distribution of the nurse shark, ginglymostoma cirratum, in the northern gulf of mexico

BULLETIN OF MARINE SCIENCE 88(1)73ndash80 2012httpdxdoiorg105343bms20111033

73Bulletin of Marine Sciencecopy 2012 Rosenstiel School of Marine and Atmospheric Science of the University of Miami

Distribution of the nurse shark GinGlymostoma cirratum in the

northern Gulf of Mexico

Kristin m Hannan William B Driggers iii David s Hanisko lisa m Jones

and amanda B canning

abstract

The distribution of nurse sharks Ginglymostoma cirratum (bonaterre 1778) in the northern Gulf of Mexico was examined using fisheries-independent bottom longline data collected from 1995 to 2009 results indicate that nurse sharks are most abundant in the eastern Gulf of Mexico and their occurrence is relatively rare west of the Mississippi river Delta While the eastern Gulf of Mexico had significantly different abiotic characteristics than the central and western regions it is hypothesized that nurse shark distribution in the Gulf of Mexico is most associated with hard bottom substrate Most nurse sharks were captured in groups of two or more individuals indicating this species frequently aggregates nurse sharks in the region likely aggregate in response to prey availability rather than for predator avoidance or mating purposes

Most sharks occupying pelagic waters within the neritic zone are highly mo-bile and as a result can range over broad areas within a limited temporal period conversely benthic sharks are relatively sluggish and are frequently associated with specific areas over more protracted periods (limbaugh 1975 standora and nelson 1977) The distribution of benthic sharks could therefore be more associated with distinct habitat types than more vagile species making them important components of spatially discrete ecosystems and potentially more susceptible to deleterious local-ized phenomena

in the coastal waters of the western atlantic ocean nurse sharks Ginglymostoma cirratum (bonaterre 1778) represent one of the few benthic species within the super-order Galeomorphii This species is known to inhabit continental and insular shelf waters primarily from north carolina to brazil including The bahamas caribbean sea and the Gulf of Mexico (castro 2011) While nurse sharks are frequently en-countered by divers and often captured in commercial and recreational fisheries information regarding their distribution on a finer scale is notably lacking

Within the Gulf of Mexico nurse sharks are described as occurring throughout the region (compagno 1984) More thorough accounts are essentially limited to the eastern Gulf of Mexico where nurse sharks are described as year-round residents along the West florida shelf from tampa florida to the florida keys moving sea-sonally into the northern Gulf of Mexico (bigelow and schroeder 1948 clark and von schmidt 1965 castro 2011) to our knowledge all records in the primary literature concerning nurse shark occurrence in the western Gulf of Mexico are attributable to baughman and springer (1950) who reported a single specimen off Port aransas texas and included reference to anecdotal accounts of its presence off south texas

BULLETIN OF MARINE SCIENCE VOL 88 NO 1 201274

and more southerly waters off the coasts of the Yucataacuten campeche and Vera cruz The purpose of the present study was to examine the distribution of nurse sharks in the northern Gulf of Mexico based on captures during fisheries-independent surveys and describe abiotic characteristics associated with the occurrence of the species

Materials and Methods

from 1995ndash2009 nurse sharks were captured during annual fisheries-independent bottom longline surveys conducted throughout the northern Gulf of Mexico by the national Marine fisheries service southeast fisheries science center Mississippi laboratories (Mslabs fig 1) surveys were conducted annually from approximately august to september however por-tions of the study area were not sampled in all years due to logistical constraints or adverse weather such as hurricanes bottom longline gear was set at locations with depths ranging from 6 to 449 m [mean = 6971 m plusmn 6595 (sD) fig 1] locations were randomly selected by stratified-random sampling with proportional allocation strata were defined by water depth (eg 9ndash55 55ndash183 and 183ndash366 m) with stratum size determined by continental shelf area longline gear consisted of 1852 m of 4 mm diam monofilament mainline and 100 gangions which were constructed of a snap 37 m of 3 mm diam monofilament leader and a hook hook type varied with number 3 J-hooks (Mustad model 34970D) used from 1995 through 1998 and 150 circle hooks (Mustad model 39960D) used from 2001 to 2009 during 1999 and 2000 both hook types were used soak times were limited to 1 hr unless circumstances dictated otherwise

The total length (tl) of each nurse shark captured was measured to the nearest mm from the rostrum to the posterior tip of the caudal fin or estimated by experienced personnel for those individuals that could not be landed sex of each captured individual was recorded in most cases however on a limited number of occasions it was not possible to identify the sex of excessively active sharks that were left in the water or those that escaped prior to sex de-termination a chi-square test was used to test for differences among observed and expected sex ratios (Zar 1999)

at each longline station environmental data were collected throughout the water column with a seabird sbe-911 ctD abiotic data utilized for analyses were limited to those values collected at the maximum depth of ctD deployment at each station and included tempera-ture (degc) salinity oxygen (mg lminus1) and water clarity ( transmissivity) Potential differences in abiotic parameters among regions were examined using one-way analysis of Variance (anoVa) tests followed by tukey multiple comparison tests (Zar 1999) although abiotic data were not normally distributed and heteroscedatic deviations from the underlying as-sumptions of anoVa were considered acceptable due to the large sample size associated with these data (Zar 1999 minimum n = 1118)

figure 1 locations (+) of bottom longline sets (n = 2419) conducted during national Marine fisheries service southeast fisheries science center Mississippi laboratories bottom long-line surveys in the northern Gulf of Mexico from 1995 to 2009 The 60 100 and 200 m iso-baths are shown

hannan et al nurse shark distribution in northern gulf of mexico 75

bottom type data for the location of each longline set were extracted from the Gulf of Mexico seabed Geodatabase (rester 2009) based on the starting latitude and longitude of each set The seabed Geodatabase classifies bottom type as mud dominant mud very dominant sand dominant sand very dominant gravel dominant gravel very dominant rock dominant and rock very dominant These classifications were condensed into bottom type classifica-tions of mud sand gravel or rock logistic regression was used to examine the relationship between binomial catch rate (ie no catch positive catch) environmental characteristics and bottom type with insignificant variables being excluded in the final model using a backward selection procedure for the purpose of analyses and discussion the eastern central and western regions of the Gulf of Mexico were defined by the longitudinal bounds 81degWndash88degW 88degWndash93degW and 93degWndash98degW respectively

results

over the course of the study 2419 longline sets were completed and resulted in the capture of 209 nurse sharks with 54 occurring in groups ranging from two to nine individuals nurse sharks ranged from 59 to 300 cm tl (mean = 203 cm tl plusmn 34 the largest directly measured shark was 263 cm tl) and based on the sizes at birth and maturity reported by castro (2000) were all considered juveniles or adults no neonates or young-of-the-year were captured (fig 2) The observed female to male ratio (1102) was not significantly different than the expected sex ratio of 11 (χ2 = 002 P gt 005) and 62 of aggregations consisted of both sexes nurse sharks were

figure 2 length frequency of nurse sharks Ginglymostoma cirratum caught in the northern Gulf of Mexico from 1995 to 2009 The largest directly measured nurse shark was 263 cm tl

figure 3 Distribution of nurse sharks Ginglymostoma cirratum captured from 1995 to 2009 in the northern Gulf of Mexico closed circles represent single individuals + represents two individuals open circles represent three to nine individuals The 60 m isobath is indicated 88degW longitude separates the transition from terrigenous clay-dominated substrate to the west and sand to the east


captured primarily in the eastern Gulf of Mexico along the West florida shelf (962 of captures) with few catches extending into the central Gulf of Mexico off the coasts of alabama and Mississippi (38 of captures fig 3) no nurse sharks were caught on longline gear west of the Mississippi river Delta (~89degW) or during any of the 924 longline sets conducted at depths gt 60 m (figs 1 4) of the nurse sharks that were caught depth of capture ranged from 11 to 55 m (mean = 2570 m plusmn 828)

Differences in abiotic variables were observed among the three defined regions in the Gulf of Mexico with the eastern Gulf having the statistically highest salin-ity (anoVa f = 3596 P lt 001) dissolved oxygen (anoVa f = 27277 P lt 001) and water clarity (anoVa f = 15583 P lt 001) Mean temperature was signifi-cantly higher in the eastern and western Gulf of Mexico than in the central region (anoVa f = 929 P lt 001 table 1) habitats where nurse sharks were captured were characterized by relatively high temperature salinity dissolved oxygen and water clarity

The majority of the longline survey effort was conducted over sand (409) and mud (427) bottom types The predominate bottom type for longline stations changed across the survey area from mud bottom in the west and central Gulf of Mexico (637 and 661 respectively) to hard bottom types (gravel rock and sand) in the eastern Gulf of Mexico (887) over 93 of the longline stations where nurse sharks were caught occurred over hard bottom types (fig 5) There was a significant relationship between temperature (χ2 = 401 P = 004) depth (χ2 = 496 P = 003)

figure 4 number of longline sets by depth conducted during national Marine fisheries service southeast fisheries science center Mississippi laboratories bottom longline surveys in the northern Gulf of Mexico from 1995 to 2009

figure 5 Percent of mud bottom (shaded) and hard bottom (unshaded) type for locations in the northern Gulf of Mexico where nurse sharks were and were not caught hard bottom type includes gravel rock and sand bottom types


Tabl

e 1

Ran

ge a

nd m

ean

of a

biot

ic c

hara

cter

istic

s as

soci

ated

with

long

line

sets

con

duct

ed in

eas

tern

cen

tral

and

wes

tern

por

tions

of

the

north

ern

Gul

f of

M

exic

o A

lso

incl

uded

are

the

sam

e va

lues

for l

ocat

ions

whe

re n

urse

sha

rks

Gin

gym

osto

ma

cirr

atum

wer

e ca

ptur

ed L

ette

rs li

sted

with

mea

n va

lues

indi

cate

st

atis

tical

gro

upin

gs w

ithin

each

hea

ding

Dom

inan

t bot

tom

type

for e

ach

regi

on is

list

ed w

ith p

erce

ntag

e of l

ongl

ine s

ets c

ompl

eted

ove

r giv

en b

otto

m ty

pe H

ard

botto

m ty

pe in

clud

es ro

ck g

rave

l an

d sa

nd b

otto

m ty

pes

Reg

ion

of G

ulf o

f Mex

ico

Tem

pera

ture

(degC

)Sa

linity

Dis

solv

ed o

xyge

n (m

g Lminus

1 )W

ater

cla

rity

( tr

ansm

issi

vity

)B

otto

m ty

pes

East

ern

rang

e15

45ndash

315

029

84ndash

375

80

80ndash8

70

231

6ndash92

39

Har

d (8

87)

mea

n plusmn

SD26

28

plusmn 3

21A

359

4 plusmn

079

A5

31 plusmn

11

9A78

49

plusmn 9

39A

Cen

tral

rang

e18

40ndash

311

426

31ndash

366

20

00ndash8

30

001

ndash87

90M

ud (6

61)

mea

n plusmn

SD25

41

plusmn 2

97B

353

2 plusmn

165

B3

23 plusmn

16

7 C

541

4 plusmn

227

2B

Wes

tern

ra

nge

145

6ndash30

45

264

9ndash36

92

000

ndash89

00

01ndash8

780

Mud

(63

7)m

ean

plusmn SD

259

7 plusmn

322

A35

25

plusmn 1

94B

488

plusmn 1

37B

574

3 plusmn

200

7B

Loca

tions

of n

urse

shar

k ca

ptur

esra

nge

221

2ndash31

21

319

0ndash37

55

360

ndash80

059

17ndash

904

1m

ean

plusmn SD

281

4 plusmn

201

357

4 plusmn

089

544

plusmn 0

91

787

9 plusmn

663


water clarity (χ2 = 2473 P lt 001) and bottom type (χ2 = 1182 P lt 001) and positive catches (deviance = 15413 P lt 001 goodness of fit = 0982 P = 032)

Discussion

results of our study demonstrate that nurse sharks are not equally abundant throughout the northern Gulf of Mexico and that the distribution of these sharks is heavily skewed toward the east The majority of nurse sharks were caught over hard bottom types in relatively warm clear shallow waters There were significant differ-ences in abiotic parameters among the regions sampled however it is more likely that the observed distribution of nurse sharks in the northern Gulf of Mexico is re-lated to bottom type While nurse sharks were captured at locations with a mud sub-strate 93 were collected on bottom longline gear set over hard substrate consisting of gravel rock or sand While the mean abiotic parameters over which nurse sharks were collected in the eastern Gulf of Mexico differed significantly from the western and central Gulf of Mexico the ranges of those conditions overlapped among the three areas indicating that if abiotic thresholds drive shark distributions then nurse sharks should be present wherever appropriate conditions exist

habitat regions defined by depth latitude and sediment type have been shown to regulate benthic community structure across the Gulf of Mexico producing at times distinctive boundaries among regions (rabalais et al 1999 roberts et al 1999) one such boundary characterized by changing sediment grain sizes extends south from Mobile bay (~88degW longitude) and corresponds to the area where nurse shark catches abruptly decline That bottom sediments in this area transition from terrigenous clay to quartz sand (balsam and beeson 2003) could be a major causative factor in why nurse sharks are less abundant west of this region

although no nurse sharks were collected in the western Gulf of Mexico during our study a query of data resulting from 12334 bottom trawls conducted from 1987 to 2009 by Mslabs resulted in the identification of two captures of nurse sharks west of the Mississippi river Delta a comparison of trawl capture location and the Gulf of Mexico seabed Geodatabase indicated that the two nurse sharks were caught over mud bottom in areas known to have relict shorelines and reefs with patchy occur-rences of sand bottom (rabalais et al 1999) These relict areas could have provided appropriate habitat for the nurse sharks and explain their occurrence in the mud-dominated western Gulf of Mexico regardless the extremely low incidence of nurse sharks during such extensive sampling effort further suggests their presence in the western Gulf of Mexico is relatively rare

in addition to being localized in the eastern Gulf of Mexico the distribution of nurse sharks also appears to be somewhat clumped rather than uniformly distrib-uted along the coastline While there were incidences of single catches most nurse shark catches were from longline stations resulting in the capture of multiple indi-viduals This could be indicative of aggregating behavior of nurse sharks to preferred habitat a similar observation was made by castro (2000) who noted that nurse sharks often occur in large groups resting on the bottom around structure carraro and Gladstone (2006) found evidence for another orectolobiform orectolobus or-natus (De Vis 1883) forming aggregations in association with structurally complex habitat and suggested that this selection of habitat could be indicative of prey avail-ability or a predator avoidance behavior Prey availability could explain nurse shark


aggregation behavior in the eastern Gulf of Mexico as prey organisms would be more readily available in association with complex structure than over featureless bottom Predator avoidance however is unlikely to influence nurse shark distributions in the northern Gulf of Mexico as there is no evidence particularly for adults of strong pre-dation pressure alternatively as adult males and females occurred together groups of nurse sharks in the eastern Gulf of Mexico could represent mating aggregations however this is also unlikely as mating nurse sharks are commonly observed in the southern extent of the eastern Gulf of Mexico during early summer (eg Gudger 1912 carrier et al 1994)

an understanding of a species distribution and the underlying processes that de-termine those patterns is a necessary component to defining critical habitat and thus evaluating the potential effects of exploitation (bell et al 1991 Dicken et al 2007) in the past nurse sharks were harvested for their hides and liver oil as well as for hu-man consumption however on a large-scale these uses were short-lived as demand for shark leather and oil diminished and relative to other shark species nurse shark meat and fins are considered to have little to no market value (Vannuccini 1999) The relatively low fishing pressure exerted on nurse sharks compared to other shark spe-cies provides data on natural distributions of this coastal shark which can serve as baseline information for management purposes if nurse shark populations or their habitat become more impacted in the future

acknowledgments

We thank M Grace a Pollock the crews of the noaa research vessels caretta Gandy Gunter and oregon ii and all the participants on the nMfsMslabs bottom longline surveys We also acknowledge J castro e hoffmayer and three anonymous reviewers for their useful comments

literature cited

balsam Wl beeson JP 2003 sea-floor sediment distribution in the Gulf of Mexico Deep-sea res Part 1 501421ndash1444 httpdxdoiorg101016jdsr200306001

baughman Jl springer s 1950 biological and economic notes on sharks of the Gulf of Mexico with special reference to those in texas and with a key for their identification am Midl nat 4496ndash152 httpdxdoiorg1023072421758

bell ss Mccoy eD Mushinsky hr 1991 habitat structure the physical arrangement of ob-jects in space london chapman and hall

bigelow hb schroeder Wc 1948 fishes of the western north atlantic Part 1 lancelets cy-clostomes and sharks new haven Mem sears found Mar res 576 p

carraro r Gladstone W 2006 habitat preferences and site fidelity of the ornate wobbegong shark (orectolobus ornatus) on rocky reefs of new south Wales Pac sci 60(2)207ndash223 httpdxdoiorg101353psc20060003

carrier Jc Pratt Jr hl Martin lk 1994 Group reproductive behaviors in free-living nurse sharks Ginglymostoma cirratum copeia 1994646ndash656 httpdxdoiorg1023071447180

castro Ji 2000 The biology of the nurse shark Ginglymostoma cirratum off the florida east coast and the bahama islands env biol fish 581ndash22 httpdxdoiorg101023a1007698017645

castro Ji 2011 The sharks of north america new York oxford university Pressclark e von schmidt k 1965 sharks of the central Gulf coast of florida bull Mar sci 1513ndash83


compagno lJV 1984 fao species catalog Vol 4 sharks of the world an annotated and illus-trated catalogue of shark species known to date Part 1 hexanchiformes to lamniformes fao fish synop 4202ndash207

Dicken Ml booth aJ smale MJ cliff G 2007 spatial and seasonal distribution patterns of juvenile and adult raggedtooth sharks (carcharias taurus) tagged off the east coast of south africa Mar freshwat res 58127ndash134 httpdxdoiorg101071Mf06018

Gudger eW 1912 summary of work done on the fi shes of the Dry tortugas carnegie inst Wash Year 11148ndash150

limbaugh c 1975 field notes on sharks in Gilbert PW editor sharks and survival lexington Massachusetts Dc heath and company p 63ndash94

rabalais nn carney rs escobar-briones eG 1999 overview of continental shelf benthic communities of the Gulf of Mexico in kumpf h steidinger k sherman k editors Th e Gulf of Mexico large marine ecosystem oxford blackwell science inc p 171ndash195

rester Jk 2009 Distribution of bottom habitat information in the Gulf of Mexico unpublished report Marfin final report Project number na05nMf4331073

roberts hh Mcbride ra coleman JM 1999 outer shelf and slope geology of the Gulf of Mexico an overview in kumpf h steidinger k sherman k editors Th e Gulf of Mexico large marine ecosystem oxford blackwell science inc p 93ndash112

standora ea nelson Dr 1977 a telemetric study of free swimming Pacifi c angel sharks squatina californica bull south california acad sci 76193ndash201

Vannuccini s 1999 shark utilization marketing and trade fao fisheries technical Paper 389 rome fao 470 p

Zar Jh 1999 biostatistical analysis 4th edition upper saddle river nJ Prentice hall

Date suBmitted 16 May 2011Date accePted 24 august 2011aVailaBle online 16 november 2011

addresses national marine Fisheries service southeast Fisheries science center mississippi laboratories Po Drawer 1207 Pascagoula mississippi 39567 corresPonding author (kMh) E-mail ltkristinhannannoaagovgt


and more southerly waters off the coasts of the Yucataacuten campeche and Vera cruz The purpose of the present study was to examine the distribution of nurse sharks in the northern Gulf of Mexico based on captures during fisheries-independent surveys and describe abiotic characteristics associated with the occurrence of the species

Materials and Methods

from 1995ndash2009 nurse sharks were captured during annual fisheries-independent bottom longline surveys conducted throughout the northern Gulf of Mexico by the national Marine fisheries service southeast fisheries science center Mississippi laboratories (Mslabs fig 1) surveys were conducted annually from approximately august to september however por-tions of the study area were not sampled in all years due to logistical constraints or adverse weather such as hurricanes bottom longline gear was set at locations with depths ranging from 6 to 449 m [mean = 6971 m plusmn 6595 (sD) fig 1] locations were randomly selected by stratified-random sampling with proportional allocation strata were defined by water depth (eg 9ndash55 55ndash183 and 183ndash366 m) with stratum size determined by continental shelf area longline gear consisted of 1852 m of 4 mm diam monofilament mainline and 100 gangions which were constructed of a snap 37 m of 3 mm diam monofilament leader and a hook hook type varied with number 3 J-hooks (Mustad model 34970D) used from 1995 through 1998 and 150 circle hooks (Mustad model 39960D) used from 2001 to 2009 during 1999 and 2000 both hook types were used soak times were limited to 1 hr unless circumstances dictated otherwise

The total length (tl) of each nurse shark captured was measured to the nearest mm from the rostrum to the posterior tip of the caudal fin or estimated by experienced personnel for those individuals that could not be landed sex of each captured individual was recorded in most cases however on a limited number of occasions it was not possible to identify the sex of excessively active sharks that were left in the water or those that escaped prior to sex de-termination a chi-square test was used to test for differences among observed and expected sex ratios (Zar 1999)

at each longline station environmental data were collected throughout the water column with a seabird sbe-911 ctD abiotic data utilized for analyses were limited to those values collected at the maximum depth of ctD deployment at each station and included tempera-ture (degc) salinity oxygen (mg lminus1) and water clarity ( transmissivity) Potential differences in abiotic parameters among regions were examined using one-way analysis of Variance (anoVa) tests followed by tukey multiple comparison tests (Zar 1999) although abiotic data were not normally distributed and heteroscedatic deviations from the underlying as-sumptions of anoVa were considered acceptable due to the large sample size associated with these data (Zar 1999 minimum n = 1118)

figure 1 locations (+) of bottom longline sets (n = 2419) conducted during national Marine fisheries service southeast fisheries science center Mississippi laboratories bottom long-line surveys in the northern Gulf of Mexico from 1995 to 2009 The 60 100 and 200 m iso-baths are shown



results











Tabl

e 1

Ran

ge a

nd m

ean

of a

biot

ic c

hara

cter

istic

s as

soci

ated

with

long

line

sets

con

duct

ed in

eas

tern

cen

tral

and

wes

tern

por

tions

of

the

north

ern

Gul

f of

M

exic

o A

lso

incl

uded

are

the

sam

e va

lues

for l

ocat

ions

whe

re n

urse

sha

rks

Gin

gym

osto

ma

cirr

atum

wer

e ca

ptur

ed L

ette

rs li

sted

with

mea

n va

lues

indi

cate

st

atis

tical

gro

upin

gs w

ithin

each

hea

ding

Dom

inan

t bot

tom

type

for e

ach

regi

on is

list

ed w

ith p

erce

ntag

e of l

ongl

ine s

ets c

ompl

eted

ove

r giv

en b

otto

m ty

pe H

ard

botto

m ty

pe in

clud

es ro

ck g

rave

l an

d sa

nd b

otto

m ty

pes

Reg

ion

of G

ulf o

f Mex

ico

Tem

pera

ture

(degC

)Sa

linity

Dis

solv

ed o

xyge

n (m

g Lminus

1 )W

ater

cla

rity

( tr

ansm

issi

vity

)B

otto

m ty

pes

East

ern

rang

e15

45ndash

315

029

84ndash

375

80

80ndash8

70

231

6ndash92

39

Har

d (8

87)

mea

n plusmn

SD26

28

plusmn 3

21A

359

4 plusmn

079

A5

31 plusmn

11

9A78

49

plusmn 9

39A

Cen

tral

rang

e18

40ndash

311

426

31ndash

366

20

00ndash8

30

001

ndash87

90M

ud (6

61)

mea

n plusmn

SD25

41

plusmn 2

97B

353

2 plusmn

165

B3

23 plusmn

16

7 C

541

4 plusmn

227

2B

Wes

tern

ra

nge

145

6ndash30

45

264

9ndash36

92

000

ndash89

00

01ndash8

780

Mud

(63

7)m

ean

plusmn SD

259

7 plusmn

322

A35

25

plusmn 1

94B

488

plusmn 1

37B

574

3 plusmn

200

7B

Loca

tions

of n

urse

shar

k ca

ptur

esra

nge

221

2ndash31

21

319

0ndash37

55

360

ndash80

059

17ndash

904

1m

ean

plusmn SD

281

4 plusmn

201

357

4 plusmn

089

544

plusmn 0

91

787

9 plusmn

663



Discussion








acknowledgments


literature cited
























results











Tabl

e 1

Ran

ge a

nd m

ean

of a

biot

ic c

hara

cter

istic

s as

soci

ated

with

long

line

sets

con

duct

ed in

eas

tern

cen

tral

and

wes

tern

por

tions

of

the

north

ern

Gul

f of

M

exic

o A

lso

incl

uded

are

the

sam

e va

lues

for l

ocat

ions

whe

re n

urse

sha

rks

Gin

gym

osto

ma

cirr

atum

wer

e ca

ptur

ed L

ette

rs li

sted

with

mea

n va

lues

indi

cate

st

atis

tical

gro

upin

gs w

ithin

each

hea

ding

Dom

inan

t bot

tom

type

for e

ach

regi

on is

list

ed w

ith p

erce

ntag

e of l

ongl

ine s

ets c

ompl

eted

ove

r giv

en b

otto

m ty

pe H

ard

botto

m ty

pe in

clud

es ro

ck g

rave

l an

d sa

nd b

otto

m ty

pes

Reg

ion

of G

ulf o

f Mex

ico

Tem

pera

ture

(degC

)Sa

linity

Dis

solv

ed o

xyge

n (m

g Lminus

1 )W

ater

cla

rity

( tr

ansm

issi

vity

)B

otto

m ty

pes

East

ern

rang

e15

45ndash

315

029

84ndash

375

80

80ndash8

70

231

6ndash92

39

Har

d (8

87)

mea

n plusmn

SD26

28

plusmn 3

21A

359

4 plusmn

079

A5

31 plusmn

11

9A78

49

plusmn 9

39A

Cen

tral

rang

e18

40ndash

311

426

31ndash

366

20

00ndash8

30

001

ndash87

90M

ud (6

61)

mea

n plusmn

SD25

41

plusmn 2

97B

353

2 plusmn

165

B3

23 plusmn

16

7 C

541

4 plusmn

227

2B

Wes

tern

ra

nge

145

6ndash30

45

264

9ndash36

92

000

ndash89

00

01ndash8

780

Mud

(63

7)m

ean

plusmn SD

259

7 plusmn

322

A35

25

plusmn 1

94B

488

plusmn 1

37B

574

3 plusmn

200

7B

Loca

tions

of n

urse

shar

k ca

ptur

esra

nge

221

2ndash31

21

319

0ndash37

55

360

ndash80

059

17ndash

904

1m

ean

plusmn SD

281

4 plusmn

201

357

4 plusmn

089

544

plusmn 0

91

787

9 plusmn

663



Discussion








acknowledgments


literature cited





























Tabl

e 1

Ran

ge a

nd m

ean

of a

biot

ic c

hara

cter

istic

s as

soci

ated

with

long

line

sets

con

duct

ed in

eas

tern

cen

tral

and

wes

tern

por

tions

of

the

north

ern

Gul

f of

M

exic

o A

lso

incl

uded

are

the

sam

e va

lues

for l

ocat

ions

whe

re n

urse

sha

rks

Gin

gym

osto

ma

cirr

atum

wer

e ca

ptur

ed L

ette

rs li

sted

with

mea

n va

lues

indi

cate

st

atis

tical

gro

upin

gs w

ithin

each

hea

ding

Dom

inan

t bot

tom

type

for e

ach

regi

on is

list

ed w

ith p

erce

ntag

e of l

ongl

ine s

ets c

ompl

eted

ove

r giv

en b

otto

m ty

pe H

ard

botto

m ty

pe in

clud

es ro

ck g

rave

l an

d sa

nd b

otto

m ty

pes

Reg

ion

of G

ulf o

f Mex

ico

Tem

pera

ture

(degC

)Sa

linity

Dis

solv

ed o

xyge

n (m

g Lminus

1 )W

ater

cla

rity

( tr

ansm

issi

vity

)B

otto

m ty

pes

East

ern

rang

e15

45ndash

315

029

84ndash

375

80

80ndash8

70

231

6ndash92

39

Har

d (8

87)

mea

n plusmn

SD26

28

plusmn 3

21A

359

4 plusmn

079

A5

31 plusmn

11

9A78

49

plusmn 9

39A

Cen

tral

rang

e18

40ndash

311

426

31ndash

366

20

00ndash8

30

001

ndash87

90M

ud (6

61)

mea

n plusmn

SD25

41

plusmn 2

97B

353

2 plusmn

165

B3

23 plusmn

16

7 C

541

4 plusmn

227

2B

Wes

tern

ra

nge

145

6ndash30

45

264

9ndash36

92

000

ndash89

00

01ndash8

780

Mud

(63

7)m

ean

plusmn SD

259

7 plusmn

322

A35

25

plusmn 1

94B

488

plusmn 1

37B

574

3 plusmn

200

7B

Loca

tions

of n

urse

shar

k ca

ptur

esra

nge

221

2ndash31

21

319

0ndash37

55

360

ndash80

059

17ndash

904

1m

ean

plusmn SD

281

4 plusmn

201

357

4 plusmn

089

544

plusmn 0

91

787

9 plusmn

663



Discussion








acknowledgments


literature cited
























Discussion








acknowledgments


literature cited

























acknowledgments


literature cited






















distribution of the nurse shark, ginglymostoma cirratum, in the northern gulf of mexico

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