure 1. Sectioned otolith from a 1 year old male
spot.
VMRC final report on finfish ageing, 2002 spot
Center for Quantitative Fisheries Ecology Old Dominion University
Page 11
Readings Two different readers aged all
sectioned otoliths using a Leica MZ-12
dissecting microscope with transmitted light
and dark-field polarization at between 8 and
100 times magnification (Figure 1).
All samples were aged in chronological
order based on collection date, without
knowledge of previously estimated ages or
the specimen lengths. When the readers’
ages agreed, that age was assigned to the
fish. When the two readers disagreed, both
readers sat down together and re-aged the
fish, again without any knowledge of
previously estimated ages or lengths, and
assigned a final age to the fish. When the
readers were unable to agree on a final age,
the fish was excluded from further analysis.
Comparison Tests Age estimates from
reader 1 were plotted against age estimates
from reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test
for symmetry was used to detect any
systematic difference between the two
readers (Hoenig et al. 1995). Also, a random
sub-sample of 50 fish was selected for
second readings to measure reader precision
and age reproducibility. To detect any
changes or drift in our ageing methods, both
readers re-aged otoliths of 50 randomly
selected fish previously aged in 2000. We
considered a reader to be biased if the
readings revealed consistent over or under
ageing.
RESULTS
Measurements of reader precision were
high, with age disagreements for only 3 out
of 405 fish aged. Figure 2 illustrates the
between readers’ precision of age estimates.
Both reader 1 and reader 2 had 100 %
agreement for precision age reading. There
was no evidence of drift in age
determination, with 100% agreement on the
2000 precision fish for both readers.
Of the 405 fish aged with otoliths, 6 age
classes were represented (Table 1). The
average age for the sample was 1.5 year old,
Figure 2. Between-reader comparison of otolith age
estimates for spot.
0 1 2 3 4 5
Reader 1
0
1
2
3
4
5
Reader 2
(26)
(247)(1)
(2)(57)
(56)
(15)
(1)
Otolith Ages (years)
Figure 3. Year-class distribution for spot collected
for ageing in 2002. Distribution is
broken down by sex.
199719981999200020012002
Year Class
0
40
80
150
190
230
270
Number of Fish
Females n = 261
Males n = 118
Unknown n = 26
VMRC final report on finfish ageing, 2002 spot
Center for Quantitative Fisheries Ecology Old Dominion University
Page 12
and the standard deviation and standard
error were 1.0 and 0.04, respectfully.
Year-class data (Figure 3) show that the
fishery was comprised of 6 year-classes,
with fish spawned in 2001 dominating the
catch.
Age-Length-Key In Table 2 we present
an age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using
otolith ages. The table is based on VMRC’s
stratified sampling of landings by total
length inch intervals.
REFERENCES
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining the
consistency of age determinations.
Trans. Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A. Brown.
1995. Analysing differences between
two age determination methods by
tests of symmetry. Can. J. Fish. Aquat.
Sci. 52:364-368.
S-Plus. 1999. S-Plus 4.5 Guide to Statistics.
Data Analysis Products Division.
Math Soft, Inc. Seattle, Washington.
VMRC final report on finfish ageing, 2002 spot
Center for Quantitative Fisheries Ecology Old Dominion University
Page 13
Table 1. The number of spot assigned to each totallength-at-age category for 405 fish sampled for agedetermination in Virginia during 2002 (Length not reported for 2 fish).
Length Age
1-inch 0 1 2 3 4 5
intervals Totals
6 - 6.99 2 0 0 0 0 0 2
7 - 7.99 14 32 0 0 0 0 46
8 - 8.99 9 87 1 0 0 0 97
9 - 9.99 0 79 9 0 0 0 88
10 - 10.99 0 37 19 1 0 0 57
11 - 11.99 0 12 21 26 3 0 62
12 - 12.99 0 1 7 17 10 1 36
13 - 13.99 0 0 1 12 2 0 15
Totals 25 248 58 56 15 1 403
Table 2. Age-Length key, as proportions-at-age ineach 1 inch length-class, based on otolith ages for spotsampled for age determination in Virginia during 2002(Length not reported for 2 fish).
Length Age
1-inch 0 1 2 3 4 5
intervals N
6 - 6.99 1.000 0.000 0.000 0.000 0.000 0.000 2
7 - 7.99 0.304 0.696 0.000 0.000 0.000 0.000 46
8 - 8.99 0.093 0.897 0.010 0.000 0.000 0.000 97
9 - 9.99 0.000 0.898 0.102 0.000 0.000 0.000 88
10 - 10.99 0.000 0.649 0.333 0.018 0.000 0.000 57
11 - 11.99 0.000 0.194 0.339 0.419 0.048 0.000 62
12 - 12.99 0.000 0.028 0.194 0.472 0.278 0.028 36
13 - 13.99 0.000 0.000 0.067 0.800 0.133 0.000 15
Sample Size 403
VMRC final report on finfish ageing, 2002 spotted seatrout
Center for Quantitative Fisheries Ecology Old Dominion University
Page 14
Chapter 9 Spotted Seatrout
Cynoscion nebulosus
INTRODUCTION
A total of 354 spotted seatrout, Cynoscion
nebulosus, was collected by the VMRC’s
Stock Assessment Program for age and
growth analysis in 2002. We were unable to
age one fish due to the poor quality of
otoliths. The average age for the sample
was 1.5 years old, and the standard deviation
and standard error were 1.1 and 0.06,
respectfully. Six age classes (0 to 5) were
represented, comprising fish from the 1997-
2002 year-classes, with fish primarily from
the 2000 and 2001 year-classes.
METHODS
Handling of collection Otoliths were
received by the Age & Growth Laboratory
in labeled coin envelopes. They were sorted
based on date of capture, their envelope
labels were verified against VMRC’s
collection data, and each fish assigned a
unique Age and Growth Laboratory sample
number. All otoliths were stored dry in
labeled cell well trays.
Preparation The first step in seatrout
otolith preparation was to make a transverse
cut just off center of the otolith with a
Hillquist thin section machine’s cut-off saw
equipped with an HCR-100 diamond blade.
To prevent distortion of the reading surface,
the cut surface of the otolith half containing
the focus was ground down on a Hillquist
thin section machine’s 320 mesh diamond
cup wheel until perpendicular to the reading
plane. The otolith’s ground surface was
then placed face down in a drop of Loctite
349 photo-active adhesive on a labeled glass
slide and placed under ultraviolet light to
allow the adhesive to harden (approximately
ten minutes). The Hillquist thin section
machine’s cup wheel was used again to
grind the otolith, embedded in Loctite, to a
thickness of 0.3 to 0.5mm. Finally, a thin
layer of Flo-texx mounting medium was
applied to the otolith section to increase
light transmission through the translucent
zones, which provided enhanced contrast
and greater readability.
VMRC final report on finfish ageing, 2002 spotted seatrout
Center for Quantitative Fisheries Ecology Old Dominion University
Page 15
Readings Two different readers aged all
sectioned otoliths using a Leica MZ-12
dissecting microscope with transmitted light
and dark-field polarization at between 8 and
100 times magnification (Figure 1). All
samples were aged in chronological order
based on collection date, without knowledge
of previously estimated ages or the specimen
lengths. When the readers’ ages agreed, that
age was assigned to the fish. When the two
readers disagreed, both readers sat down
together and re-aged the fish, again without
any knowledge of previously estimated ages
or lengths, and assigned a final age to the
fish. When the readers were unable to agree
on a final age, the fish was excluded from
further analysis.
Comparison Tests Age estimates from
reader 1 were plotted against age estimates
from reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test
for symmetry was used to detect any
systematic difference between the two
readers (Hoenig et al. 1995). Also, both
readers aged all 50 fish a second time to
measure reader precision and age
reproducibility. To detect any changes or
drift in our ageing methods, both readers re-
aged otoliths of 50 randomly selected fish
previously aged in 2000. We considered a
reader to be biased if the readings revealed
consistent over or under ageing.
RESULTS
No bias was discovered in any of the self-
precision tests of otolith age estimates, with
both readers equally able to reproduce the
ages of previously read samples. There was
also 100 percent agreement between reader
age estimates. Figure 2 illustrates the
between readers’ precision of age estimates.
There was no evidence of drift in age
determination from the 2000 precision fish
with 100% agreement for both readers.
Of the 353 fish aged with otoliths, 6 age
classes were represented (Table 1). The
average age for the sample was 1.5 years old,
and the standard deviation and standard
error were 1.1 and 0.06, respectfully.
Figure 1. Sectioned otolith from an 8 year old
male spotted seatrout.
VMRC final report on finfish ageing, 2002 spotted seatrout
Center for Quantitative Fisheries Ecology Old Dominion University
Page 16
Year-class data (Figure 3) show that the
fishery was comprised of 6 year-classes,
comprising fish from the 1997-2002 year-
classes, with fish primarily from the 2000
and 2001 year-classes.
Age-Length-Key In Table 2 we present
an age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using
otolith ages. The table is based on VMRC’s
stratified sampling of landings by total
length inch intervals.
REFERENCES
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining the
consistency of age determinations.
Trans. Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A. Brown.
1995. Analysing differences between
two age determination methods by
tests of symmetry. Can. J. Fish. Aquat.
Sci. 52:364-368.
S-Plus. 1999. S-Plus 4.5 Guide to Statistics.
Data Analysis Products Division.
Math Soft, Inc. Seattle, Washington.
Figure 2. Between-reader comparison of otolith
age estimates for spotted seatrout.
0 1 2 3 4 5
Reader 1
0
1
2
3
4
5
Reader 2
(61)
(121)
(129)
(19)
(17)
(6)
Otolith Ages (years)
Figure 3. Year-class distribution for spotted
seatrout collected for ageing in 2002.
Distribution is broken down by sex.
199719981999200020012002
Year Class
0
40
80
120
Number of Fish
Females n = 174
Males n = 179
VMRC final report on finfish ageing, 2002 spotted seatrout
Center for Quantitative Fisheries Ecology Old Dominion University
Page 17
Table 1. The number of spotted seatrout assigned to each total length-at-age category for
353 fish sampled for age determination in Virginia during 2002 (no length for 1 fish).
Length Age (years)
1-inch 0 1 2 3 4 5
intervals Totals
11 - 11.99 36 0 0 0 0 0 36
12 - 12.99 24 2 0 0 0 0 26
13 - 13.99 1 5 0 0 0 0 6
14 - 14.99 0 2 0 0 0 0 2
15 - 15.99 0 13 0 0 0 0 13
16 - 16.99 0 17 2 0 0 0 19
17 - 17.99 0 30 6 0 0 0 36
18 - 18.99 0 24 5 0 0 0 29
19 - 19.99 0 14 25 0 0 0 39
20 - 20.99 0 11 29 4 0 0 44
21 - 21.99 0 2 9 3 1 0 15
22 - 22.99 0 0 20 5 4 0 29
23 - 23.99 0 0 19 2 4 1 26
24 - 24.99 0 0 12 1 6 3 22
25 - 25.99 0 0 2 2 1 1 6
26 - 26.99 0 0 0 1 1 0 2
28 - 28.99 0 0 0 1 0 0 1
29 - 29.99 0 0 0 0 0 1 1
Totals 61 120 129 19 17 6 352
VMRC final report on finfish ageing, 2002 spotted seatrout
Center for Quantitative Fisheries Ecology Old Dominion University
Page 18
Table 2. Age-Length key, as proportions-at-age in each 1 inch length-class, based on
otolith ages for spotted seatrout sampled for age determination in Virginia during 2002.
Length Age (years)
1-inch 0 1 2 3 4 5
intervals N
11 - 11.99 1.000 0.000 0.000 0.000 0.000 0.000 36
12 - 12.99 0.923 0.077 0.000 0.000 0.000 0.000 26
13 - 13.99 0.167 0.833 0.000 0.000 0.000 0.000 6
14 - 14.99 0.000 1.000 0.000 0.000 0.000 0.000 2
15 - 15.99 0.000 1.000 0.000 0.000 0.000 0.000 13
16 - 16.99 0.000 0.895 0.105 0.000 0.000 0.000 19
17 - 17.99 0.000 0.833 0.167 0.000 0.000 0.000 36
18 - 18.99 0.000 0.828 0.172 0.000 0.000 0.000 29
19 - 19.99 0.000 0.359 0.641 0.000 0.000 0.000 39
20 - 20.99 0.000 0.250 0.659 0.091 0.000 0.000 44
21 - 21.99 0.000 0.133 0.600 0.200 0.067 0.000 15
22 - 22.99 0.000 0.000 0.690 0.172 0.138 0.000 29
23 - 23.99 0.000 0.000 0.731 0.077 0.154 0.038 26
24 - 24.99 0.000 0.000 0.545 0.045 0.273 0.136 22
25 - 25.99 0.000 0.000 0.333 0.333 0.167 0.167 6
26 - 26.99 0.000 0.000 0.000 0.500 0.500 0.000 2
29 - 29.99 0.000 0.000 0.000 0.000 0.000 1.000 1
Sample Size 352
VMRC final report on finfish ageing, 2002 striped bass
Center for Quantitative Fisheries Ecology Old Dominion University
Page 19
Chapter 10 Striped Bass
Morone
saxatilis
INTRODUCTION
A total of 863 striped bass, Morone saxatilis,
was collected by the VMRC’s Stock
Assessment Program for age and growth
analysis in 2002. Only otoliths were
collected from 12 fish and only scales were
collected from 565 fish, leaving 284 fish
from which both scales and otoliths were
collected. The average scale age was 8.0
years, with 17 age classes (2 to 16, 18, and
19) comprising fish from the 1983, 1984,
and 1986-2000 year-classes. The average
otolith age was 7.9 years, with 17 age
classes (2 to 14, 18, and 20 to 22)
comprising fish from the 1980-1982, 1984,
and 1988-2000 year-classes.
METHODS
Handling of collection Otoliths and
scales were received by the Age & Growth
Laboratory in labeled coin envelopes. Once
in our hands, they were sorted based on date
of capture, their envelope labels were
verified against VMRC’s collection data,
and each fish assigned a unique Age and
Growth Laboratory sample number. All
otoliths were stored dry in labeled cell well
plates, while scales were stored in their
original coin envelopes.
Preparation
Scales – Striped bass scales were prepared
for age and growth analysis by making
acetate impressions of the scale
microstructure. Due to extreme variation in
the size and shape of scales from individual
fish, we selected only those scales that had
even margins and uniform size. We selected
a range of four to six preferred scales (based
on overall scale size) from each fish, making
sure that only non-regenerated scales were
used. Scale impressions were made on
extruded clear 020 acetate sheets (25mm x
75mm) with a Carver Laboratory Heated
Press (model “C”). The scales were pressed
with the following settings:
Pressure: 15000 psi
Temperature: 77°C (170°F)
Time: 5 to 10 min
VMRC final report on finfish ageing, 2002 striped bass
Center for Quantitative Fisheries Ecology Old Dominion University
Page 20
Striped bass scales that were the size of a
quarter (coin) or larger, were pressed
individually for up to twenty minutes. After
pressing, the impressions were viewed with
a Bell and Howell microfiche reader and
checked again for regeneration and
incomplete margins. Impressions that were
too light, or when all scales were
regenerated a new impression was made
using different scales from the same fish.
Otoliths We used a “thin-section and
bake” technique to process striped bass
otoliths for age determination. Otolith
preparation began by randomly selecting
either the right or left otolith. The otolith
was mounted with Crystal Bond onto a
standard microscope slide with its distal
surface orientated upwards. Once mounted,
a small mark was placed on the otolith
surface directly above the otolith focus. The
slide, with attached otolith, was then secured
to an Isomet saw equipped with two
diamond wafering blades separated by a 0.5
mm spacer, which was slightly smaller in
diameter than the diamond blades. The
otolith was positioned so that the wafering
blades straddled each side of the otolith
focus ink mark. It was crucial that this cut be
perpendicular to the long axis of the otolith.
Failure to do so resulted in “broadening”
and distortion of winter growth zones. A
proper cut resulted in annuli that were
clearly defined and delineated. Once cut,
the otolith section was placed into a ceramic
“Coors” spot plate well and baked in a
Thermolyne 1400 furnace at 400oC. Baking
time was otolith size dependent and gauged
by color, with a light caramel color desired.
Once a suitable color was reached the baked
thin-section was placed on a labeled glass
slide and covered with a thin layer of Flo-
texx mounting medium, which provided
enhanced contrast and greater readability by
increasing light transmission through the
sections.
Readings By convention, a birthdate of
January 1 is assigned to all Northern
Hemisphere fish species. We use a system
of age determination that assigns age class
according to the date of sacrifice with
respect to this international accepted
birthdate and the timing of annulus
formation, which occurs between the months
of May and June for striped bass. Once the
reader decides how many annuli are visible
on the ageing structure, the year class is
assigned. The year class designation, or age,
is written first followed by the actual
number of annuli visible listed within
brackets (e.g. 3(3)). The presence of a “+”
after the number in the brackets indicates
new growth, or “plus growth” visible on the
structure’s margin. Using this method, a
fish sacrificed in January before annulus
formation with three visible annuli would be
assigned the same age, 4(3), as a fish with
four visible annuli sacrificed in July after
annulus formation, 4(4).
Two different readers aged all samples in
chronological order based on collection date,
without knowledge of previously estimated
ages or the specimen lengths. When the
readers’ ages agreed, that age was assigned
to the fish. When the two readers disagreed,
both readers sat down together and re-aged
the fish, again without any knowledge of
previously estimated ages or lengths, and
assigned a final age to the fish. When the
age readers were unable to agree on a final
VMRC final report on finfish ageing, 2002 striped bass
Center for Quantitative Fisheries Ecology Old Dominion University
Page 21
age, the fish was excluded from further
analysis.
Scales - We determined fish age by viewing
acetate impressions of scales (Figure 1) with
a standard Bell and Howell R-735
microfiche reader equipped with 20 and 29
mm lenses.
Annuli on striped bass scales are identified
based on two scale microstructure features,
“crossing over” and circuli disruption.
Primarily, “crossing over” in the lateral
margins near the posterior\anterior interface
of the scale is used to determine the origin
of the annulus. Here compressed circuli
(annulus) “cross over” the previously
deposited circuli of the previous year’s
growth. Typically annuli of the first three
years can be observed transversing this
interface as dark bands. These bands remain
consistent throughout the posterior field and
rejoin the posterior\anterior interface on the
opposite side of the focus. Annuli can also
be observed in the anterior lateral field of
the scale. Here the annuli typically reveal a
pattern of discontinuous and suddenly
breaking segmented circuli. This event can
also be distinguished by the presence of
concentric white lines, which are typically
associated with the disruption of circuli.
Annuli can also be observed bisecting the
perpendicular plain of the radial striations in
the anterior field of the scale. Radii emanate
out from the focus of the scale towards the
outer corner margins of the anterior field.
These radial striations consist mainly of
segmented concave circuli. The point of
intersection between radii and annuli results
in a “straightening out” of the concave
circuli. This straightening of the circuli
should be consistent throughout the entire
anterior field of the scale. This event is
further amplified by the presence of concave
circuli neighboring both directly above and
below the annulus.
The first year’s annulus can be difficult to
locate on some scales. It is typically best
identified in the lateral field of the anterior
portion of the scale. The distance from the
focus to the first year’s annulus is typically
larger with respect to the following few
annuli. For the annuli two through six,
summer growth generally decreases
proportionally. For ages greater than six, a
crowding effect of the annuli near the outer
margins of the scale is observed. This
crowding effect creates difficulties in edge
interpretation. At this point it is best to
focus on the straightening of the circuli at
the anterior margins of the scale.
When ageing young striped bass, zero
through age two, extreme caution must be
taken as not to over age the structure.
Young fish have no point of reference to aid
in the determination of the first year; this
invariably results in over examination of the
scale and such events as hatching or
Figure 1. Scale impression of a 5-year-old male
striped bass.
VMRC final report on finfish ageing, 2002 striped bass
Center for Quantitative Fisheries Ecology Old Dominion University
Page 22
saltwater incursion marks (checks) may be
interpreted as the first year.
Otoliths – Sectioned otoliths were aged by
two different readers using a Leica MZ-12
dissecting microscope with transmitted light
and dark-field polarization at between 8 and
100 times magnification (Figure 2).
By convention an annulus is identified as the
narrow opaque zone, or winter growth.
Typically the first year’s annulus can be
determined by first locating the focus of the
otolith. The focus is generally located,
depending on preparation, in the center of
the otolith, and is visually well defined as a
dark oblong region. The first year’s annulus
can be located directly below the focus,
along the outer ridge of the sulcal groove on
the ventral and dorsal sides of the otolith.
This insertion point along the sulcal ridge
resembles a check mark (not to be confused
with a false annulus). Here the annulus can
be followed outwards along the ventral and
dorsal surfaces where it encircles the focus.
Subsequent annuli also emanate from the
sulcal ridge, however, they do not encircle
the focus, but rather travel outwards to the
distal surface of the otolith. To be
considered a true annulus, each annulus
must be rooted in the sulcus and travel
without interruption to the distal surface of
the otolith. The annuli in striped bass have a
tendency to split as they advance towards
the distal surface. As a result, it is critical
that reading path proceed in a direction
down the sulcal ridge and outwards to the
distal surface.
Comparison Tests Age estimates from
reader 1 were plotted against age estimates
from reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test
for symmetry was used to detect any
systematic difference between the two
readers (Hoenig et al. 1995). Also, a random
sub-sample of 50 fish was selected for
second readings to measure reader precision
and age reproducibility. We considered a
reader to be biased if the readings revealed
consistent over or under ageing.
RESULTS
Figure 2. Otolith thin-section of a 5-year-old
male striped bass.
VMRC final report on finfish ageing, 2002 striped bass
Center for Quantitative Fisheries Ecology Old Dominion University
Page 23
Scales Measurements of reader self-
precision were high, with both readers able
to reproduce the ages of previously read
scales (reader 1’s CV = 4.4% and reader 2’s
CV = 3.5%). In Figure 3 we present a graph
of the results for between-reader scale
ageing precision. There was good between-
reader agreement for scale age readings,
with age differences between the two
readers one year or less for 93.2% of all
aged fish. The average between-reader
coefficient of variation (CV) of 4.3% was
not significant, and comparable to the CV of
4.4% from 2001. There was evidence of
systematic disagreement between reader 1
and reader 2 (test of symmetry, 2 = 78.4,
df = 37, P < 0.01), with reader 1 tending to
age fish older than reader 2.
Of the 849 striped bass aged with scales, 17
age classes (2 to 16, 18, and 19) were
represented. The average age for the sample
was 8.0 years. The standard deviation and
standard error were 2.5 and 0.1, respectfully.
Year-class data (Figure 4) indicates that
recruitment into the fishery typically begins
at age 3, which corresponds to the 1999
year-class for striped bass caught in 2002.
Striped bass appear to fully recruit to the
fishery at age 6 (1996 year-class).
Otoliths There was good between-reader
agreement for otolith age readings using
sectioned otoliths, with age differences
between the two readers one year or less for
97.6% of all aged fish (Figure 5). The
between reader average CV for otolith age
estimates was only 1.5%, slightly less than
the CV of 2.4% reported for 2001 fish.
Unlike scale ages, there was no evidence of
systematic disagreement between reader 1
and reader 2 (test of symmetry, 2 = 25.8,
df = 17, P = 0.08.
Figure 4. Year-class frequency distribution for
striped bass collected for ageing in
2002. Distribution of scale ages is
broken down by sex.
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000Year Class
0
50
100
150
Number of Fish
Females n = 242
Males n = 267
Unknown n = 340
Figure 3. Between-reader comparison of scale
age estimates for striped bass.
2345678910111213141516171819
Reader 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Reader 2
(2)
(1)(13)(1)(1)
(7)(24)(2)
(8)(45)(9)(1)
(1)(18)(113)(15)(1)(1)
(1)(36)(91)(10)(6)
(4)(36)(52)(28)(2)
(6)(19)(88)(12)(2)(1)
(1)(1)(35)(32)(5)(1)(1)
(7)(10)(26)(5)(3)
(1)(2)(6)(13)(6)(1)(3)
(1)(1)(2)(10)(1)(1)
(1)(2)(2)(3)(1)
(1)(1)(1)
(1)
(1)
(1)(1)
(1)
scale Ages (years)
VMRC final report on finfish ageing, 2002 striped bass
Center for Quantitative Fisheries Ecology Old Dominion University
Page 24
Measurements of reader self-precision were
high, with both readers able to reproduce the
ages of previously read otoliths (Reader 1’s
CV = 0.2% and Reader 2’s CV = 1.7%).
Seventeen age classes (2-14, 18, and 20-22)
were represented for striped bass aged with
otoliths. The average age for the sample was
7.8 years. The standard deviation and
standard error were 2.8 and 0.2, respectfully.
Comparison of Scale and Otolith Ages
While the CV of otolith and scales age
estimates was only 6.3%, there was
significant evidence of systematic
disagreement between otolith and scale ages
(test of symmetry, 2 = 64.3, df = 28, P <
0.01). Scales were assigned a lower age
than otoliths for 32% of the fish. Only
14.8% of the time were scales assigned a
higher age than otoliths (Figure 6). There
was also evidence of bias between otolith
and scale ages using an age bias plot (Figure
7), again with scales generally assigned
lower ages than otoliths.
Age-Length-Key In Table 2 we present
an age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using
scale ages. The table is based on VMRC’s
stratified sampling of landings by total
length inch intervals.
RECOMMENDATIONS
We recommend that VMRC and ASMFC
use otoliths for ageing striped bass.
Although there is more preparation time for
otoliths compared to scales, as the mean age
of striped bass increases in the recovering
fishery, otoliths should provide more
reliable estimates of age. We will continue
to compare the age estimates between
otoliths and scales.
Figure 5. Between-reader comparison of
otolith age estimates for striped
bass.
2345678910111213141516171819202122
Reader 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22Reader 2
(4)(1)(12)(1)(9)(1)(4)(1)(1)(67)(4)(43)(1)(4)(2)(16)(3)(2)(62)(9)(13)(1)(1)(10)(1)(3)(2)(1)(8)(1)(1)(1)(1)(2)
(1)(1)(1)
(1)
Otolith Ages (years)
Figure 6. Comparison of otolith and scale age
estimates for striped bass.
2345678910111213141516171819202122
Otolith Age (years)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Scale Age (years)
(1)(3)(7)(4)(6)(3)
(1)
(4)(1)(10)(45)(14)(2)(1)
(2)(13)(25)(5)(1)
(4)(4)(6)(1)
(1)
(2)(8)(18)(39)(3)(1)
(1)(4)(10)(1)(4)(5)(1)
(2)(3)(2)(3)(2)(3)(1)
(2)(1)
(1)
(1)
(1)
(1)
VMRC final report on finfish ageing, 2002 striped bass
Center for Quantitative Fisheries Ecology Old Dominion University
Page 25
REFERENCES
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining the
consistency of age determinations.
Trans. Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A. Brown.
1995. Analysing differences between
two age determination methods by
tests of symmetry. Can. J. Fish. Aquat.
Sci. 52:364-368.
S-PLUS. 1999. Guide to Statistics, Vol 1.
Data Analysis and Products Division.
MathSoft, Inc. Seattle, Washington.
Figure 7. Age-bias plot for striped bass scale
and otolith age estimates.
246810121416182022
Otolith Age (years)
2
4
6
8
10
12
14
16
18
20
22
Scale Age +/- 95% Confidence Intervals
VMRC final report on finfish ageing, 2002 striped bass
Center for Quantitative Fisheries Ecology Old Dominion University
Page 26
Table 1. The number of striped bass assigned to each total length-at-age category for fish
collected for age determination in Virginia during 2002.
Length Age (years)
1-inch 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 18 19
intervals Totals
12 - 12.99 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
13 - 13.99 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
14 - 14.99 0 3 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4
15 - 15.99 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2
16 - 16.99 1 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4
17 - 17.99 0 1 2 1 1 0 0 0 0 0 0 0 0 0 0 0 0 5
18 - 18.99 1 2 8 2 3 1 0 0 0 0 0 0 0 0 0 0 0 17
19 - 19.99 1 3 5 16 8 1 0 0 0 0 0 0 0 0 0 0 0 34
20 - 20.99 0 0 5 11 17 10 0 2 0 0 0 0 0 0 0 0 0 45
21 - 21.99 0 1 3 5 16 12 2 3 0 0 0 0 0 0 0 0 0 42
22 - 22.99 0 1 2 8 20 15 3 4 0 1 0 0 0 0 0 0 0 54
23 - 23.99 0 0 2 6 15 10 5 8 2 0 0 0 0 0 0 0 0 48
24 - 24.99 0 1 1 4 8 8 11 4 1 0 0 0 0 0 0 0 0 38
25 - 25.99 0 0 0 2 12 10 7 13 2 0 0 0 0 0 0 0 0 46
26 - 26.99 0 0 0 1 10 8 5 4 4 1 0 0 0 0 0 0 0 33
27 - 27.99 0 0 0 1 12 10 8 4 1 0 0 0 0 0 0 0 0 36
28 - 28.99 0 0 0 1 13 12 4 4 1 0 1 0 0 0 0 0 0 36
29 - 29.99 0 0 0 0 6 8 5 7 1 0 0 0 0 0 0 0 0 27
30 - 30.99 0 0 0 0 3 13 5 4 2 0 0 0 0 1 1 0 0 29
31 - 31.99 0 0 0 0 1 8 5 8 2 0 0 1 0 0 1 0 0 26
32 - 32.99 0 0 0 0 1 5 9 20 1 1 0 1 0 1 0 0 0 39
33 - 33.99 0 0 0 0 0 5 5 19 8 2 3 1 1 0 0 0 0 44
34 - 34.99 0 0 0 0 0 3 6 22 10 4 8 2 0 0 0 1 0 56
35 - 35.99 0 0 0 0 1 0 9 19 9 4 6 1 0 0 0 0 0 49
36 - 36.99 0 0 0 0 1 1 2 12 10 10 3 5 0 0 0 0 0 44
37 - 37.99 0 0 0 0 0 1 1 5 8 9 1 3 1 0 1 0 0 30
38 - 38.99 0 0 0 0 1 0 1 4 2 5 4 1 0 0 0 0 0 18
39 - 39.99 0 0 0 0 0 0 0 1 3 3 0 2 0 2 0 0 0 11
40 - 40.99 0 0 0 0 0 0 0 0 0 3 0 1 2 0 1 0 0 7
41 - 41.99 0 0 0 0 0 0 0 0 0 2 1 0 0 1 0 0 0 4
42 - 42.99 0 0 0 0 0 0 0 1 0 2 0 1 1 0 0 1 0 6
43 - 43.99 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 0 0 2
44 - 44.99 0 0 0 0 0 0 0 0 0 0 1 0 0 1 1 0 0 3
45 - 45.99 0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 2
46 - 46.99 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 1
49 - 49.99 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1
Totals 3 18 30 58 149 141 93 169 67 48 28 19 5 7 7 2 1 845
VMRC final report on finfish ageing, 2002 striped bass
Center for Quantitative Fisheries Ecology Old Dominion University
Page 27
Table 2. Age-Length key, as proportions-at-age in each 1 inch length-class, based on scale ages
for striped bass sampled for age determination in Virginia during 2002.Length Age (years)
1-inch 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 18 19
intervals N
12 - 12.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
13 - 13.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
14 - 14.99 0.000 0.750 0.250 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 4
15 - 15.99 0.000 0.500 0.500 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
16 - 16.99 0.250 0.750 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 4
17 - 17.99 0.000 0.200 0.400 0.200 0.200 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 5
18 - 18.99 0.059 0.118 0.471 0.118 0.176 0.059 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 17
19 - 19.99 0.029 0.088 0.147 0.471 0.235 0.029 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 34
20 - 20.99 0.000 0.000 0.111 0.244 0.378 0.222 0.000 0.044 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 45
21 - 21.99 0.000 0.024 0.071 0.119 0.381 0.286 0.048 0.071 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 42
22 - 22.99 0.000 0.019 0.037 0.148 0.370 0.278 0.056 0.074 0.000 0.019 0.000 0.000 0.000 0.000 0.000 0.000 0.000 54
23 - 23.99 0.000 0.000 0.042 0.125 0.313 0.208 0.104 0.167 0.042 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 48
24 - 24.99 0.000 0.026 0.026 0.105 0.211 0.211 0.289 0.105 0.026 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 38
25 - 25.99 0.000 0.000 0.000 0.043 0.261 0.217 0.152 0.283 0.043 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 46
26 - 26.99 0.000 0.000 0.000 0.030 0.303 0.242 0.152 0.121 0.121 0.030 0.000 0.000 0.000 0.000 0.000 0.000 0.000 33
27 - 27.99 0.000 0.000 0.000 0.028 0.333 0.278 0.222 0.111 0.028 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 36
28 - 28.99 0.000 0.000 0.000 0.028 0.361 0.333 0.111 0.111 0.028 0.000 0.028 0.000 0.000 0.000 0.000 0.000 0.000 36
29 - 29.99 0.000 0.000 0.000 0.000 0.222 0.296 0.185 0.259 0.037 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 27
30 - 30.99 0.000 0.000 0.000 0.000 0.103 0.448 0.172 0.138 0.069 0.000 0.000 0.000 0.000 0.034 0.034 0.000 0.000 29
31 - 31.99 0.000 0.000 0.000 0.000 0.038 0.308 0.192 0.308 0.077 0.000 0.000 0.038 0.000 0.000 0.038 0.000 0.000 26
32 - 32.99 0.000 0.000 0.000 0.000 0.026 0.128 0.231 0.513 0.026 0.026 0.000 0.026 0.000 0.026 0.000 0.000 0.000 39
33 - 33.99 0.000 0.000 0.000 0.000 0.000 0.114 0.114 0.432 0.182 0.045 0.068 0.023 0.023 0.000 0.000 0.000 0.000 44
34 - 34.99 0.000 0.000 0.000 0.000 0.000 0.054 0.107 0.393 0.179 0.071 0.143 0.036 0.000 0.000 0.000 0.018 0.000 56
35 - 35.99 0.000 0.000 0.000 0.000 0.020 0.000 0.184 0.388 0.184 0.082 0.122 0.020 0.000 0.000 0.000 0.000 0.000 49
36 - 36.99 0.000 0.000 0.000 0.000 0.023 0.023 0.045 0.273 0.227 0.227 0.068 0.114 0.000 0.000 0.000 0.000 0.000 44
37 - 37.99 0.000 0.000 0.000 0.000 0.000 0.033 0.033 0.167 0.267 0.300 0.033 0.100 0.033 0.000 0.033 0.000 0.000 30
38 - 38.99 0.000 0.000 0.000 0.000 0.056 0.000 0.056 0.222 0.111 0.278 0.222 0.056 0.000 0.000 0.000 0.000 0.000 18
39 - 39.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.091 0.273 0.273 0.000 0.182 0.000 0.182 0.000 0.000 0.000 11
40 - 40.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.429 0.000 0.143 0.286 0.000 0.143 0.000 0.000 7
41 - 41.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.500 0.250 0.000 0.000 0.250 0.000 0.000 0.000 4
42 - 42.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.167 0.000 0.333 0.000 0.167 0.167 0.000 0.000 0.167 0.000 6
43 - 43.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.500 0.000 0.000 0.000 0.000 0.500 0.000 0.000 2
44 - 44.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.333 0.000 0.000 0.333 0.333 0.000 0.000 3
45 - 45.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.500 0.000 0.000 0.000 0.000 0.000 0.000 0.500 0.000 0.000 2
46 - 46.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 1
49 - 49.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 1
Sample Size 845
VMRC final report on finfish ageing, 2002 summer flounder
Center for Quantitative Fisheries Ecology Old Dominion University
Page 28
Chapter 11 Summer Flounder
Paralichthys
dentatus
INTRODUCTION
A total of 324 summer flounder,
Paralichthys dentatus, was collected by the
VMRC’s Stock Assessment Program for age
and growth analysis in 2002. Only otoliths
were collected from 5 fish and only scales
were collected from 21 fish, leaving 303 fish
for which both scales and otoliths were
collected. The average scale age was 2.9
years, representing 9 year-classes (1992, and
1995 to 2002). Fish from the 1998-2001
year-classes dominated the collection. The
average otolith age was 3.0 years,
representing 9 year-classes (1992, and 1994
to 2001).
METHODS
Handling of collection Otoliths and
scales were received by the Age & Growth
Laboratory in labeled coin envelopes. Once
in our hands, they were sorted based on date
of capture, their envelope labels were
verified against VMRC’s collection data,
and each fish assigned a unique Age and
Growth Laboratory sample number. All
otoliths were stored dry in labeled cell well
plates, while scales were stored in their
original coin envelopes.
Preparation
Scales – Summer flounder scales were
prepared for age and growth analysis by
making acetate impressions of the scale
microstructure. Due to extreme variation in
the size and shape of scales from individual
fish, we selected only those scales that had
even margins and uniform size. We selected
a range of five to ten preferred scales (based
on overall scale size) from each fish, making
sure that only non-regenerated scales were
used. Scale impressions were made on
extruded clear 020 acetate sheets (25mm x
75mm) with a Carver Laboratory Heated
Press (model “C”). The scales were pressed
with the following settings:
Pressure: 12000 to 15000 psi
Temperature: Room temperature
Time: two to three minutes
VMRC final report on finfish ageing, 2002 summer flounder
Center for Quantitative Fisheries Ecology Old Dominion University
Page 29
Otoliths – The left otoliths of summer
flounder are symmetrical in relation to the
otolith nucleus, while right otoliths are
asymmetrical (Figure 1). The right otolith
was mounted with Crystal Bond onto a
standard microscope slide with its distal
surface orientated upwards. Once mounted,
a small mark was placed on the otolith
surface directly above the otolith focus. The
slide, with attached otolith, was then secured
to an Isomet saw equipped with two
diamond wafering blades separated by a 0.5
mm spacer, which was slightly smaller in
diameter than the diamond blades. The
otolith was positioned so that the wafering
blades straddled each side of the otolith
focus ink mark. It was crucial that this cut be
perpendicular to the long axis of the otolith.
Failure to do so resulted in “broadening”
and distortion of winter growth zones. A
proper cut resulted in annuli that were
clearly defined and delineated. Once cut,
the otolith section was placed into a ceramic
“Coors” spot plate well and baked in a
Thermolyne 1400 furnace at 400oC. Baking
time was otolith size dependent and gauged
by color, with a light caramel color desired.
Once a suitable color was reached the baked
thin-section was placed on a labeled glass
slide and covered with a thin layer of Flo-
texx mounting medium, which provided
enhanced contrast and greater readability by
increasing light transmission through the
sections.
Readings By convention, a birthdate of
January 1 is assigned to all Northern
Hemisphere fish species. The Age and
Growth Lab uses a system of age
determination that assigns age class
according to the date of sacrifice with
respect to this international accepted
birthdate and the timing of annulus
formation, which occurs in Virginia’s waters
between the months of February and April.
Using this method, a fish sacrificed in
January before annulus formation with three
visible annuli will be assigned the same age
as a fish with four visible annuli sacrificed
in July after annulus formation. Once the
reader has decided how many annuli are
visible on the ageing structure, the year class
is assigned. The year class designation, or
age, is written first followed by the actual
number of annuli visible listed within
brackets (e.g. 3(3)). The presence of a “+”
after the number in the brackets indicates
new growth, or “plus growth” visible on the
structure’s margin.
Two different readers aged all samples in
chronological order based on collection date,
without knowledge of previously estimated
ages or the specimen lengths. When the
readers’ ages agreed, that age was assigned
to the fish. When the two readers disagreed,
both readers sat down together and re-aged
the fish, again without any knowledge of
previously estimated ages or lengths, and
(a) (b)
Figure 1. Whole otoliths from a 485 mm (total
length) female summer flounder. (a) left otolith
(b) right otolith.
VMRC final report on finfish ageing, 2002 summer flounder
Center for Quantitative Fisheries Ecology Old Dominion University
Page 30
assigned a final age to the fish. When the
readers were unable to agree on a final age,
the fish was excluded from further analysis.
Scales - We determined fish age by viewing
the acetate impressions of scales (Figure 2)
with a standard Bell and Howell R-735
microfiche reader equipped with 20 and 29
mm lenses.
Annuli on summer flounder scales are
primarily identified by the presence of
crossing over of circuli. Crossing over is
most evident on the lateral margins near the
posterior/anterior interface of the scale.
Here compressed circuli (annulus) “cross
over” the deposited circuli of the previous
year’s growth. Typically the annulus will
protrude partially into the ctenii of the
posterior field, but not always. Following
the annulus up into the anterior field of the
scale reveals a pattern of discontinuous and
suddenly breaking segmented circuli. This
event can also be distinguished by the
presence of concentric white lines, which
are associated with the disruption of circuli.
This pattern should be continuous
throughout the entire anterior field of the
scale. Locating the first annulus can be
difficult due to latitudinal differences in
growth rates and changes in the size of the
first annulus due to a protracted spawning
season. We consider the first annulus to be
the first continuous crossing over event
formed on the scale.
Otoliths – Sectioned otoliths were aged by
two different readers using a Leica MZ-12
dissecting microscope with transmitted light
and dark-field polarization at between 8 and
100 times magnification (Figure 3).
Summer flounder otoliths are composed of
visually distinct summer and winter growth
zones. By convention, an annulus is
identified as the narrow opaque zone, or
winter growth band. With sectioned otoliths,
to be considered a true annulus, these
growth bands must be rooted in the sulcus
and able to be followed, without interruption
to the distal surface of the otolith. The
annuli in summer flounder have a tendency
to split as they advance towards the distal
surface. As a result, it is critical that the
reading path proceeds in a direction from the
sulcus to the proximal surface. The first
annulus is located directly below the focus
and near the upper portion of the sulcal
groove. The distance from the focus to the
first year is moderate, with translucent zone
deposition gradually becoming smaller as
consecutive annuli are deposited towards the
outer edge.
Figure 2. Scale impression of a 590 mm female
summer flounder collected in
November and aged as 4-years-old
with scales. The question mark is
located at a possible “3rd” annulus.
VMRC final report on finfish ageing, 2002 summer flounder
Center for Quantitative Fisheries Ecology Old Dominion University
Page 31
Comparison Tests Age estimates from
reader 1 were plotted against age estimates
from reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test
for symmetry was used to detect any
systematic difference between the two
readers (Hoenig et al. 1995). Also, a random
sub-sample of 50 fish was selected for
second readings to measure reader precision
and age reproducibility. We considered a
reader to be biased if the readings revealed
consistent over or under ageing.
RESULTS
Scales Measurements of reader self-
precision were high, with both readers able
to reproduce the ages of previously read
scales (reader 1’s CV = 1.6% and reader 2’s
CV = 1.9%). There was no evidence of
systematic disagreement between reader 1
and reader 2 (test of symmetry, 2 = 12.1,
df = 9, P = 0.2). In Figure 4 we present a
graph of the results for between-reader scale
ageing precision. The average between-
reader coefficient of variation (CV) of 3.5%
was not significant, and slightly lower than
the CV of 4.2% from 2001.
Of the 324 fish aged with scales, 9 age-
classes (0 to 7, and 10) were represented
(Table 1). The average scale age was 2.9
years, and the standard deviation and
standard error were 1.4 and 0.1,
respectfully.
Year-class data (Figure 5) indicate that
recruitment into the fishery began at age 0,
which corresponds to the 2002 year-class for
summer flounder caught in 2002. Year-class
abundance was high for the 1999–2001
year-classes, but declined sharply in the
1998 year-class and remained low for the
Figure 3. Otolith section from a 590 mm, 6-
year-old female summer flounder
collected in November. Same fish as
Figure 2.
Figure 5. Scale year-class distribution for
summer flounder collected in 2002.
Distribution is broken down by sex.
19921993199419951996199719981999200020012002
Year Class
0
20
40
60
80
100
Number of Fish
Females n = 305
Males n = 14
Unknown n = 5
Figure 4. Between-reader comparison of scale
age estimates for summer flounder.
012345678910
Reader 1
0
1
2
3
4
5
6
7
8
9
10
Reader 2
(1)
(44)(3)
(5)(74)(2)
(6)(99)(3)
(11)(31)(2)
(1)(2)(16)(1)
(1)(4)(11)(2)
(3)(1)
(1)
scale Ages (years)
VMRC final report on finfish ageing, 2002 summer flounder
Center for Quantitative Fisheries Ecology Old Dominion University
Page 32
earlier years.
Otoliths Measurements of reader self-
precision were high, with both readers able
to reproduce the ages of previously read
otoliths (reader 1’s CV = 0.0% and reader
2’s CV = 0.6%). There was no evidence of
systematic disagreement between reader 1
and reader 2 (test of symmetry, 2 = 3, df
= 4, P = 0.56). In Figure 6 we present a
graph of the results for between-reader
otolith ageing precision. The average
between-reader coefficient of variation (CV)
of 0.3% was not significant, and less than
the CV of 1.6% reported in 2001.
Of the 308 fish aged with otoliths, 9 age-
classes (1 to 8, and 10) were represented.
The average age for the sample was 3.0
years. The standard deviation and standard
error were 1.5 and 0.1, respectfully.
Comparison of Scale and Otolith Ages
Otolith and scales ages were similar, with an
average CV of 3.2% for the 303 fish for
which both otoliths and scales were
collected. There was no evidence of
systematic disagreement between otolith and
scale ages (test of symmetry, 2 = 9.5, df
= 12, P = 0.66). In Figure 7 we present a
graph of the results for between-reader
otolith/scale ageing precision. There was
some evidence of bias between otolith and
scale ages for the oldest fish in the sample
(Figure 8), but this could be due to the
extremely small number of fish in these age
categories.
Age-Length-Key In Table 2 we present
an age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using
otolith ages. The table is based on VMRC’s Figure 6. Between-reader comparison of otolith
age estimates for summer flounder.
12345678910
Reader 1
1
2
3
4
5
6
7
8
9
10
Reader 2
(47)
(70)(1)
(1)(112)(1)
(36)(1)
(18)
(16)
(2)(1)
(2)
(1)
Otolith Ages (years)
Figure 7. Comparison of otolith and scale age
estimates for summer flounder.
12345678910
Otolith Age (years)
1
2
3
4
5
6
7
8
9
10
Scale Age (years)
(44)
(1)
(1)
(1)
(63)
(4)
(7)
(100)
(5)
(1)
(3)
(30)
(3)
(1)
(1)
(14)
(2)
(1)
(3)
(11)
(1)
(1)
(1)
(2)
(1)
(1)
Figure 8. Age-bias plot for summer flounder
scale and otolith age estimates.
12345678910
Otolith Age (years)
1
2
3
4
5
6
7
8
9
10
Scale Age +/- 95% Confidence Intervals
VMRC final report on finfish ageing, 2002 summer flounder
Center for Quantitative Fisheries Ecology Old Dominion University
Page 33
stratified sampling of landings by total
length inch intervals.
REFERENCES
Bolz, G., R. Monaghan, K. Lang, R. Gregory,
and J. Burnett. 1999. Proceedings of
the summer flounder ageing workshop,
1-2 February 1999, Woods Hole, MA. .
NOAA Tech. Memo, in press.
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining the
consistency of age determinations.
Trans. Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A. Brown.
1995. Analysing differences between
two age determination methods by
tests of symmetry. Can. J. Fish. Aquat.
Sci. 52:364-368.
S-Plus. 1999. S-Plus 4.5 Guide to Statistics.
Data Analysis Products Division.
Math Soft, Inc. Seattle, Washington.
VMRC final report on finfish ageing, 2002 summer flounder
Center for Quantitative Fisheries Ecology Old Dominion University
Page 34
Table 1. The number of summer flounder assigned to each total length-at-age
category for 324 fish sampled for age determination in Virginia during 2002.
Length Age (years)
1-inch 0 1 2 3 4 5 6 7 10
intervals Totals
9 - 9.99 1 0 0 0 0 0 0 0 0 1
11 - 11.99 0 1 0 0 0 0 0 0 0 1
12 - 12.99 0 2 0 0 0 0 0 0 0 2
13 - 13.99 0 8 1 0 0 0 0 0 0 9
14 - 14.99 0 24 15 0 0 0 0 0 0 39
15 - 15.99 0 10 19 9 0 2 1 0 0 41
16 - 16.99 0 2 19 19 2 2 0 0 0 44
17 - 17.99 0 1 15 38 6 1 0 0 0 61
18 - 18.99 0 0 9 29 10 1 1 0 0 50
19 - 19.99 0 0 1 13 7 6 1 0 0 28
20 - 20.99 0 0 1 1 8 4 2 0 0 16
21 - 21.99 0 0 0 3 3 2 2 1 0 11
22 - 22.99 0 0 0 1 2 0 4 0 1 8
23 - 23.99 0 0 0 0 1 3 1 0 0 5
24 - 24.99 0 0 0 0 0 0 4 1 0 5
25 - 25.99 0 0 0 0 0 1 0 0 0 1
26 - 26.99 0 0 0 0 0 0 1 1 0 2
Totals 1 48 80 113 39 22 17 3 1 324
VMRC final report on finfish ageing, 2002 summer flounder
Center for Quantitative Fisheries Ecology Old Dominion University
Page 35
Table 2. Age-Length key, as proportions-at-age in each 1 inch length-class, based on
scale ages for summer flounder sampled for age determination in Virginia during 2002.
Length Age (years)
1-inch 0 1 2 3 4 5 6 7 10
intervals N
9 - 9.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
11 - 11.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
12 - 12.99 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
13 - 13.99 0.000 0.889 0.111 0.000 0.000 0.000 0.000 0.000 0.000 9
14 - 14.99 0.000 0.615 0.385 0.000 0.000 0.000 0.000 0.000 0.000 39
15 - 15.99 0.000 0.244 0.463 0.220 0.000 0.049 0.024 0.000 0.000 41
16 - 16.99 0.000 0.045 0.432 0.432 0.045 0.045 0.000 0.000 0.000 44
17 - 17.99 0.000 0.016 0.246 0.623 0.098 0.016 0.000 0.000 0.000 61
18 - 18.99 0.000 0.000 0.180 0.580 0.200 0.020 0.020 0.000 0.000 50
19 - 19.99 0.000 0.000 0.036 0.464 0.250 0.214 0.036 0.000 0.000 28
20 - 20.99 0.000 0.000 0.063 0.063 0.500 0.250 0.125 0.000 0.000 16
21 - 21.99 0.000 0.000 0.000 0.273 0.273 0.182 0.182 0.091 0.000 11
22 - 22.99 0.000 0.000 0.000 0.125 0.250 0.000 0.500 0.000 0.125 8
23 - 23.99 0.000 0.000 0.000 0.000 0.200 0.600 0.200 0.000 0.000 5
24 - 24.99 0.000 0.000 0.000 0.000 0.000 0.000 0.800 0.200 0.000 5
25 - 25.99 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 1
26 - 26.99 0.000 0.000 0.000 0.000 0.000 0.000 0.500 0.500 0.000 2
Sample Size 324
VMRC final report on finfish ageing, 2002 tautog
Center for Quantitative Fisheries Ecology Old Dominion University
Page 36
Chapter 11 Tautog
Tautoga
onitis
INTRODUCTION
A total of 374 tautog, Tautoga onitis, was
collected by the VMRC’s Stock Assessment
Program for age and growth analysis in
2002. Otoliths were not collected from 3 fish
and opercula were not collected from 2 fish,
leaving 369 fish for which both otoliths and
opercula were collected. Our results and
analyses are based on operculum ages,
unless otherwise noted, to allow our data to
be directly comparable to other tautog age
and growth studies. The average operculum
age for the sample was 4.8 years, and the
standard deviation and standard error were
2.8 and 0.1, respectfully. Fourteen age-
classes (2-14, and 16) were represented,
comprising fish from the 1986 and 1988
through 2000 year-classes.
METHODS
Handling of collection Otoliths and
opercula were received by the Age &
Growth Laboratory in labeled coin
envelopes. Once in our hands, they were
sorted based on date of capture, their
envelope labels were verified against
VMRC’s collection data, and each fish
assigned a unique Age and Growth
Laboratory sample number. All otoliths
were stored dry in labeled cell well plates,
while opercula were stored frozen in their
original coin envelopes until processed.
Preparation
Opercula – Tautog opercula were boiled for
several minutes to remove any attached skin
and muscle tissue. After boiling, opercula
were examined to determine whether they
were collected whole or in some way
damaged. Opercula were allowed to dry and
finally stored in new labeled coin envelopes.
Otoliths – Because of the small size of a
tautog otolith, it required extra steps in
preparation for ageing. An otolith was first
baked in a Thermolyne 1400 furnace at
400 C for one to two minutes until it turned
a medium brown color (caramel). The
location of the core of the otolith was
marked with a felt pen and the entire otolith
was embedded in Loctite 349 adhesive,
placed under UV light, and allowed to
harden overnight. The otolith was then
transversely sectioned through the felt pen
mark with a low speed Buehler Isomet saw
equipped with double wafering blades
separated by a 0.5 mm spacer. The sectioned
side of the otolith was then placed face
VMRC final report on finfish ageing, 2002 tautog
Center for Quantitative Fisheries Ecology Old Dominion University
Page 37
down in a drop of Loctite 349 photo-active
adhesive on a labeled glass slide and placed
under ultraviolet light to allow the adhesive
to harden (approximately ten minutes). The
otolith section was then polished using a
Buehler Ecomet 3 variable speed grinder-
polisher with Mark V Laboratory 30-micron
polishing film. After polishing, a thin layer
of Flo-texx mounting medium was applied
to the otolith section to increase light
transmission through the translucent zones,
which provided enhanced contrast and
greater readability.
Readings
table with no magnification (Figure 1).
Sectioned otoliths were aged by two
different readers using a Leica MZ-12
dissecting microscope with transmitted light
and dark-field polarization at between 8 and
100 times magnification (Figure 2).
Two different readers aged all samples in
chronological order based on collection
date, without knowledge of previously
estimated ages or the specimen lengths.
When the readers’ ages agreed, that age
was assigned to the fish. When the two
readers disagreed, both readers sat down
together and re-aged the fish, again without
any knowledge of previously estimated ages
or lengths, and assigned a final age to the
fish. When the readers were unable to agree
on a final age, the fish was excluded from
further analysis.
Comparison Tests Age estimates from
reader 1 were plotted against age estimates
from reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test
for symmetry was used to detect any
systematic difference between the two
readers (Hoenig et al. 1995). Also, a random
sub-sample of 50 fish was selected for
second readings to measure reader precision
and age reproducibility. We considered a
reader to be biased if the readings revealed
consistent over or under ageing
RESULTS
Figure 1. Operculum from a 13 year-old male
tautog.
Figure 2. Otolith section from a 13 year-old
male tautog. Same fish as Figure 1.
VMRC final report on finfish ageing, 2002 tautog
Center for Quantitative Fisheries Ecology Old Dominion University
Page 38
Opercula Measurements of reader self-
precision were high, with both readers able
to reproduce the ages of previously read
opercula (Reader 1’s CV = 1.0% and Reader
2’s CV = 3.8%). In Figure 3 we present a
graph of the results for between-reader
operculum ageing precision. There was no
evidence of systematic disagreement
between reader 1 and reader 2 (test of
symmetry, 2 = 24.1, df = 17, P = 0.12).
The average between-reader coefficient of
variation (CV) of 3.6% was slightly lower
than the 4.3% reported last year.
The average operculum age for the sample
was 4.8 years, and the standard deviation
and standard error were 2.8 and 0.1,
respectfully.
Year-class data (Figure 4) indicate that full
recruitment into the fishery occurred at age
three, which corresponds to the 1999 year-
class for tautog caught in 2002. Year-class
abundance was high for the 1996–2000
year-classes.
Otoliths Measurements of reader self-
precision were high, with both readers able
to reproduce the ages of previously read
otoliths (reader 1’s CV = 0.7% and reader
2’s CV = 0.9%). There was no evidence of
systematic disagreement between reader 1
and reader 2 (test of symmetry, 2 = 12.5,
df = 13, P = 0.48). In Figure 5 we present a
graph of the results for between-reader
otolith ageing precision. The average
between-reader coefficient of variation (CV)
of 0.9 % was not significant, and slightly
smaller than the CV of 1.4% reported last
year.
Figure 3. Between-reader comparison of
operculum age estimates for tautog.
2345678910111213141516
Reader 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Reader 2
(55)(9)
(7)(85)(5)
(1)(10)(44)(9)
(2)(40)(3)
(4)(23)(1)
(4)(4)(1)
(1)(19)(4)
(7)(3)
(2)(10)(1)(1)
(1)(2)(1)(1)
(1)(2)(1)(1)
(1)(2)(1)
(2)
(1)
Opercle Ages (years)
Figure 4. Operculum year-class distribution for
tautog collected in 2002.
Distributions are broken down by sex.
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
Year Class
0
20
40
60
80
100
Number of Fish
Females n = 173
Males n = 198
Unknown n = 1
Figure 6. Comparison of otolith and operculum
age estimates for tautog.
23456789101112131415161718
Otolith Age (years)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Operculum Age (years)
(54)
(6)
(1)
(7)
(93)
(11)
(1)
(38)
(4)
(1)
(4)
(45)
(7)
(2)
(22)
(1)
(2)
(2)
(1)
(2)
(19)
(1)
(1)
(1)
(7)
(1)
(3)
(13)
(1)
(1)
(3)
(1)
(4)
(3)
(1)
(1)
(1)(2)
(1)
VMRC final report on finfish ageing, 2002 tautog
Center for Quantitative Fisheries Ecology Old Dominion University
Page 39
Of the 374 fish aged with otoliths, 14 age-
classes (2 through 14, and 18) were
represented. The average age for the sample
was 4.8 years. The standard deviation and
standard error were 2.8 and 0.2, respectfully.
Comparison of Operculum and Otolith
Ages The between-structure average CV
of 3.1% was comparable to the within
structure CV’s. There was no evidence of
systematic disagreement between otolith and
operculum ages (test of symmetry, 2 =
26.8, df = 19, P = 0.1), with operculum ages
generally younger than otolith ages. In
Figure 6 we present a graph of the results for
between-reader otolith ageing precision.
There was no evidence of bias between
otolith and operculum ages for tautog
(Figure 7).
Age-Length-Key In Table 2 we present
an age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using
operculum ages. The table is based on
VMRC’s stratified sampling of landings by
total length inch intervals.
REFERENCES
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining the
consistency of age determinations.
Trans. Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A. Brown.
1995. Analysing differences between
two age determination methods by
tests of symmetry. Can. J. Fish. Aquat.
Sci. 52:364-368.
S-Plus. 1999. S-Plus 4.5 Guide to Statistics.
Data Analysis Products Division.
Math Soft, Inc. Seattle,
Washington.
White, G.G., J.E. Kirkley, and J.A. Lucy.
1997. Quantitative assessment of
fishing mortality for tautog, Tautoga
onitis, in Virginia. Preliminary report
23456789101112131415161718
Otolith Age (years)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Operculum Age +/- 95% Confidence IntervalsFigure 7. Age-bias plot for tautog otolith and
operculum age estimates.
Figure 5. Between-reader comparison of otolith
age estimates for tautog.
23456789101112131415161718
Reader 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Reader 2
(61)(1)
(110)
(2)(42)(3)
(52)(1)
(2)(25)
(5)(1)
(1)(18)(2)
(2)(6)(2)
(11)(2)(1)
(1)(3)(4)
(5)(1)(1)
(1)(2)
(1)(1)
(1)
Otolith Ages (years)
VMRC final report on finfish ageing, 2002 tautog
Center for Quantitative Fisheries Ecology Old Dominion University
Page 40
to the Virginia Marine Recreational
Advisory Board and Virginia Marine
Resources Commission. Newport
News, VA.
VMRC final report on finfish ageing, 2002 tautog
Center for Quantitative Fisheries Ecology Old Dominion University
Page 41
Table 1. The number of tautog assigned to each total length-at-age category for 372 fish
sampled for operculum age determination in Virginia during 2002.
Length Age (years)
1-inch 2 3 4 5 6 7 8 9 10 11 12 13 14 16
intervals Totals
10 - 10.99 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1
11 - 11.99 2 5 0 0 0 0 0 0 0 0 0 0 0 0 7
12 - 12.99 1 9 3 0 0 0 0 0 0 0 0 0 0 0 13
13 - 13.99 22 19 11 5 2 0 0 0 0 0 0 0 0 0 59
14 - 14.99 28 27 13 4 3 0 0 0 0 0 0 0 0 0 75
15 - 15.99 8 13 6 10 8 0 6 3 0 0 0 1 0 0 55
16 - 16.99 0 16 13 10 5 2 6 1 4 1 1 1 0 0 60
17 - 17.99 0 10 6 10 8 1 1 1 2 1 3 0 0 0 43
18 - 18.99 0 2 2 6 2 0 4 3 4 2 1 0 0 0 26
19 - 19.99 0 0 2 5 2 1 1 1 2 0 0 1 1 0 16
20 - 20.99 0 0 0 1 1 1 1 1 0 0 0 1 2 0 8
21 - 21.99 0 0 0 0 0 0 1 0 1 0 1 0 0 0 3
22 - 22.99 0 0 0 0 0 0 0 1 0 0 0 0 0 0 1
23 - 23.99 0 0 0 0 0 1 1 0 0 0 0 0 0 0 2
24 - 24.99 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1
25 - 25.99 0 0 0 0 0 0 0 1 0 0 0 0 0 1 2
Totals 62 101 56 51 31 6 21 12 14 4 6 4 3 1 372
VMRC final report on finfish ageing, 2002 tautog
Center for Quantitative Fisheries Ecology Old Dominion University
Page 42
Table 2. Age-Length key, as proportions-at-age in each 1 inch length-class, based on
operculum ages for tautog sampled for age determination in Virginia during 2002.Length Age (years)
1-inch 2 3 4 5 6 7 8 9 10 11 12 13 14 16
intervals N
10 - 10.99 1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1
11 - 11.99 0.286 0.714 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 7
12 - 12.99 0.077 0.692 0.231 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 13
13 - 13.99 0.373 0.322 0.186 0.085 0.034 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 59
14 - 14.99 0.373 0.360 0.173 0.053 0.040 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 75
15 - 15.99 0.145 0.236 0.109 0.182 0.145 0.000 0.109 0.055 0.000 0.000 0.000 0.018 0.000 0.000 55
16 - 16.99 0.000 0.267 0.217 0.167 0.083 0.033 0.100 0.017 0.067 0.017 0.017 0.017 0.000 0.000 60
17 - 17.99 0.000 0.233 0.140 0.233 0.186 0.023 0.023 0.023 0.047 0.023 0.070 0.000 0.000 0.000 43
18 - 18.99 0.000 0.077 0.077 0.231 0.077 0.000 0.154 0.115 0.154 0.077 0.038 0.000 0.000 0.000 26
19 - 19.99 0.000 0.000 0.125 0.313 0.125 0.063 0.063 0.063 0.125 0.000 0.000 0.063 0.063 0.000 16
20 - 20.99 0.000 0.000 0.000 0.125 0.125 0.125 0.125 0.125 0.000 0.000 0.000 0.125 0.250 0.000 8
21 - 21.99 0.000 0.000 0.000 0.000 0.000 0.000 0.333 0.000 0.333 0.000 0.333 0.000 0.000 0.000 3
22 - 22.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 0.000 1
23 - 23.99 0.000 0.000 0.000 0.000 0.000 0.500 0.500 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2
24 - 24.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 0.000 0.000 1
25 - 25.99 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.500 0.000 0.000 0.000 0.000 0.000 0.500 2
Sample Size 372
VMRC final report on finfish ageing, 2002 weakfish
Center for Quantitative Fisheries Ecology Old Dominion University
Page 43
Chapter 13 Weakfish
Cynoscion regalis
INTRODUCTION
A total of 646 weakfish, Cynoscion regalis,
was collected by the VMRC’s Stock
Assessment Program for age and growth
analysis in 2002. The average age was 3.6
year old, and the standard deviation and
standard error were 1.5 and 0.06,
respectfully. Nine age classes (1 to 8, and
10) were represented, comprising fish from
the 1992 and 1994-2001 year-classes, with
fish primarily from the 1998 through 2001
year-classes.
METHODS
Handling of collection Otoliths were
received by the Age & Growth Laboratory
in labeled coin envelopes. Once in our
hands, they were sorted based on date of
capture, their envelope labels were verified
against VMRC’s collection data, and
assigned unique Age and Growth
Laboratory sample numbers. All otoliths
were stored dry in labeled cell well trays.
Preparation The first step in otolith
preparation was to grind down the otolith in
a transverse plane to its core using a
Hillquist thin section machine’s 320-mesh
diamond cup wheel cup wheel. To prevent
distortion of the reading surface, the otolith
was ground exactly perpendicular to the
reading plane. The otolith’s ground surface
was then placed face down in a drop of
Loctite 349 photo-active adhesive on a
labeled glass slide and placed under
ultraviolet light to allow the adhesive to
harden (approximately ten minutes). The
Hillquist thin section machine’s cup wheel
was used again to grind the otolith,
embedded in Loctite, to a thickness of 0.3 to
0.5mm. Finally, a thin layer of Flo-texx
mounting medium was applied to the otolith
section to increase light transmission
through the translucent zones, which
provided enhanced contrast and greater
readability.
VMRC final report on finfish ageing, 2002 weakfish
Center for Quantitative Fisheries Ecology Old Dominion University
Page 44
Readings Two different readers aged all
sectioned otoliths using a Leica MZ-12
dissecting microscope with transmitted light
and dark-field polarization at between 8 and
100 times magnification (Figure 1). Each
reader aged all of the otolith sections using
ageing criteria listed in Lowerre-Barbieri et
al. (1994). All samples were aged in
chronological order based on collection date,
without knowledge of previously estimated
ages or the specimen lengths. When the
readers’ ages agreed, that age was assigned
to the fish. When the two readers disagreed,
both readers sat down together and re-aged
the fish, again without any knowledge of
previously estimated ages or lengths, and
assigned a final age to the fish. When the
readers were unable to agree on a final age,
the fish was excluded from further analysis.
Comparison Tests Age estimates from
reader 1 were plotted against age estimates
from reader 2 to assess deviation from 1:1
equivalence (Campana et al. 1995). A test
for symmetry was used to detect any
systematic difference between the two
readers (Hoenig et al. 1995). Also, a random
sub-sample of 50 fish was selected for
second readings to measure reader precision
and age reproducibility. We considered a
reader to be biased if the readings revealed
consistent over or under ageing.
RESULTS
No bias was discovered in any of the self-
precision tests of otolith age estimates, with
both readers equally able to reproduce the
ages of previously read samples. There was
also 100 percent agreement between reader
age estimates. Figure 2 illustrates the
between readers’ precision of age estimates.
Figure 1. Sectioned otolith from a 7 year old
female weakfish.
Figure 3. Year-class frequency distribution for
weakfish collected for ageing in 2002.
Distribution is broken down by sex.
1992199319941995199619971998199920002001
Year Class
0
50
100
150
200
Number of Fish
Females n = 409
Males n = 236
Unknown n = 1
VMRC final report on finfish ageing, 2002 weakfish
Center for Quantitative Fisheries Ecology Old Dominion University
Page 45
Of the 646 fish aged with otoliths, nine age
classes were represented (Table 1). The
average age was 3.0 years old, and the
standard deviation and standard error were
1.6 and 0.06, respectfully.
Year-class data (Figure 3) show that the
fishery was comprised of nine year-classes,
comprising fish from the 1992 and 1994
through 2001 year-classes, with fish
primarily from the 1998 through 2001 year-
classes.
Age-Length-Key In Table 2 we present
an age-length-key that can be used in the
conversion of numbers-at-length in the
estimated catch to numbers-at-age using
otolith ages. The table is based on VMRC’s
stratified sampling of landings by total
length inch intervals.
REFERENCES
Campana, S.E., M.C. Annand, and J.I.
McMillan. 1995. Graphical and
statistical methods for determining the
consistency of age determinations.
Trans. Am. Fish. Soc. 124:131-138.
Hoenig, J.M., M.J. Morgan, and C.A. Brown.
1995. Analysing differences between
two age determination methods by
tests of symmetry. Can. J. Fish. Aquat.
Sci. 52:364-368.
Lowerre-Barbieri, S.K., M.E. Chittenden Jr.,
and C.M. Jones. 1994. A comparison
of a validated otolith method to age
weakfish, Cynoscion regalis, with the
traditional scale method. Fish Bull.
92:555-568.
S-Plus. 1999. S-Plus 4.5 Guide to Statistics.
Data Analysis Products Division.
Math Soft, Inc. Seattle, Washington.
Figure 2. Between-reader comparison of otolith age
estimates for weakfish.
12345678910
Reader 1
1
2
3
4
5
6
7
8
9
10
Reader 2
(119)
(141)
(189)
(87)
(55)
(32)
(21)
(1)
(1)
Otolith Ages (years)
VMRC final report on finfish ageing, 2002 weakfish
Center for Quantitative Fisheries Ecology Old Dominion University
Page 46
Table 1. The number of weakfish assigned to each total length-at-age category for
644 fish sampled for age determination in Virginia during 2002 (no lengths for 2 fish).
Length Age (years)
1-inch 1 2 3 4 5 6 7 8 10
intervals Totals
8 - 8.99 17 1 0 0 0 0 0 0 0 18
9 - 9.99 70 12 1 0 0 0 0 0 0 83
10 - 10.99 22 82 27 1 0 0 0 0 0 132
11 - 11.99 3 35 31 3 0 0 0 0 0 72
12 - 12.99 6 7 14 3 0 0 0 0 0 30
13 - 13.99 1 2 19 7 0 0 0 0 0 29
14 - 14.99 0 1 16 12 1 0 0 0 0 30
15 - 15.99 0 1 13 13 5 1 1 0 0 34
16 - 16.99 0 0 13 7 4 4 0 0 0 28
17 - 17.99 0 0 4 4 4 2 0 0 0 14
18 - 18.99 0 0 6 7 3 0 0 0 0 16
19 - 19.99 0 0 6 4 6 2 0 0 0 18
20 - 20.99 0 0 8 3 7 4 0 0 0 22
21 - 21.99 0 0 9 6 7 5 0 0 0 27
22 - 22.99 0 0 10 7 5 3 1 0 0 26
23 - 23.99 0 0 6 5 6 3 3 0 0 23
24 - 24.99 0 0 4 1 3 3 2 0 0 13
25 - 25.99 0 0 0 3 2 1 1 0 0 7
26 - 26.99 0 0 1 0 0 1 2 0 0 4
27 - 27.99 0 0 1 0 1 2 3 0 1 8
28 - 28.99 0 0 0 0 1 1 2 0 0 4
29 - 29.99 0 0 0 0 0 0 3 1 0 4
30 - 30.99 0 0 0 0 0 0 2 0 0 2
Totals 119 141 189 86 55 32 20 1 1 644
VMRC final report on finfish ageing, 2002 weakfish
Center for Quantitative Fisheries Ecology Old Dominion University
Page 47
Table 2. Age-Length key, as proportions-at-age in each 1 inch length-class, based on
otolith ages for weakfish sampled for age determination in Virginia during 2002.
Length Age (years)1-inch 1 2 3 4 5 6 7 8 10
intervals N
8 - 8.99 0.944 0.056 0.000 0.000 0.000 0.000 0.000 0.000 0.000 18
9 - 9.99 0.843 0.145 0.012 0.000 0.000 0.000 0.000 0.000 0.000 83
10 - 10.99 0.167 0.621 0.205 0.008 0.000 0.000 0.000 0.000 0.000 132
11 - 11.99 0.042 0.486 0.431 0.042 0.000 0.000 0.000 0.000 0.000 72
12 - 12.99 0.200 0.233 0.467 0.100 0.000 0.000 0.000 0.000 0.000 30
13 - 13.99 0.034 0.069 0.655 0.241 0.000 0.000 0.000 0.000 0.000 29
14 - 14.99 0.000 0.033 0.533 0.400 0.033 0.000 0.000 0.000 0.000 30
15 - 15.99 0.000 0.029 0.382 0.382 0.147 0.029 0.029 0.000 0.000 34
16 - 16.99 0.000 0.000 0.464 0.250 0.143 0.143 0.000 0.000 0.000 28
17 - 17.99 0.000 0.000 0.286 0.286 0.286 0.143 0.000 0.000 0.000 14
18 - 18.99 0.000 0.000 0.375 0.438 0.188 0.000 0.000 0.000 0.000 16
19 - 19.99 0.000 0.000 0.333 0.222 0.333 0.111 0.000 0.000 0.000 18
20 - 20.99 0.000 0.000 0.364 0.136 0.318 0.182 0.000 0.000 0.000 22
21 - 21.99 0.000 0.000 0.333 0.222 0.259 0.185 0.000 0.000 0.000 27
22 - 22.99 0.000 0.000 0.385 0.269 0.192 0.115 0.038 0.000 0.000 26
23 - 23.99 0.000 0.000 0.261 0.217 0.261 0.130 0.130 0.000 0.000 23
24 - 24.99 0.000 0.000 0.308 0.077 0.231 0.231 0.154 0.000 0.000 13
25 - 25.99 0.000 0.000 0.000 0.429 0.286 0.143 0.143 0.000 0.000 7
26 - 26.99 0.000 0.000 0.250 0.000 0.000 0.250 0.500 0.000 0.000 4
27 - 27.99 0.000 0.000 0.125 0.000 0.125 0.250 0.375 0.000 0.125 8
28 - 28.99 0.000 0.000 0.000 0.000 0.250 0.250 0.500 0.000 0.000 4
29 - 29.99 0.000 0.000 0.000 0.000 0.000 0.000 0.750 0.250 0.000 4
30 - 30.99 0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.000 2
Sample Size 644