correcting aerial survey counts of harbor seals (phoca vitulina richardsi) in washington and oregon

MARINE MAMMAL. SCIENCE, 17(2):276-293 (April 2001) 0 2001 by the Society for Marine Mammalogy

CORRECTING AERIAL SURVEY COUNTS OF HARBOR SEALS (PHOCA VITULINA

RICHARDSI) IN WASHINGTON AND OREGON HARRIET R. HUBER

National Marine Mammal Laboratory, NMFS/Alaska Fisheries Science Center,

7600 Sand Point Way, N.E., Seattle, Washington 981 15, U.S.A.

E-mail: [email protected]

STEVEN J. JEFFRIES

Washington Department of Fish and Wildlife, Marine Mammal Investigations,

7801 Phillips Road, S.W., Tacoma, Washington 98498, U.S.A.

ROBIN F. BROWN Oregon Department of Fish and Wildlife,

Wildlife Diversity Program, 7818 NE Vandenberg Avenue,

Corvallis, Oregon 97330, U.S.A.

ROBERT L. DELONG National Marine Mammal Laboratory, NMFS/Alaska Fisheries Science Center,

7600 Sand Point Way, N.E., Seattle, Washington 981 15, U.S.A.

GLENN VANBLARICOM U.S. Geological Survey, Biological Resources Division,

Washington Cooperative Fish and Wildlife Research Unit, School of Fisheries, University of Washington,

Seattle, Washington 98105, U.S.A.

ABSTRACT

Aerial surveys of harbor seals on land produce only a minimum assessment of the population; a correction factor to account for the missing animals is necessary to estimate total abundance. In 1991 and 1992, VHF radio tags were deployed on harbor seals (n = 124) at six sites in Washington and Oregon. During aerial surveys a correction factor to account for seals in the water was determined from the proportion of radio-tagged seals on shore during the pupping season. This proportion ranged from 0.54 to 0.74.

276

HUBER ET AL.: AERIAL SURVEYS 277

Among the six sites there was no significant difference in the proportion of animals on shore nor was there a difference in agelsex categories of seals on shore between sites. The pooled correction factor for determining total population abundance was 1.53. An additional 32 seals were radio tagged in 1993 at one of the sites used in 1991. Comparing data from the two years, we found no interannual variation. Aerial surveys of all known harbor seal haul-out sites in Washington ( n = 319) and Oregon (n = 68) were flown during the peak of the pupping season, 1991-1993. The Washington and Oregon harbor seal population was divided into two stocks based on pupping phenology, morphometics, and genetics. Mean counts for the Washington inland stock were 8,710 in 1991, 9,018 in 1992, and 10,092 in 1993. Oregon and Washington coastal stock mean counts were 18,363 in 1991, 18,556 in 1992, and 17,762 in 1993. Multiplying the annual count by the correction factor yielded estimates of harbor seal abundance in the Washing- ton inland stock of 13,326 (95% CI = 11,637-15,259) for 1991, 13,798 (95% CI = 11,980-15,890) for 1992, and 15,440 (95% CI = 13,382- 17,814) for 1993. In the Oregon and Washington coastal stock the corrected estimate of harbor seal abundance was 28,094 (95% CI = 24,697-31,960) in 1991, 28,391 (95% CI = 24,847-32,440) for 1992, and 27,175 (95% CI = 23,879-30,926) for 1993.

Key words: harbor seal, Pboca uittllina ricbardsi, census methods, population estimate, aerial survey, radio-tagging, correction factor.

Harbor seals are the most abundant pinniped in the Pacific Northwest: their distribution includes the outer coast and the coastal estuaries of Washington and Oregon and the inland waters of Washington (Fig. 1). They pup and breed in all of these regions and are present in both states year-round; they are the only pinniped which breeds in Washington (Jeffries 1985). Through the 1960s, Pacific harbor seals (Phoca vitzllinu richmdsi) were killed by state- financed bounty programs in Washington and Oregon as a method of population control because the seals were considered fish predators in conflict with commercial and sport fisheries (Pearson and Verts 1970, Newby 1973).

Although historical levels of harbor seal abundance are unknown, Scheffer and Slipp (1944) estimated the number of harbor seals in Washington as 5,000-10,000 in the early 1940s. Other estimates for the late 1930s are lower-5,000-6,000 (Newby 1973). Between 1965 and 1972, Newby (1973) reported a minimum population estimate of 1,7 10 harbor seals in Washington compiled from state-wide counts. Pearson and Verts (1970) estimated 500 harbor seals in Oregon based on counts of 307 on land in May 1968. These estimates are likely to be low because the land counts were not complete and not necessarily done at the optimal time. Since the termination of the harbor seal bounty program in the 1960s and the passage of the Marine Mammal Protection Act in 1972, harbor seal numbers in Washington and Oregon have increased (Jeffries et al. 1997, Brown 1997).

The timing of harbor seal pupping follows a cline along the west coast of North America, with pups born earlier south to north from Mexico to Canada (Bigg 1973, Bigg and Fisher 1975). In Oregon pupping begins earlier in the southern half of the state, but the peak of pupping (point at which the greatest

278 MARINE MAMMAL SCIENCE, VOL. 17, NO. 2 , 2001

Fzgzire 1. Map of Washington and Oregon, showing eight survey regions, stock boundaries, and radio-tagging sites.

number of pups is present on the haul-out) is similar throughout the state (late May to early June). In Washington the timing of pupping is variable within the state: a slightly later pupping along the coast (mid-June) than in the coastal estuaries (early June), a considerably later pupping in the inland waters (July-August), and an extended pupping season in Hood Canal (Au- gust-January).

Evidence for at least two stocks of harbor seals in the Pacific Northwest is based on significant differences in cranial morphology (Temte 1991), in timing of pupping (Temte 1986), and preliminary studies of mitochondria1 DNA

HUBER E T A L . : AERIAL SURVEYS 279

(Huber et al. 1995, Lamont et al. 1996). The coastal stock is composed of seals in Oregon, the Columbia River, the Washington coastal estuaries of Grays Harbor and Willapa Bay, and the outer Olympic Peninsula coast; the inland stock is composed of seals in the Strait of Juan de Fuca, the waters around the San Juan Islands, Eastern Bays, Puget Sound, and Hood Canal (Fig. 1). The separation between inland and coastal stocks was arbitrarily set as north of Cape Flattery (Fig. 1) and the southern boundary for the coastal stock was set at the Oregon/California border (Barlow et al. 1995).

Pupping, breeding, molting, and food availability all contribute to seasonal variation in the number of harbor seals hauled out on land (Brown and Mate 1983, Stewart and Yochem 1983, Jeffries 1985). Other, more local, influences on the number of seals hauled out are tide level, time of day, weather, and disturbance (Stewart 1984, Allen et al. 1985, Harvey 1987, Thompson 1989). Highest counts in most regions of Washington and Oregon are obtained at low tide during the pupping and molt periods, but at no time is the entire population ashore Ueffries 1985, Miller 1988). As an unknown and variable proportion of seals is in the water during surveys, only a minimum estimate of the population size is possible from direct counts.

Previous research to estimate harbor seal abundance from numbers hauled out during surveys has relied on the proportion of radio-tagged animals hauled out to correct survey counts (Pitcher and McAllister 1981; Stewart and Yoch- em 1983; Herder 1986; Yochem et al. 1987; Harvey 1987; Thompson and Harwood 1990; Withrow and Loughlin 1995, 1996; Hanan 1996). However, most of these studies have had a small or skewed sample (in regard to age or sex) of radio-tagged seals (Table 1).

This study is the first large-scale estimate of harbor seal abundance to in- corporate a correction factor to account for seals in the water. Although this correction factor is specific to the pupping season in Washington and Oregon, the method used to determine the correction factor is applicable to other harbor seal populations. We also evaluated seasonal effects on haul-out behavior for the different age and sex classes of harbor seals.

METHODS

Study Area

For this study Washington state was divided into six regions: Coastal Es- tuaries, Outer Olympic Peninsula, Strait of Juan de Fuca and the San Juan Islands, Hood Canal, Eastern Bays, and Puget Sound; Oregon was divided into two regions: north and south of Newport (Fig. 1). The divisions were based on timing of pupping and on the number of regional sites that could be surveyed in three to four hours during the low-tide window when maximum numbers of seals are on shore. All known harbor seal haul-out sites (319 in Washington, 68 in Oregon) were surveyed when the highest number of pups were on land. The period with the peak number of pups present varies with the timing of pupping, but i t has been identified in Oregon (Wilson 1993,

Tabl

e 1.

Su

mm

ary

of s

tudi

es o

f tim

e as

hore

for

radi

o-ta

gged

har

bor

seal

s. A

= A

dult,

I =

Im

mat

ure,

P =

Pup

, F =

Fem

ale,

M =

Mal

e.

Num

ber

of s

eals

Cor

rect

ion

Stud

y ar

ea

tagg

ed

fact

or

App

ropr

iate

app

licat

ion

Sour

ce

Tugi

dak

Isla

nd, A

K

24 A

F 2.

0 lo

w t

ide,

pup

ping

sea

son

(June

) 5

IF

5 A

M

2.44

lo

w t

ide,

mol

t se

ason

(Aug

/Sep

t) 1

IM

35 t

otal

6M

10

tota

l K

lam

ath

Riv

er, C

A

2 A

F 2

IF

6AM

2

IM

1 PM

San

Nic

olas

Isl

and,

CA

4

F

65%

ash

ore

each

day

in

May

58

% a

shor

e ea

ch d

ay i

n Ju

ne

41%

ash

ore

each

day

in

July

se

als

asho

re 5

6% o

f da

ys i

n A

pri

seal

s as

hore

63%

of

days

in M

ay

13 to

tal

Ork

ney

Isla

nd, S

cotla

nd

5 A

F

San

Mig

uel

Isla

nd, C

A

13 M

5

F

18 to

tal

Als

ea B

ay,

OR

22

F

4M

26

tot

al

24 A

F 28

AM

4

IF

7 PF

8

PM

7 1 to

tal

Sant

a B

arba

ra C

ount

y, C

A

1.69

11.1

1.87

1.

2

mea

n pr

opor

tion

of t

ime

asho

re 0

.59

(95%

C.I.

, 0.

42-0

.75)

pr

opor

tion

of s

eals

ash

ore

in d

aylig

ht

hour

s ra

nged

fro

m 0

.14

to 0

.19

(23

Oct

-6

Dec

) lo

w t

ide,

Oct

-Feb

low

tid

e, M

arch

-Jul

y M

ay, J

une,

Jul

y

Pitc

her a

nd M

cAlli

ster

19

81

Stew

art

and

Yoc

hem

19

83

Her

der,

1986

Thom

pson

and

Har

-

Yoc

hem

et a

l. 19

87

woo

d 19

90

Har

vey

1987

Han

an 1

996

HUBER ET AL. : AERIAL SURVEYS 281

282 MARINE MAMMAL SCIENCE, VOL. 17, NO. 2, 2001

[-+-South Oregon -North Oregon -+-Coast Washington z San Juans 0 Puget Sound 1

4 C

7

- a 7

n v)

.d

8 Figare 2. Difference in pupping phenology between Oregon and Washington coast-

al and inland stocks.

Oregon Department of Fish and Wildlife (ODFW), and National Marine Mammal Laboratory (NMML) data), at the Washington coastal estuaries (Stein 1989), outer Olympic Peninsula coast (Moss 1992), Strait of Juan de Fuca (Everitt 1980, Gearin 1979), the San Juan Islands (Suryan 1995), and Puget Sound (Washington Department of Fish and Wildlife (WDFW) data; Fig. 2). The Eastern Bays and Hood Canal were surveyed during the presumed peak of pupping based on data collected by WDFW and Cascadia Research Col- lective.

Aerial SurveyJ

We flew surveys in a single engine, high-winged airplane (Cessna 172, 182, or 185) at 250-111 altitude at 80 kn from two hours before low tide to two hours after low tide except in Hood Canal where surveys were flown at high tide because maximum numbers of seals haul out then (Calambokidis et al, 1979). To minimize variation associated with counts, surveys were flown at morning or midday tides during the peak pupping season (which varied between May and September depending on region; Fig. 2) and under comparable weather conditions. Three to five surveys were scheduled in each region; some flights were canceled or incomplete because of bad weather. In 1991, in addition to the June surveys, three surveys in July and two surveys in August were flown at Grays Harbor (Fig. 1) to determine if seasonal changes occurred in the proportion of age classes hauled out. Photographs were taken with a hand-held 35-mm SLR camera equipped with a 70-210-mm zoom lens and polarizing filter using Kodak High Speed Ektachrome film (ASA 200 or 400). The primary observer (right front seat) estimated the total number of animals

HUBER ETAL.: AERIAL SURVEYS 283

and photographed sites; the secondary observer (right rear seat) recorded sites, estimates, and comments. At each haul-out site photographs were taken, as well as a visual estimate of the total number of seals hauled out, including PUPS.

Small groups ( 2 2 5 seals), which were possible to count accurately from the plane, were not always photographed. Transparencies from the aerial surveys were projected onto a whiteboard in the laboratory and a mark was made for each animal to prevent under- or overcounting. Photo counts were repeated at least twice to ensure accuracy.

Captzrel Tagging

Seals were captured using the beach seine technique Ueffries et al. 1993). Handling (sex determination, weighing, measuring, tagging, and VHF radio transmitter attachment) took approximately 15 min for each seal, after which the seal was released into the water. The radio tag was attached to the seal’s hind flipper, consequently the signal could be detected only when the seal was on land. The radio transmitter, weighing 22 g (ATS, Isanti, MN), was glued to a Temple brand cattle ear tag with epoxy Ueffries et a/. 1993). The battery life for transmitters was 90+ d in 1991 and 360+ d in 1992 and 1993. The range of the transmitters was 1.5-3 km from the ground and 8-10 km in an aircraft at an altitude of 250 m.

Multiple radio telemetry surveys to detect the proportion of tagged seals hauled out were conducted simultaneously with aerial count surveys to produce a correction factor for estimates of the total harbor seal population. VHF radio tags were deployed on adult males, adult females, juveniles, and pups in the approximate proportion to their estimated abundance in the population (determined by review of published life tables: Pitcher and Calkins 1979, Bigg 1969). Radio tags were deployed in six areas (three sites within the coastal stock: Grays Harbor, Tillamook Bay, and Umpqua River and three sites within the inland waters stock: Gertrude Island, Protection/Smith Islands, and Boundary Bay, BC) to determine any regional haul-out variability (Fig. 1). In 1991, 63 VHF radio transmitters were attached to harbor seals: 34 in Grays Harbor (coastal stock), 16 at Protection/Smith Islands (inland stock), and 13 at Gertrude Island (inland stock; Table 2). In 1992, 61 harbor seals were instrumented with VHF radio transmitters, 26 in Boundary Bay, B.C. (inland stock), 17 at Tillamook Bay, Oregon, and 17 at the Umpqua River, Oregon (coastal stock; Table 2). To assess interannual variation in the proportion of seals hauled out at a specific site, an additional 32 harbor seals were instrumented at Gertrude Island in 1993 (Table 2).

Automated receivers and data loggers (ATS, Isanti MN) were set up within five days of radio tag deployment to serve as a check on detection of radio tags during aerial surveys and to monitor haul-out sites continuously to determine which seals were present. A tag was considered inactive if it was not detected by the automated systems either because the tag had stopped work- ing, it had fallen off, or the tagged seal had moved out of the area. Automated

Tabl

e 2.

D

istri

butio

n of

rad

io t

ags

depl

oyed

on

harb

or s

eals

in W

ashi

ngto

n, O

rego

n, a

nd B

ritis

h C

olum

bia,

199

1, 1

992,

and

199

3.

Adu

lt Y

ear

Tagg

ing

date

s A

rea

Adu

lt m

ale

fem

ale

Sub-

adul

t Pu

p To

tal

1991

25

-27

May

5

July

, 5

Aug

19

-20

Aug

1992

1 2

-1 4

Jun

e,

6 A

ugus

t 8-

9 M

ay,

14 M

ay

5-6

May

1993

23

-26

Aug

Gra

ys H

arbo

r, W

A

Smith

/Pro

tect

ion

Isla

nds,

WA

G

ertru

de I

slan

d, W

A

Bou

ndar

y Ba

y, B

.C.

Tilla

moo

k Ba

y, O

R

Um

pqua

Riv

er, O

R

Ger

trude

Isl

and,

WA

TOTA

L

TOTA

L

7 4 3 14 6 4 5 15 8

9 5 4 18 8 5 4 17 8

10 3 2 15 5 4 3 12 9

8 4 4 16 7 4 6 17 7

34

16

13

63

26

17

18

61

32

Tabl

e 3.

C

orre

ctio

n fa

ctor

to

acco

unt

for

harb

or s

eals

in t

he w

ater

dur

ing

aeria

l sur

veys

con

duct

ed d

urin

g pu

ppin

g se

ason

, 199

1-19

92.

Seal

s at

Gra

ys H

arbo

r, Ti

llam

ook,

and

Um

pqua

are

par

t of

the

coa

stal

sto

ck. S

eals

at P

rote

ctio

n/Sm

ith,

Ger

trude

, and

Bou

ndar

y Ba

y ar

e pa

rt of

the

inla

nd s

tock

.

Prop

ortio

n as

hore

R

egio

nal

Act

ive

Num

ber

of

Are

a Y

ear

trans

mitt

ers

surv

eys

ij (R

ange

) co

rrec

tion

fact

or

Gra

ys H

arbo

r, W

A

1991

34

4

0.70

(0

.67-

0.79

) 1.

43

Tilla

moo

k Ba

y, O

R

1992

17

2

0.62

(0

.5 9-

0.65

) 1.

62

Prot

ectio

n/Sm

ith, W

A

1991

16

3

0.54

(0

.50-

0.63

) 1.

85

Ger

trud

e Is

land

, WA

19

91

13

3 0.

73

(0.6

1-0.

84)

1.36

B

ound

ary

Bay,

BC

1992

26

5

0.66

(0

.54-

0.73

) 1.

51

Com

bine

d co

rrec

tion

fact

or (S

E)

Um

pqua

Riv

er, O

R

1992

18

2

0.69

(0

.61-

0.77

) 1.

44

1.53

(0.

1)

h)

W

A

r

c 0 r z 0

N

N

0

0

w

HUBER ET AL.: AERIAL SURVEYS 285

data loggers ran 24 h/d and scanned each frequency for 10 sec every 20 min. A reference transmitter was placed at each tagging site to insure that the receiver was functioning properly. The receiver for the automated data logger was connected to a four-element Yagi antenna. Antennae were placed <2 km from the seal haul-out at an elevation between 12 and 75 m above the haul- out site. The receiver in the survey aircraft was attached either to a single two-element Yagi antenna inside the plane or to a pair of two-element Yagi antennae attached to the wing struts on each side of the plane. There was no difference in reception between antennae in the air or on the ground, nor between the antennae inside or outside the plane.

Data AnalysiJ

The correction factor to account for seals in the water is the reciprocal of the proportion of radio-tagged harbor seals ashore during multiple aerial surveys at each tagging site. Surveys in each area were combined to obtain an average correction factor for the site. The average proportion of animals ashore at all six sites in 1991 and 1992 was used for a combined correction factor. Variance around the combined correction factor was estimated by the delta method (Mood et al. 1974, p. 181) using the equation:

where var(p) = p(l - p)/n, p = proportion ashore, and n = number of seals with active radio tags.

The arcsine transformation was applied to all proportion data before testing. A one-way ANOVA was used to determine if there were differences in the proportion ashore of the different agehex categories at the six areas, in the proportion of total seals ashore among the six areas, and in the proportion ashore between the two stocks. The ANOVA procedure was also used to test if there were changes in the composition of age classes ashore during pupping season (June), premolt (July), and molt (August). Where differences occurred, the Tukey test for multiple comparisons was used. A two-way ANOVA was used to test for an interaction effect between the proportion of age classes on shore and area. A t-test was used to determine if there were interannual differences in the proportion ashore at Gertrude Island in 1991 and 1993. Only data from active radio tags were used.

The total abundance of harbor seals in Washington, accounting for seals in the water, was estimated as the product of the mean number of seals in each year and the combined correction factor: fi = ($/e. Variance around total seal abundance was estimated assuming independence of variables (Goodman 1960) using the equation:

var(fi> = var(f)/jP + f*.var( 1 p) - var( 1 p)-var(n),

where f is the mean number of seals counted (Goodman 1960). The 95%

286 MARINE MAMMAL SCIENCE, VOL. 17, NO. 2 , 2001

confidence interval was computed based on a log normal distribution (Johnson and Kott 1970):

(filc, fi x c),

where = e 1 . 9 6 ~ 1 n [ l + c v 2 ( ~ ) l

and CV2(fi) = var(fi)/fi*. Multiple surveys were attempted in all regions in all years. Low regional

counts due to incomplete surveys, disturbance, or weather were discarded. The mean (2) count and its standard error (SE(2)) were computed for all regions for all three years. In each year one or two regions had only one complete survey. In that situation the SE($ was estimated by one of two methods. In the first method, where there were multiple surveys but the surveys were incomplete on all but one day, the SE($ for the region was estimated by using the variance around the counts in areas within a region which were surveyed on multiple flights (at least 25% of the region). In the second method, when there was only one complete survey for a region in a year, the SE($ was estimated using the CV for that region from the previous year. With both methods we assumed the SE(2) was a constant fraction of the mean count. Because the CV of counts in all regions was < O . l , these methods of estimating SE@) for one to two regions each year had little influence on the variance estimate of total abundance. The mean total of harbor seals for each year was calculated by summing the means from all survey regions. The SE($ for each year was calculated by summing the squares of the SE(f) for each survey region and taking the square root of the sum.

RESULTS

The average proportion of radio-tagged harbor seals ashore during surveys conducted during the pupping season ranged from 0.54 to 0.73 at the six areas where radio tags were deployed in 1991 and 1992; consequently, the correction factor ranged from 1.36 to 1.85 (Table 3). Pups were ashore in higher proportions than all other agehex categories and adult females were ashore in higher proportions than adult males and subadults ( F = 9.27, df = 3, 20, P = 0.0005; Fig. 3). There was no difference in proportion of agelsex categories ashore at the six areas (adult males F = 1.12, df = 5, 13, P = 0.4; adult females F = 0.53, df = 5, 13, P = 0.75; subadults F = 0.51, P = 0.76; pups F = 0.41, df = 5 , 13, P = 0.84), no difference in total proportion ashore among the six areas (F = 2.33, df = 5, 13, P = 0.10), and no difference between the coastal and inland stocks ( F = 0.88, df = 1, 17, P = 0.36). There was no interaction effect between agelsex categories and area (F = 1.55, P = 0.12). Therefore, the proportion of seals ashore on all surveys for all areas was combined. The combined proportion was 0.655, thus the combined correction factor was 1.53 (SE = 0.1; Table 3).

The proportion of radio-tagged seals ashore at Gertrude Island was 0.61,


t 0

PUP Adult Female Adult Male Subadult

Figure 3. Mean proportion on shore of four agehex categories of radio tagged harbor seals during pupping season aerial surveys at six sites. Error bars indicate 95% confidence limits.

0.73, and 0.84 in 1991 (three surveys) and was 0.71, 0.71, and 0.81 in 1993 (three surveys). There was no significant difference between the two years in the mean proportion of seals ashore (t = -0.15, df = 4, P = 0.89).

Between 1991 and 1993, 18-20 aerial surveys were flown each year to assess the harbor seal population in Washington and Oregon. The coastal stock of harbor seals is composed of seals in Oregon, the Columbia River, the Wash- ington coastal estuaries of Grays Harbor and Willapa Bay, and the outer Olym- pic Peninsula coast; mean counts for all three years ranged from 17,744 to 18,667 (Table 4). For the inland stock, composed of harbor seals in the Strait of Juan de Fuca, the waters around the San Juan Islands, Eastern Bays, Puget Sound, and Hood Canal, mean counts increased from 8,420 to 10,623 (Table 5).

The corrected estimate for the Washington and Oregon coastal stock ranged from 27,148 to 28,561 between 1991 and 1993 with the 95% CI between 23,839 and 32,598 seals (Table 4). For the Washington inland waters stock,

Table 4. Harbor seal total abundance estimates for Washington and Oregon coastal stock, 1991-1993. CF = correction factor, calculated by combining correction factors for all regions.

Total Count Count abundance

Year mean SE CF CF SE estimate Total SE 95% CI 1991 18,635 302 1.53 0.1 28,512 1,919 24,990-32,529 1992 18,667 318 1.53 0.1 28,561 1,928 24,973-32,598 1993 17,744 207 1.53 0.1 ' 27,148 1,802 23,839-30,917


0.8 --

Table 5. Harbor seal total abundance estimates for Washington inland stock, 1991- 1993. CF = correction factor, calculated by combining correction factors for all areas.

Total Count Count abundance

Year mean SE CF CF SE estimate Total SE 95% CI 1991 8,420 301 1.53 0.1 12,883 959 11,135-14,906

1993 10,623 209 1.53 0.1 16,253 1,109 14,220-18,577 1992 8,834 268 1.53 0.1 13,516 973 11,743-15,557

the corrected estimate ranged from 12,833 in 1991 to 16,253 in 1993 with the 95% CI between 11,135 and 18,577 seals (Table 5).

There were seasonal changes in the composition of age classes of harbor seals on shore at Grays Harbor. In June (the period of peak pupping) adult females and pups were present on land in the highest proportion and adult males in the lowest proportion (Fig. 4). In July (the premolt period) the proportion of adult males increased slightly, the proportion of adult females decreased, and weaned pups were nearly absent from the haul-outs (Fig. 4). By August (the molt period) the proportion of radio-tagged adult males increased to 0.6, of adult females to 0.55, and pups to 0.25 (Fig. 4). The proportion of subadults remained constant at about 0.5 during all three months. There was no significant difference in proportion of adult male or subadult harbor seals ashore during the three months (adult males: F = 4.36, df = 2, 6, P = 0.07; subadults: F = 0.11, df = 2, 6, P = 0.89). For adult females and for pups

' T

t Adult male Adult female PUP Subadult

Age/Sex Category

Figare 4. Proportion of four age/sex categories of harbor seals on shore at Grays Harbor, Washington, during June (pupping season), July (premolt) and August (molt), 1991.


the proportion hauled out was significantly greater in June than in July or August (adult females: F = 5.52, df = 2, 6, P = 0.04; pups: F = 34.39, df 2, 6, P = 0.001). The overall proportion of seals with active tags ashore during surveys changed from 0.70 in June to 0.39 in July and 0.45 in August with regional correction factors of 1.43, 2.56 and 2.22, respectively.

DISCUSSION

Although harbor seals have been studied in the Pacific Northwest for the past 40 yr, an accurate estimate of their abundance was unknown. In recent years numbers of harbor seals in Washington and Oregon have increased dra- matically (Brown 1997, Jeffries et al. 1997). This study is the first to estimate the total abundance of harbor seals in Washington and Oregon using a correction factor to account for seals in the water during surveys. It differs from previous radio-tagging studies in several ways: the sample of radio-tagged seals was proportioned by age and sex to reflect the composition of the population, thus avoiding age and sex bias in the data; radio tags were placed on the hind flippers, so that the signal was detected only when the seal was on shore. In addition, to avoid the biases associated with local sampling and small sample sizes, the correction factor was derived from the proportion of seals ashore at multiple areas, the survey area was of large geographic extent, and the sample of radio-tagged seals was much larger than in other studies.

This correction factor is applicable to harbor seals in Washington and Oregon only during the pupping season because the proportion of harbor seal age classes ashore changes seasonally. Biological and physiological constraints on harbor seal haul-out behavior such as rest, nursing, thermoregulation, foraging, and molting are constant, but local conditions (e.g., available space, disturbance, weather, prey availability, and predator presence) are specific to an area. Consequently, although the correction factor may differ in other sea- sons or other localities, the techniques used to obtain the correction factor are likely appropriate for any season or locality.

The distribution of radio tags among age and sex categories can have a strong influence on the magnitude of the correction factor because of seasonal differences in haul-out behavior. Because nursing almost always occurs on land, lactating females and their pups are the most likely segment of the population to be on shore during the pupping season. Pups, although capable of swim- ming at birth, are not skilled swimmers during the first weeks of life and mainly venture into the water only when haul-out areas are covered by high tide or during disturbance, whereas adult females that are not pregnant or have lost their pups are less likely to be on shore. Adult males and subadults, not tied to the haul-out site by the constraints of care of offspring are present in low proportions. After the pupping season, changes occur in the proportion of age classes on shore. After pups are weaned, many remain in the area, but they ate infrequently hauled out, presumably because they are inefficient at capturing prey and must spend most of their time foraging. During the molt season, more changes occur because seals undergoing molt spend more time


ashore than seals which have not yet begun to molt or have completed their molt, and because there are differences in timing of molt among ages and sexes (Thompson and Rothery 1987). As a result, the timing of surveys within the molt season may also have a strong influence on the correction factor, with a higher proportion of females present early in the season and a higher proportion of males present at the end of the season. In winter, it is apparent that harbor seals haul out less frequently than during pupping or molt (Table I), consequently a correction factor in winter will be greater than during pupping or molt season.

The highest regional correction factor that we found was 1.85 in the Strait/ San Juan Islands region and the lowest was 1.36 in the Puget Sound region. However, when we applied regional correction factors to count data from each region, the difference in the total abundance estimate was less than 2% com- pared to applying the combined correction factor to the sum of counts in all regions.

Because we applied the combined correction factor to all regions in Wash- ington and Oregon, possible differences in haul-out behavior between areas where we radio tagged seals and areas where we did not instrument animals may have influenced the results. Where food is less abundant, seals may haul out less frequently because they must spend more time foraging. Where haul- out space is limited, seals which are not constrained to be ashore to care for offspring may also spend more time in the water. In areas of high disturbance like the San Juan Islands (Suryan 1995), more seals may be in the water at any given time than in areas of low disturbance like Grays Harbor (Stein 1989). In addition, haul-out patterns, and thus the proportion of seals ashore, may be different at sites such as Hood Canal which are available only at high tide or at sites such as docks and log booms which are available at all tides. Haul-out sites that are available only at high tide or are available regardless of tide are relatively low in number and account for <lo% of Washington and Oregon harbor seals.

Despite the caveats attached to the correction factor in this discussion, the correction factor itself appears to be a valid multiplier to account for seals in the water during surveys. That there was no difference in the proportion of seals ashore during surveys among areas, between stocks, or between years gives credence to this number as a correction factor which can legitimately be applied to counts of harbor seals in Washington and Oregon during the pupping season.

ACKNOWLEDGMENTS

Time spent in the field and in the lab would not have been so productive, nor so much fun without: Dyanna Lambourn, Kim Forbes, Jim Thomason, Mike Wilson, Monique L. Wilson, Susan Riemer, Rob Suryan, Kim Raum-Suryan, and others who flew on survey planes, checked on DCCs, and captured seals. Thanks to Tim Ragen, Richard Merrick, and John Bengtson for lending DCCs and receivers to this project. Thanks to Peter Olesiuk and the Canadian Department of Oceans and Fisheries for permission to work in British Columbia. For permission to place automated data log-


gers and scanning receivers on their property, thanks to Clarence De Boer (Boundary Bay), Joan and David Empfield (Ocean Shores), Garibaldi Harbor Master (Tillamook Bay), International Paper Company (Umpqua River), U. S. Fish and Wildlife Service (San Juan Islands, Protection Island), U. S. Coast Guard (Clements Reef buoy, Smith Island), WDFW, and Washington Department of Corrections (McNeil Island). All research was conducted under NMFS scientific research permit 475 issued to WDFW, or NMFS scientific research permit 835 issued jointly ~3 WDFW, ODFW, and NMML. The manuscript was greatly improved by comments from Jeff Laake, Anne York, M. Hammill, and an anonymous reviewer.

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Received: 12 September 1999 Accepted: 18 July 2000

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correcting aerial survey counts of harbor seals (phoca vitulina richardsi) in washington and oregon

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