Movement and activity patterns of Harbor seals (Phoca Vitulina) from the Drakes Estero Population, California, 1985-86

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NOAA Technical Memorandum
NOS MEMD 6

MOVEMENT AND ACTIVITY PATTERNS OF HARBOR SEALS
(PHOCA VITULINA) FROM THE DRAKES ESTERO
POPULATION, CALIFORNIA, 1985-86

Sarah G. Allen, David G. Ainley, Lyman Fancher, and David Shuford

Point Re es Bird Observatory, 4990 Shoreline Highway, Stinson Beach,
CA 94970

Washington, D.CS. DEPARTMENT OF COMMERCE NOAA
August 1987 COASTAL SERVICES CENTER
2234 SOUTH HOBSON AVENUE
CHARLESTON, SC 29405-2i:

UNITED STATES National Oceani and National Ocean Service
DEPARTMENT OF COMMERCE Atmospheric Administ'att .

i ate
4 0=:

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MEMD 1 M.M. Littler, D.S. Littler and B.E. Lapointe. 1986. Baseline Studies of Herbivory and Eutrophication
on Dominant Reef Communities of Looe Key National Marine Sanctuary.

MEMD 2 M.M. Croom and N. Stone, Eds. 1987. Current Research Topics in the Marine Environment.

MEMD 3 C.E. Birkeland, R.H. Randall, R.C. Wass, B. Smith and S. Wilkens. 1987. Biological Resource
Assessment of the Fagatele Bay National Marine Sanctuary

MEMD 4 H. Huber. 1987. Reproduction in Northern Sea Lions on Southeast Farallon Island, 1973-1985.

MEMD 5 J.A. Bohnsack, D.E. Harper, D.B. McClellan, D.L. Sutherland, and M.W. White. 1987. Resource
Survey of Fishes within Looe Key National Marine Sanctuary.

MEMD 6 S.G. Allen, D.G. Ainley, L. Fancher, and D. Shuford. 1987. Movement Patterns of Harbor
Seals (Phoca vitulina) from the Drakes Estero Population, California, 1985-86.

National Marine Sanctuary Program
Marine and Estuarine Management Division
Office of Ocean and Coastal Resource Management
National Ocean Service
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REPORT TO
NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION
U.S. DEPARTMENT OF COMMERCE

NOAA TECHNICAL MEMORANDA SERIES NOS/MEMD

Movement and Activity Patterns of Harbor Seals (Phoca vitulina)
from the Drakes Estero Popoulation, California, 1985-86

Sarah G. Allen, David G. Ainle�,, Lyman Fancher, and David Shuford

August 1987

U.S. DEPARTMENT OF COMMERCE
NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION
NATIONAL OCEAN SERVICE
OFFICE OF OCEAN AND COASTAL RESOURCE MANAGEMENT
MARINE AND ESTUARINE MANAGEMENT DIVISION
WASHINGTON, D.C.

REPORT TO
NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION
U.S. DEPARTMENT OF COMMERCE

NOAA TECHNICAL MEMORANDA SERIES NOS/MEMD

Movement and Activity Patterns of Harbor Seals (Phoca vitulina)
from the Drakes Estero Population, California, 1985-86

Sarah G. Allen, David G. Ainley, Lyman Fancher, and David Shuford

Point Reyes Bird Observatory, 4990 Shoreline Highway, Stinson
Beach, CA 94970

This work is the result of research sponsored by the U.S.
Department of Commerce, National Oceanic and Atmospheric
Administration, National Ocean Service, Office of Ocean and
Coastal Resource Management, Marine and Estuarine Management Division
Under Interagency Agreement Number NA-81-AA-H-CZ151

INTRODUCTION

Harbor seal colonies in the Point Reyes area, Marin County, contribute

about 20% of the mainland California breeding population (Allen and Huber

1984, Hanan et al. 1986) and consequently deserve attention when devising

regional management policies. Estimates for the size of the state and

Point Reyes populations are based on direct ground and aerial counts,

without corrections for the number of seals hauled out on a given day

relative to the total number of seals pres~ent in the area. Consequently,

estimates are rough, and are likely low. In addition, it is difficult to

relate numbers among seasons. This problem is not unique to the Point

Reyes population.

Several researchers have addressed the problem using marked animals and

have derived correction factors with varying attendance patterns based on

location, time of day, and season of year (Brown and Mate 1981, Pitcher and

McAllister 1981, Stewart and Yochem 1983, and Sullivan 1979). Stewart and

Yochem (1983) for the Channel Islands and Sullivan (1979) for Humboldt Bay,

California, found that seals spent about 40% of each day hauled out.

Stewart and Yachem (1983) determined that marked seals hauled out on 65% of

the days in May, 58% in June, and 35% from October through December.

Pitcher and McAllister (1981) estimated that seals in Alaska hauled out on

50% of the days in June and 41% in August, and Herder (1986) found that

seals in the Klamath River area hauled out on average 56% of the days in

April and 64% in May.

These researchers determined that the differences between total number

hauled out and the estimated population were a function of variations in

daily haul-out patterns as well as movements to other areas. Brown and

Mate (1983) determined that dispersal accounted for seasonal changes in the

number of seals hauled out in Oregon. Movements appeared to be associated

with seasonal changes in reproductive status and in response to feeding

strategies (Brown and Mate 1983, Herder 1986). Seals at the Klamath River,

California, moved locally to alternate haul out sites year round and

dispersed long distances in winter months (Herder 1986). With the

exception of one inter-island movement, Stewart and Yochem (1983) found

seal movements were local, leading them to conclude that reduced hauling

frequency accounted for the decline in seal numbers during the fall and

winter months.

In the Point Reyes area we determined previously that seals displayed

diurnal and seasonal variation in haul-out patterns (Allen and Huber 1983,

1984). Diurnal and tidal effects on seal haul-out behavior varied within

an optimum range with most seal hauled out from mid-day to late afternoon

at low to medium tides, depending on the physical attributes of each

location. The diurnal pattern was similar to that of the southern Channel

Islands (Stewart 1984), Mowry Slough in San Francisco Bay (Fancher 1979),

Southeast Farallon Island (Ainley et al. 1977), and Bolinas Lagoon (Allen

et al. 1985). Seals were seasonally most abundant during the spring and

summer, coincident with the breeding and molt periods. Maximum numbers

reached 2449 in May and 2502 in June 1983-84. During the fall and winter,

the maximum number of seals was around 1000 animals. It appeared that

seals were either moving to other haul-out sites outside of Point Reyes in

the winter, or spending more time at sea.

Information on seasonal activity patterns and fall/winter movements

would be invaluable for any long-term management program for seals in the

Point Reyes area, and, in conjunction with results accumulated elsewhere

(Stewart and Yochem 1983; Herder 1986), could provide more accurate

estimates of the California harbor seal population.

In an effort to identify movement and seasonal activity patterns, we

undertook a radio-tagging program of harbor seals in Point Reyes. We chose

this method based on the successful results obtained from radio-telemetry

studies by Brown and Mate (1983), Herder (1986), Pitcher and McAllister

(1983), and Stewart and Yochem (1983). Our main objectives were to

determine 1) if the apparent decline in seal numbers during the winter

months was a function of dispersal and seasonal movements, or a reduction

in the frequency of the haul-out pattern, 2) the destination of departing

animals, 3) daily and seasonal variability in haul-out behavior, and 4) if

gender accounted for variations in the above.

STUDY AREA AND METHODS

Drakes Estero is an estuary where seals haul out on tidal mud flats

exposed at low, medium, and medium-high tides, as well as on the tip of

Limantour Beach exposed at all tide levels. This location is one of the

main breeding areas in the Point Reyes area (Figure 1). Significantly

more seals were present during the spring and su.ner than during fall or

winter; however, in contrast to the two other important areas, Tomales

Point and Double Point, seals were present in substantial numbers year

round (Allen and Huber 1983, 1984). A maximum of 122 pups was counted in

May 1983, accounting for 19% (122/656) of the total numbers of seals

present. Twenty-three percent (122/527) of all pups in the Point Reyes

area occurred at Drakes Estero in 1983.

We chose to tag seals at Drakes Estero because of its significance to

the Point Reyes population, because seals there exhibited the seasonal and

diurnal activity patterns characteristic of seals elsewhere in the area,

and because the capture technique we wished to use was only possible in an

estuarine environment.

Over a two-day period (July 31 - August 1) in 1985, we successfully

captured about 60 seals and affixed radio transmitters to 17 (nine males

and eight females). Our intention was to obtain a sample size of 10

females and 10 males, rather than to randomly capture and tag whatever

seals happened to be caught; therefore, we released unmolted and immature

seals. Personnel from the Point Reyes National Seashore, the Point

Reyes/Farallon Islands National Marine Sanctuary, the National Marine

Fisheries Service, the California Department of Fish and Game, the Oregon

t..
trena

St. t Poant r

Double Point

Forollon Sn Pro ncic coo
Isaonds

Pillar Pt

Holf Meon Boy---

Pigoan Pi-
Pt. Ana Nuev-

Pi Sonto Cruz

Kilometers
0 10 20 30
Pt. Sur

FIGURE 1. Capture site and locations of movement of harbor seals in Califomia. 195-1986.

Department of Fish and Wildlife, and the Point Reyes Bird Observatory

assisted in the capture. We captured seals in a 60-fathom long gill net,

with 8-inch mesh, set off the haul-out site and then pulled ashore. Two

power boats were used to deploy the net adjacent to the haul-out site. The

lead boat carried the net on a platform set above the transom. This boat

approached the hauled out seals as rapidly as possible, set the net as the

seals entered the water, and then landed on the far side of the haul-out

beach. The second boat'retrieved the other end of the net and landed. A

third boat carried supplies and additional personnel for handling the

seals. This method of capture was successfully developed and used in

Washington and Oregon for capturing harbor seals in estuaries and enabled

the researcher to capture a large number of seals over a short period of

time, thereby minimizing the duration of disturbance days to the herd

(Beach et al. 1985).

Captured seals were removed from the large net and individually placed

in small hoop nets. This temporarily restrained the animals before they

were tagged and reduced aggressive interactions between seals, thus

reducing injury. Seventeen people were involved in handling the seals.

One person recorded data, three weighed seals, two monitored the condition

of seals retained in hoop nets, two marked seals with flipper-tags, two to

three restrained seals being tagged, one attached the radio transmitter,

and one supplied the equipment (tags, resin, etc.). All 17 people were

needed to pull the net ashore and place seals in hoop nets.

Captured seals were weighed, measured (length and girth), sexed,

flipper-tagged, dye-marked, and radio-tagged. The average time required to

accomplish this was 15-20 minutes per seal. When the tide level began to

flood the haul-out site, any remaining seals were transferred by boat to

the mainland for processing. Red Woolite sheep dye which lasts for about

three months was used to mark each seal for easy visual resighting.

Fourteen seals were flipper-tagged with lime green "Riese" cattle ear tags

supplied by NMFS. Two neoprene patches with an individual number code and

a color code for gender were glued with "Lock-tight" super glue to the fur

between the shoulder blades to provide an additional visual mark. A radio

transmitter manufactured by Advanced Telemetry Systems, Inc. (Bethel,

Minnesota) was glued to the fur on the back of the seal's head with

"Devcon" 5-minute epoxy (for attachment procedures see Beach et al. 1985).

By attaching the radio tag to the head of the seal we were able to locate

seals when they were in the water. Radio transmitters were 9 x 3 x 3 cm,

weighed 60 gms, had a 13-inch flexible antenna, and a life expectancy of

300 days. Each transmitter had a frequency within the range of 164-165

Mhz, transmitting 55 pulses per minute.

We monitored seals with an automated recording station and aerial

surveys. The automated recording station was established on a bluff

overlooking Drakes Estero and consisted of an "Esterline Angus" 20-channel

strip chart recorder, a programmable scanning receiver, a 12-volt marine

battery, and a "Yagi" antenna. The station recorded seal-s within a

two-mile radius on a 24-hour basis. Each frequency was monitored by the

scanning receiver for 32 seconds and was scanned five to six times per

hour. A seal was considered hauled out if it was recorded with two strong

pen marks in a row within a half-hour period; a seal was considered in the

vicinity but in the water if a strong pen mark in conjunction with two weak

marks within a half-hour was recorded. We manually checked frequencies to

determine if radio interference from the nearby radio communication

facilities (U. S. Coast Guard Station and RCA Gobal Communications Center)

triggered pens and found that interference did not hinder distinguishing

whether a seal was hauled out. Radio interference on one frequency

prevented our distinguishing whether one animal was in the water but did

not hinder our distinguishing when it was hauled out; interpretation was

not impaired for the other frequencies. A test transmitter was placed near

the remote station to verify that the receiving system was functioning

properly. The remote receiving station was in operation from July 31

through September 5, and from October 25 through December 31, 1985; and

from January I through February 16, and February 21 through April 30, 1986.

We checked and maintained the remote station almost daily in August and

semi-weekly for the rest of the study period. During visits we scanned all

frequencies and checked for signal drift. Each signal was checked for a

period of five to ten minutes to detect seals in the water. We based this

scan period from averaging dive times of tagged seals during the first

month of the project (x = 3 min, range -5 sec - 9 min 45 sec, SD = 2 min,

n = 76). 'We made visual searches for marked animals to check for

transmitter loss or failure.

We also conducted all-day censuses at Drakes Estero to compare the

diurnal haul-out pattern of the herd with those of individually marked

seals. For both summer and winter, individual patterns were similar to

those of the herd (Table 1). Consequently, we were confident in making

Table 1. Diurnal haul-out pattern of the herd in comparison to that
of radio-tagged seals at Drakes Estero during the summer and
winter seasons; herd = total number of seals hauled out, tagged =
total number of tagged seals hauled out, and proportion - the
proportion of tagged seals hauled out to the total number of
resident, tagged seals.

TIME (PST)

0700 0800 0900 1000 11.00 1200 1300 1400 1500 1600

SUMMER
Aug 6
Herd 453 397 429 462 365 259 81 36 17 21
Tagged 8 8 8 9 7 7 3 3 0 0
Proportion 0.6 0.6 0.6 0.6 0.5 0.5 0.2 0.2 0 0
Aug 13
Herd 387 378 325 94 24 106 89 173 192 70
Tagged 5 7 7 5 3 3 4 4 5 2
Proportion 0.5 0.7 0.7 0.5 0.3 0.3 0.4 0.4 0.5 0.2
Aug 20
Herd 389 427 261 372 316 51 1 0 0 6
Tagged 8 8 5 9 8 4 0 0 0 0
Proportion 0.8 0.8 0.5 0.9 0.8 0.4 0 0 0 0
Aug 27
Herd 257 241 174 56 60 72 211 248 265 157
Tagged 3 3 4 2 2 3 8 8 9 9
Proportion 0.3 0.3 0.4 0.2 0.2 0.3 0.8 0.8 0.9 0.9
Sep 3
Total 100 230 355 252 210 48 0 0 3 24
Tagged 3 4 4 5 5 2 0 0 1 2
Proportion 0.3 0.4 0.4 0.6 0.6 0.2 0 0 0.1 0.2

WINTER
Jan 1
Herd 190 266 404 467 467 497 192 70 66 0
Tagged 4 4 7 8 8 8 3 1 1 0
Proportion 0.5 0.5 0.9 1.0 1.0 1.0 0.4 0.1 0.1 0
Jan 7
Herd 0 0 0 0 157 329 419 497 498 506
Tagged 0 0 0 0 5 6 7 7 7 7
Proportion 0 0 0 0 0.7 0.9 1.0 1.0 1.0 1.0
Jan 15
Herd 309 364 413 456 462 451 229 110 RAIN-----
Tagged 3 3 4 4 5 6 5 4 0
Proportion 0.4 0.4 0.6 0.6 0.7 0.9 1.0 0.6 0
Jan 20
Herd 0 0 14 124 310 372 464 476 467 445
Tagged 0 0 0 2 4 4 5 5 4 4
Proportion 0 0 0 0.3 0.6 0.6 0.7 0.7 0.5 0.6
Jan 18
Herd 212 210 165 79 64 100 100 110 110
Tagged 4 4 3 3 3 3 3 3 4
Proportion 0.6 0.6 0.4 0.4 0.4 0.4 0.4 0.4 0.6

assumptions regarding herd behavior from data derived from monitoring

individuals at Drakes Estero until the spring (after the month of March)

when two transmitters that were still functioning were too few to make

valuable comparisons. During the all-day censuses we also verified the

accuracy of the equipment in distinguishing the presence or absence of

seals and whether they were hauled out or in the water.

To locate dispersing seals we attempted to make weekly aerial surveys

along the California coast from Monterey Bay to the California/Oregon

border. Seals were monitored with a scanning receiver and two

wing-mounted, two-element antennae from Telonics (Mesa, Arizona), and

frequencies chosen were continuously scanned for three seconds each. When

a seal was located, activity, time, and location were recorded. Most

flights ranged from Point Arena south to Half Moon Bay and into San

Francisco Bay. The flights varied depending on which animals we were

trying to locate. We scheduled flights on days when tides were low to

medium to maximize numbers of seals hauled out.

DATA ANALYSIS

We defined seals as "resident" if they remained at Drakes Estero for

most or all of the study period, "breeders" as those which departed in the

fall but returned during the following breeding season, and "transients"

as those present at Drakes Estero during the capture but which resided away

from Drakes Estero during the rest of the study period. Seal behavior at

Drakes Estero was classified as either 1) hauled out, 2) in the water, or

3) absent. All information recorded on the Esterline Angus strip charts

was transferred to the "DBase 111" database system. The `SPSS statistical

program for micro-computers" was used for all analyses. Analyses presented

here are exploratory and primarily descriptive since this is the first of a

two-year study. Averages are expressed with + standard error. We chose

August and December data to illustrate some of the seasonal differences in

haul-out behavior.

RESULTS

We captured and radio-tagged 17 seals (eight females, nine males) and

flipper-tagged two additional males (Table 2). On average, males weighed

27.5 kg more, and were 10.2 cm longer than females. On the day after

capture, all but two of the radio-tagged seals were hauled out again at

Drakes Estero, ane on the third day after capture all but one seal was

hauled out. All but two transmitters (88%) remained attached to seals for

a minimum of five months, nine (53%) for seven months, six (35%) for eight

months, and one for nine months (Table 3). We were able to locate

regularly, either visually or with a receiver, all but one animal through

April 1986.

MOVEMENTS

All radio and flipper tagged seals were resighted at Drakes Estero at

least once during the study period. Two seals with transmitters that

ceased functioning within the first month of study were excluded from all

analyses; the two animals involved remained at Drakes for the entire year.

Radio-tagged seals moved both north and south of the capture site;

Table 2. Standard measurements of harbor seals tagged at Drakes Estero,
August 1985. (RT = Radio Tag, FT = Flipper Tag.)

Radio Tag Age Sex Length(cm) Girth(cm) Weight (kg) Comments

RT024 AD F 146 98 63.5
RT033 AD F 138 95 56.7
RT092 AD F 148 96 55.8
RT234 AD F 147 120 86.2
RT313 AD F 145 103 63.5
RT415 AD F 131 94 50.0
RT491 AD F 121 94 56.7
RT710 AD F 138 96 63.5
RT047 AD M 150 108 74.8
RT084 AD M 154 112 93.0 Old shark bite
RT124 AD M 148 125 77.1
RT176 AD M 148 118 84.4
RT256 AD M 146 105 74.8 Old shark bite
RT272 AD M 167 117 90.7
RT435 AD M 168 106 79.4
RT753 AD M 147 118 9'.0
RT974 AD M 163 114 90.7
FT212 AD M 155 111 90.7
FT222 AD M 144 107 77.1
FT220 AD M 150 - 83.9
�-- AD M 173 117 102.0

Total Average 149 + 3 108 + 2 77 + 3

Average for Females 139 + 3 100 + 3 62 + 4

Average for Males 155 + 3 113 + 2 86 + 2

Table 3. Minimum estimated length of time transmitters were functional,
and date/place seal last located visually or audibly with a
receiver.

Date Last Date Last a Minimum length of
Seal Recorded Observed/Recorded Location Attachment(days)

024 Nov/12/85 Apr/29/86 DE 101
033 Mar/3/86 Jul/10/86 DE 240
047 Mar/12/86 Jul/10/86 DE 221
084 Mar/27/86 Jul/10/86 DE 236
092 Mar/19/86 Jul/3/86 DE 228
124 Mar/6/86 Jul/ /86 DE 236
176 Jan/8/86 Jan/8/86 DE 158
234 Jan/24/86 Jul/10/86 DE 174
256 Aug/9/85 Jul/21/86 DE 7
272 Aug/28/85 Jul/10/86 DE 26
313 Dec/19/85 May/22/86 DE 138
415 Apr/18/86 Apr/18/86 RR 258
435 Jan/1/86 Jul/3/86 DE 151
491 Dec/19/85 May/10/86 DP 138
710 Mar/19/86 Jul/23/86 DE 228
753 May/3/86 Jul/10/86 DE 273
974 Apr/16/86 Jul/3/86 DE 256

a
DE = Drakes Estero, DP = Double Point, RR = Russian River.

movements were within 0.8 km of the shoreline (Table 4, Figure 1). Five

seals (33%) were recorded exclusively at Drakes Estero, six others (40%) on

a total of two haul-out sites, and four (27%) on three haul-out sites

(Table 5).

We classified eight radio-tagged seals (53%) as residents (five males,

three females). Of these, three were located once in the water near

another, nearby, haul-out site and one (a female) traveled greater than 25

km. We classified four seals (27%) as breeders (three females, one male);

all of which traveled greater than 25 km away from Drakes Estero. We

classified two females and one male as transients (20%), and these moved

both locally and further than 25 km (Table 5).

Long-range movements of greater than 25 km were recorded for 6 of 15

seals. Although females traveled longer distances than males, both sexes

moved about the same number of times (Table 5). One female moved north to

the Klamath River (480 kin) and one traveled south to Monterey Bay (210 kin)

for the winter months. Both were observed (without transmitter) at Drakes

Estero in April 1986. The seal at Monterey Bay was observed on almost a

weekly basis hauled out in front of Hopkins Marine Station from mid August

to mid-March (Alan Baldridge, Hopkins Marine Station, CA; C. Deutsch, Univ.

of Calif., Santa Cruz, CA, pers. comm.). Five seals traveled to nearby'

haul-out sites at Point Reyes; three to the Tomales Bay haul out site and

two to the Double Point haul-out site. Seals were also recorded in the

water near the Point Reyes Headland, Wildcat Beach, Bolinas Point, and

Drakes Bay near the fish docks.

More movements both local and long distance (20) were recorded during

Table 4. Location and behavior of seals that traveled from Drakes Estero recorded during aerial surveys.
Behavior is classified as A = active/swimming, I = inactive/hauled out.

FLIGHT SEAL BY RADIO FREQUENCY NUMBER
DATE/PLAN 024 033 047 084 092 124 176 234 313 415 435 491 710 753 974

1985
Aug 9 1 DE/I DE/I PRH/A DE/I DE/I DE/I DE/I
15 1 TB/I
29 1 DL/I TB/A TB/I
Sep 12 1 DE/I PRH/A DP/I DE/I DE/I TB/I DE/I DE/I TB/I
13 2 MB/I
19 1 DE/A DP/I
26 1 DE/I DE/I DE/I DE/A DE/I DE/I DP/A DE/I DE/I TB/I
Oct 4 1 DE/I DP/I TB/A PRH/A TB/A TB/I
10 3 KR/A DB/A DE/A DE/I DP/A DB/A DE/I TB/I SR/I DP/A DE/I
22 1 DE/A
23 2 MB/I
28 1 DB/A DE/A TB/I
31 1 DE/A DE/A BH/A BP/A DB/A BH/A
Nov 8 1 DE/I DP/A DE/I DE/I TB/A DE/I DE/I TB/I
21 1 DE/I D4/I DE/I DE/I DE/I DE/I DE/I TB/I
26 1 DE/I DE/I SP/I DE/I
Dec 10 equipment failure
19 1 DP/I SP/I RR/I DP/I TB/I
Feb 7 1 PRH/A RR/I TB/A
27 1 PRH/A DE/A DE/I TB/I
Mar 20 1 DE/I DE/I RR/I DE/I DE/I
23 2 DE/I DE/I MB/I DE/I
25 1 DE/I RR/I DE/I TB/I
Apr 2 1 RR/I DE/I TB/I
18 1 RR/I DE/I DE/I
May 9 1 DE/I

Locations with distances (km) from capture site: Flight Plan:

DE = Drakes Estero 0 BP = Bolinas Point 20 Half Moon Bay to Pt. Arena = 1
DB = Drakes Bay <10 BH = Bodega Head 40 Drakes Estero to Monterey Bay = 2
DP = Double Point 16 MB = Monterey Bay 210 San Francisco to Klamath R. = 3
KR = Klamath River 480 RR = Russian River 55
SP = Salt Point 65 TB = Tomales Bay 48
SR = Sea Ranch 85 PRH - Pt Reyes Head 12

Table 5. Distance of movement and locations of individual seals.

No. of Average a Number of
Seal moves distance (km) Locations Locations

Females:
024 2 480 + 0 KR DE 2
033 -0- - DE 1
092 2 210 + 0 MB DE 2
234 -0- - DE 1
313 3 47 + 11 TB SP DE 3
415 2 58 + 28 SR RR 3
491 3 24 + 4 DP PRH DP 3
710 2 48 +0 TB DE 2

Totals 14 128 + 43

Males:
047 4 12 + 0 PRH DE PRH DE 2
084 -0- - DE 1
124 4 18 + 0 PRH DP DE DP 3
176 -0- - DE 1
435 2 20 + 0 BP DE 2
753 -0- - DE 1
974 2 48 + 0 TB DE 2

Totals 12 26 + 4

a
BP = Bolinas Point, DE - Drakes Estero, DP = Double Point, KR Klamath
River, MB = Monterey Bay, PRH - Point Reyes Headlands, RR - Russian River,
SP - Salt Point, SR - Sea Ranch, TB = Tomales Bay.

the fall (August-October) than during the winter (November-February) or the

spring (March-April), i.e. eight versus six movements, respectively. Most

long distance movements were made by breeders during the fall and the

spring when seals were departing to wintering sites after the molt and

returning for the breeding season (Table 6). One transient seal that

traveled 85 km did not return to Drakes during the breeding season.

ACTIVITY PATTERNS

Resident seals exhibited distinct seasonal and circadian trends in

haul-out behavior at Drakes Estero. Resident seals were hauled out an

average of 0.92 + 0.03 of the days from August through October but only

0.77 + 0.04 of days from November through February (Tables 7, 8)

(p = .0081, n -8, Wilcoxon matched-pairs sign-rank test). The change in

number of days that seals were hauled out in winter was more pronounced for

females than males but sample sizes were too small to test for

significance. If one looks at the individual patterns, there is variation,

but the trends are similar. The proportion of tagged seals hauled out per

day relative to the total number of tagged resident seals was also greater

for August through November than from December through March (Table 9).

When resident seals were not hauled out, they were in the water in the

vicinity of the haul out or they were absent from Drakes Estero. The

length of absences averaged 1.4 to 1.9 consecutive days for over six

months; the longest absence was 10 days (Table 10). Length of absences was

similar for females and males.

When seals were present in Drakes Estero, the haul-out was strongly

a
Table 6. Movements of resident, breeding, and transient seals by season.

Resident Breeder Transient Totals

Number of seals 8 4 3 15
Number moves <25 km 11 3 7 21
Number moves >25 km 2 9 2 13

Number of moves by season:
Aug-Oct
<25 km 6 2 5 13
>25 km 2 4 1 7

Nov-Feb
(25 km 5 1 -0- 6
>25 km -0- 1 1 2

March
<25 km -0- -0- 2 2
>25 km -0- 4 -0- 4

a
Local movements not recorded after March because transmitters ceased
functioning.

Table 7. Proportion of days resident seals were hauled out at Drakes
Estero during the fall and winter and March, based on number of
days monitored. Only resident seals included.

SEAL BY RADIO FREQUENCY AVERAGE

Female Male Total
SEASON 033 234 710 047 084 176 435 753

Fall 0.9 0.9 0.9 1.0 0.8 1.0 1.0 1.0 0.9+0.0 0.9+0.0 0.9+0.0
(Aug-Oct)

Winter 0.7 0.6 0.7 0.7 0.8 0.9 0.8 0.9 0.7+0.1 0.8+0.0 0.8+0.0
(Nov-Feb)

Spring 0.7 - 0.6 0.5 0.7 - - 0.9 0.7+0.5 0.7+0.1 0.7+0.1
(March)

Table 8. Proportion of days per month each seal was hauled out at Drakes
Estero.

MONTH

Seal Aug Sep Oct Nov Dec Jan Feb Mar

Females:
RT033 0.9 1.0 1.0 0.9 0.7 0.8 0.6 0.7
RT234 0.9 0.6 0.9 0.5 0.5 0.7 *
RT491 0.5 0.4 **
RT710 0.9 0.8 0.9 0.5 0.9 0.7 0.6 0.6

Males:
RT047 0.9 1.0 1.0 0.9 0.8 0.7 0.3 0.5
RT084 0.8 0.8 0.6 0.8 0.9 0.8 0.7 0.6
RT176 1.0 1.0 1.0 0.8 0.9 **
RT272 0.8 *
RT435 0.9 1.0 1.0 0.9 0.8 *
RT753 0.9 1.0 1.0 0.9 0.9 0.8 0.9 0.9

Total Average 0.9 0.8 0.9 0.8 0.8 0.7 0.6 0.7
SE 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
n 10 9 8 8 8 6 5 5

Female
Average 0.8 0.7 0.9 0.6 0.7 0.7 0.6 0.7
SE 0.1 0.1 0.1 0.1 0.1 0.0 0.5 0.5
n 4 4 3 3 3 3 2 2

Male
Average 0.9 0.9 0.9 0.8 0.9 0.8 0.6 0.7
SE 0.0 0.1 0.1 0.1 0.0 0.0 0.2 0.1
n 6 5 5 5 5 3 3 3

* Transmitter ceased functioning.
** Seal no longer present at Drakes Estero.

Table 9. Proportion of resident radio-tagged seals hauled out each day
(seals with transmitters that are no longer functional, breeders,
and transients are excluded).

MONTH

Date Aug Sep Oct Nov Dec Jan Feb Mar

1 0.9 1.0 0.6 0.7 1.0 1.0
2 0.9 0.9 1.0 0.4 - 0.6 1.0
3 1.0 0.9 1.0 0.9 0.6 0.6 0.8
4 0.7 0.9 0.9 1.0 0.4 0.8 1.0
5 0.7 0.8 1.0 1.0 1.0 0.8 1.0
6 0.7 1.0 0.9 1.0 1.0 1.0
7 0.9 1.0 0.7 0.9 0.8 0.2
8 0.8 0.6 1.0 1.0 0.2 0.4
9 0.9 0.9 1.0 0.7 0.8 0
10 0.4 0.6 0.9 0.7 0.8 0
11 0.3 0.8 0.4 0.6 0.8 0
12 0.6 ' 0.8 - 0.6 0 1.0
13 0.8 0.5 0.7 0.7 0.6 0.8
14 0.9 0.5 1.0 0.3 0.2 1.0
15 0.9 0.6 1.0 0.9 0.6 1.0
16 0.8 0.8 1.0 0.4 1.0 0.8
17 0.9 0.8 1.0 0.7 0.8
18 1.0 0.9 1.0 0.9 0.8
19 1.0 1.0 0.9 0.4 0.8
20 1.0 0.9 1.0 0.7 0.5
21 1.0 0.7 0.7 0.8 1.0
22 1.0 0.9 0.6 0.6
23 1.0 1.0 0.7 0.4
24 1.0 0.6 0.7 0.4
25 1.0 0.9 0.6 0.3 0.6
26 1.0 0.9 0.5 0.6 0.8
27 0.8 0.9 0.2 0.6 0.6
28 0.9 0.9 0.2 0.7 0.2
29 0.9 0.9 0 -
30 1.0 0.9 0.4 0.5 0.7

x 0.9 0.9 0.9 0.8 0.7 0.6 0.6 -0.7
SE 0.0 0.0 0 0.0 0.1 0.0 0.1 0.1

Adjusted figures including breeders and transients:

x 0.6 0.5 0.5 0.4 0.4 0.3 0.2 0.3
SE 0.1 0.1 0.0 0.1 0.2 0.1 0.1 0.1

Table 10. Average length of time resident seals were absent from Drakes
Estero, measured in consecutive days. Seals were neither in
the vicinity in the water nor hauled out. Insufficient data
were available for September and October. Averages and modes
for days absent are presented for seal and for each month.

AVERAGE BY MONTH AVERAGE ALL MONTHS

Seal Aug Nov Dec Jan Feb Mar x SE mode range n

033 1 1 2 1 2 1 1.4 0.2 1 1-3 19
234 1 2 3 0 * 2.6 0.8 1 1-10 10
710 2 1 0 1 0 2 1.4 0.3 1 1-3 6
047 1 1 2 1 2 * 1.5 0.3 1 1-4 13
084 2 0 1 1 2 1 1.3 0.2 1 1-3 14
176 0 0 1 * 1.0 0 1 1 2
435 1 0 1 * 1.0 0 1 1 5

Average days by month (all seals combined):

x 1.5 1.4 1.9 1.0 1.8 1.4
SE 0.3 0.3 0.6 0.0 0.5 0.2
mode 1 1 1 1 1 1
range 1-3 1-4 1-10 1 1-4 1-2
n 10 9 16 13 13 8

* Transmitter ceased functioning.

diurnal in all months monitored, and most hauled out during mid-day

regardless of season. The proportion of tagged seals hauled out over a

24-hr period was largest 0800 to 1400 hr in August, and 1000 to 1600 hr in

December (Table 11). On average, seals hauled out for a total of seven

hours per day; however, seals would often haul-out more than once per day.

The duration of a single hauling bout averaged 4.8 to 5.6 hrs (Table 12).

The averages for hours-per-day and length-of-hauling-bout were not

significantly different between August and December (hour, p = .88; length,

p = .84; Wilcoxon matched-pairs sign-ranks test, m - 3, n = 5). Our sample

was too small to test for differences between males and females; however,

in December two pregnant females (RT234 and RT710) that gave birth in April

spent fewer hours on shore per day than males. Individuals hauled out for

up to 34 hrs in August, but never more than 17 hrs in December. The

average number of hauling bouts per day was similar in August (x = 1.6 +

0.05) and December (x = 1.4 + = 0.05) and for both sexes (females in

August, x = 1.7 + = 0.08, and December, x = 1.4 + 0.09; males in August, x

x = 1.5 + 0.06; and December, x = 1.4 + 0.07) and ranged between one and

four. Ninety-five percent of the 372 hauling bouts in August, and 96% of

the 212 in December were 12 hrs or less; 20% of the hauling bouts were less

than two hrs in both August and December.

The length of a hauling bout was affected by the time-of initiation

in August such that seals that first hauled out between 0500 and 0900 hr

were on shore longer than seals that initiated a haul-out between 0900 and

2400 hr; in December, though, time of initiation did not appear to

influence length of bout (Table 13). Though seals initiated a hauling bout

Table 11. The proportion of resident seals hauled out per day during
two-hour intervals.

August December

Hours x SE- n x SE n

0000-2000 0.1 0.0 30 0.1 0.0 28

0200-0400 0.1 0.0 30 0.1 0.0 28

0400-0600 0.2 0.0 30 0.2 0.1 28

0600-0800 0.5 0.1 30 0.2 0.1 28

0800-1000 0.7 0.1 30 0.3 0.1 28

1000-1200 0.7 0.1 30 0.5 0.1 28

1200-1400 0.6 0.0 30 0.6 0.1 28

1400-1600 0.5 0.1 30 0.5 0.1 28

1600-1800 0.5 0.0 30 0.5 0.1 28

1800-2000 0.3 0.1 30 0.3 0.1 28

2000-2200 0.2 0.0 30 0.2 0.0 28

2200-2400 0.1 0.0 30 0.2 0.0 28

Table 12. Average number of hours per day and the average duration of
haul-out bouts presented for individuals and by season.

FEMALES MALES

033 234 710 047 084 176 435 753 ALL SEALS

AUGUST:
Hours/Day
x 6.7 8.0 7.1 7.2 7.8 7.6 7.6 7.8 7.5
SE 0.6 0.7 0.7 0.8 0.9 0.8 0.7 0.7 0.2
Median 6.5 8.2 6.5 6.3 8.0 7.5 7.5 7.0 7.0
Range 1-13 1-20 1-17 1-19 1-19 1-19 1-16 1-17 1-20
n 28 27 27 28 24 30 27 28 219

Length of Bout
x 4.7 5.4 4.6 6.5 6.3 6.9 6.0 5.5 4.8
SE 0.7 0.7 0.4 1.1 0.9 1.1 0.8 0.7 0.2
Median 4.2 5.0 4.0 4.8 5.0 7.0 5.0 5.0 0.4
Range 1-13 1-20 1-10 1-28 1-19 1-34 1-16 1-17 1-34
n 38 49 52 32 35 40 40 53 339

DECEMBER:
Hours/Day
x 7.0 5.3 5.5 7.8 7.6 8.1 8.1 8.0 7.3
SE 0.8 1.2 0.8 0.6 0.5 0.6 0.6 0.7 0.3
Median 6.0 4.0 5.0 9.0 7.8 7.8 8.0 8.3 7.5
Range 1-15 1-16 1-14 1-13 1-13 1-13 1-16 1-15 1-16
n 19 11 21 26 22 22 23 26 170

Length of Bout
x 4.8 4.5 5.0 6.8 7.0 6.7 7.1 5.5 5.6
SE 0.6 1.3 0.8 0.8 0.6 0.9 0.6 0.7 0.2
Median 5.0 2.5 4.5 7.5 7.5 7.3 7.8 5.5 5.5
Range 1-8 1-16 1-14 1-17 1-13 1-13 1-13 1-12 1-17
n 19 11 24 28 27 30 29 40 208

a
Season is represented by data from one month.

Table 13. Length of haul-out in relationship to time of initiation.

August December

Hours n x SE x Range n x SE rx Range

0100-0500 32 6.88 .89 5.25 1-19 12 5.38 1.14 3.75 1-13

0500-0700 74 7.09 .65 6.0 1-34 33 5.30 .58 5.5 1-13

0700-0900 86 5.63 .39 5.5 1-26 29 6.65 .76 7.0 1-16

0900-1100 47 3.39 .34 3.0 1-12 36 6.96 .49 7.5 1-13

1100-1300 33 3.39 .47 2.5 1-9 44 5.71 .42 6.0 1-13

1300-1500 44 3.74 .39 3.5 1-13 19 5.26 .79 5.0 1-13

1500-1700 28 3.41 .39 3.5 1-8 14 4.89 1.4 2.0 1-17

1700-1900 20 2.83 .77 1.5 1-16 5 5.0 1.49 4.0 1-10

1900-2400 25 1.94 .44 1.0 1-12 20 3.98 .71 2.75 1-12

at all hours of the day most initiations occurred during daylight hours

between 0400 and 1600 hr for August and between 0600 and 1400 hr for

December (Table 14). The pattern was similar for both males and females.

DISCUSSION

Results from one survey year indicate that the decline in seal numbers

during the winter months is related to both a seasonal movement away from

Drakes Estero and a reduction in the number of days spent hauled out for the

remaining animals. Among radio-tagged animals, equal numbers remained in

and moved away from Drakes Estero; departees were composed of more females

than males. Similar to results in Alaska (Pitcher and McAllister 1981)

and on San Nicolas Island (Stewart and Yochem 1983), we found a high level

of site fidelity, with seals using no more than three sites. Exclusive use

of one haul-out site has also been noted in other studies (Boulva and

McLaren 1979, Herder 1986, Stewart and Yochem 1983). In contrast to seals

at the Klamath River (Herder 1986), most movements of seals from Drakes

Estero occurred from August to October; however, we have no data for the

period from April to June when Klamath seals exhibited the greatest local

movement. We found, as did Herder (1986), that females made more long

distant moves than males. Immature seals may travel greater distances than

adult animals, as suggested by Pitcher and McAllister (1981), but we have

no information on this.

In general, we observed that if a seal moved to a location greater than

Table 14. Frequency of initiation of haul-out bouts in relationship to
time of day by season for females, males, and all seals.

August December
n - 28d 1985 n = 30d

Time Female Male Total Female Male Total

0000-0200 2 5 7 4 6 10

0200-0400 3 7 10 4 1 5

0400-0600 24 17 41 8 10 18

0600-0800 35 72 107 8 21 29

0800-1000 24 36 60 7 23 30

1000-1200 10 1, 27 11 39 50

1200-1400 10 30 40 10 22 32

1400-1600 13 24 37 3 7 10

1600-1800 10 18 28 5 13 18

1800-2000 8 9 17 2 8 10

2000-2200 9 2 11 2 3 5

2200-2400 4 1 5 3 3 6

25 km away from the capture site, it remained there for an extended time

rather than continuing to travel. The majority of these long-range

movements occurred after the molt and prior to the breeding season. The

movements were to well established haul-out sites in estuaries (Tamales

Bay), at river mouths (Klamath and Russian rivers), and at coastal sites

(Double Point, Hopkins Marine Station).

The localities at which seals relocated may be preferred winter

foraging areas. However, because migrant seals were' highly individual in

the distance of their movement and their final destination, we suggest that

seals were not responding to a single prey situation as was noted in winter

movements of seals in Oregon and Washington. Instead, individuals may be

expressing a preference for location based on past experience, including

foraging success (Beach et al. 1985, Brown and Mate 1983).

The observed movements indicate that Drakes Estero is an important

breeding area for seals between Monterey Bay and the Klamath River, an

overall distance of 690 km. The three seals which did not return to Drakes

Estero in the following breeding season may visit Drakes Estero only during

the molt, or use it only as a secondary haul-out site as did one transient

which visited Drakes Estero twice after capture but resided at Double Point

through the breeding season. The movements also indicate that there is

substantial mixing among harbor seal colonies along the cenftral and

northern California coast, and that seals in central and northern

California may represent a subpopulation of harbor seals in California.

Herder (1986) and Brown and Mate (1983) also found movement of seals

between colonies in the Klamath River and Alsea Bay, Oregon,and between

seals in Oregon and Washington.

All recorded foraging activities and movements, of seals in the Point

Reyes area appeared to be limited to within 0.8 km of shore. Though

pelagic movements of harbor seals have been recorded by others (Pitcher and

McAllister 1981, Spaulding 1964, Stewart and Yochem 1983, Wahl 1977), our

results suggest nearshore feeding and migration are more typical than

offshore.

Seals that remained dt Drakes Estero were hauled out on more days and

in higher proportions of total animals in the fall than the winter months.

Our figures on the proportion of days per month spent hauled out and on the

proportion of radio-tagged seals hauled out per mionth were substantially

higher than those derived by both Stewart and Yochem (1983) and Pitcher and

MacAllister (1979); if we include in our analyses animals that departed

from Drakes Estero, our figures would be similar (see Table 9 for adjusted

figures). We eliminated these animals from analyses, though, since the

location of the departed animals had been ascertained, thereby allowing for

more accurate assessments of time spent ashore for seals in the immediate

vicinity.

When hauled out, the number of hours per day that seals spent on shore

was similar for fall and winter, as was the haul-out time interval. This

suggests that despite high tides during the day in winter, with winter

storms accentuating high tides, seals were regular in the length of time

they hauled out, but not in the number of days hauled out. Both Sullivan

(1979) and Stewart and Yachem (1983) observed a higher average time per day

that seals were hauled out, 44% and 39%, respectively, than we found for

seals at Drakes Estero (29%). Differences may be attributed to stronger

tidal influences at Drakes Estero or to exposure to disturbance from

humans; though Limantour Spit is exposed at all tide levels, seals there

are more frequently subjected to human distubance. Seals at Drakes Estero

may, therefore, be forced to spend more time in the water. Females may

haul-out for shorter periods both in number of days and number of hours per

day than males in the winter (see also Stewart and Yachem, 1983). The small

differences between means coupled with the small sample of animals result

in insignificant differences at this time. Pregnant females may require

more time feeding than males.

The strong diurnal haul-out pattern documented at Drakes Estero

previously (Allen and Huber 1983), and shown by innumerable other

researchers (Boulva and McLaren 1979, Fancher 1979, and Stewart and Yochem.

1983), was further confirmed in this study. Though seals hauled out at all

hours of a day, peak abundance, based on the proportion of tagged animals,

occurred mid-day. Stewart and Yochem (1983) determined that time of

initiation of a haul-out bout did not influence the length of the bout;

however, we found that in August, seals hauled out for longer periods when

bouts were initiated between 0100 and 0900 hr than for other time periods

during the day or night. In December, time of day did not appear to

influence the length of bout during daylight hours but did during the

night. Differences may be attributed to changes in day length, and/or

frequency of human disturbance at Drakes Estero.

Absences of resident seals from Drakes Estero, averaged about one day

suggesting that absentees were foraging fairly close by; the local movement

patterns observed during aerial surveys also support this conclusion.

SUMMARY

1. Evidence from movement patterns indicates that Drakes Estero is a

focal breeding area for seals ranging as far south as Monterey Bay and as

far north as the Klamath River, and therefore,that northern and central

California may represent a subpopulation of harbor seals in California. It

is also likely that some mixing between seals from Oregon and this

subpopulation occurs (Herder 1986).

2. A larger proportion of tagged seals hauled out on a daily basis in

August (92%) than in December (77%), and this difference appeared to be

more pronounced for females than for males.

3. The largest proportion of seals were hauled out between 0400 and

1600 hr in August and this represented between 53% and 71% of the estimated

number of seals in the area.

4. Seasonal changes in abundance are a function of the movement of

seals away from Drakes Estero and of a decrease in the number of days that

seals spent hauled out in the winter.

ACKNOWLEDGEMENTS

This study was supported by the Point Reyes - Farallon Islands National

Marine Sanctuary under the Sanctuary Programs Office of the National

Oceanic and Atmospheric Administration, the National Marine Fisheries

Service, the National Park Service, and the Point Reyes Bird Observatory.

Numerous people participated in the capture and tagging of seals at Drakes

Estero, but particular thanks are extended to R. Brown from the Oregon

Department of Fish and Wildlife, J. Harvey from Oregon State University, W.

Perryman from the National Marine Fisheries Service, J. Scholl from the

California Department of Fish and Came, P. Yochem from Hubbs Sea World

Research Institute, H. Huber, and B. Sutton, C. Young, A. Quintera of the

National Park Service. Others who assisted in this operation were R.

Stallcup, L. Spear, N. Stone, J. Rolleto and J. Plant. R. Plant generously

provided us with an additional vessel for the capture which was invaluable.

Much of the monitoring equipment was provided by the National Marine

Fisheries Service. R. Gordon provided a plane, flexible hours and

dedication in locating seals. S. Peaslee assisted in field observations;

C. Deutsch of the University of California, Santa Cruz, and A. Baldridge of

Hopkins Marine Station, California, were very helpful in providing

information on sightings.

We are also grateful to D. DeMaster of the National Marine Fisheries

Service and S. Jeffries of the Washington Department of Game for their

guidance and encouragement. Both C. Ribic and L., Stenzel gave guidance

with data analysis and project design. C. Page and E. Tuomi reviewed and

greatly improved the manuscript. Finally, I am most thankful to D. Miller

for logistical and emotional support.

This is Point Reyes Bird Observatory Contribution number 357.

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