abundance and distribution of harbor seals (phoca vitulina) in oregon, 1975-1983

Abundance and Distribution of Harbor Seals (Phoca vitulina) in Oregon, 1975-1983Author(s): James T. Harvey, Robin F. Brown and Bruce R. MateSource: Northwestern Naturalist, Vol. 71, No. 3 (Winter, 1990), pp. 65-71Published by: Society for Northwestern Vertebrate BiologyStable URL: http://www.jstor.org/stable/3536774 .

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NORTHWESTERN NATURALIST 71:65-71 WINTER 1990

ABUNDANCE AND DISTRIBUTION OF HARBOR SEALS (PHOCA VITULINA) IN OREGON, 1975-1983

JAMES T. HARVEY, ROBIN F. BROWN, AND BRUCE R. MATE

ABSTRACT-Harbor seals were observed on 32 haulout sites in Oregon during aerial counts conducted from 1975 to 1983; 90% were seen on 14 sites. The greatest number of seals seen on a haulout was 985 recorded at Cape Arago in July 1982. Counts of harbor seals in 1982 and 1983 were 38.6% greater than counts from 1975 to 1980. These data indicate an increase in numbers of harbor seals in Oregon, an increase corroborated by other information, namely, lower counts made before 1975 and increased use of new haulout sites since 1975. Between 1975 and 1983, numbers within bays increased, whereas numbers on most offshore rocks remained somewhat constant. Decreased harassment and mortality since implementation of the Marine Mammal Protection Act of 1972 doubtless allowed harbor seals to increase in numbers and to reoccupy protected haulout sites in bays.

The harbor seal (Phoca vitulina richardsi) is one of five pinniped species common in Oregon. It is the most abundant and ubiquitous pinniped in the state, found in most bays and estuaries, and on many offshore rocks. Harbor seal abundance has been determined at specific haulout sites along the Oregon coast (Graybill 1981; Brown and Mate 1983; Roffe and Mate 1984; Bayer 1985), although there is only one published account of their state-wide distribution and abundance (Pearson and Verts 1970). From 1967 to 1968, Pearson and Verts (1970) conducted four counts from land and one aerial count along the Oregon coast and concluded that there were fewer than 500 harbor seals in the state.

More than 3800 harbor seals were killed in Oregon between 1925 and 1972 by state- hired and bounty hunters (Pearson 1969). Harbor seals were harassed at inshore areas where they rested and pupped, and their number in Oregon was probably reduced. Harassment and killing of harbor seals were greatly reduced after 1972 when the Marine Mammal Protection Act became effective. Protection probably has resulted in an increase in abundance and redistribution of animals into bays and estuaries.

The objectives of this study were to determine the location and use of haulout sites and to document any changes in abundance and distribution of harbor seals in Oregon from 1975 to 1983.

METHODS Aerial photographic surveys of seals on land along the entire Oregon coast were performed

annually from a high-winged, single-engine aircraft (Cessna 172 or 182) during 1 month in the summer (June, July, or August) from 1975 to 1983 (Fig. 1). Surveys were begun approximately 2 hr before low tide and ended approximately 2 hr after low tide to maximize the number of seals visible on haulout sites (Schneider and Payne 1983; Terhune and Almon 1983). The entire Oregon coastline was scanned during annual surveys to determine locations of harbor seals ashore and to locate new haulout sites. When more than five seals were ashore, the group was photographed from an altitude of 215-305 m using a hand-held 35 mm camera and 300 mm or 70 to 210 mm zoom lens. Photographs were taken within 15° of vertical. Photographic images of seals on land were projected onto a white surface, and each seal was counted.

Photographic surveys required 2 days; flights were made from Newport to the north or south on consecutive days. Surveys of some haulout sites were not possible because of inclement weather.

65



71(3) 66 NORTHWESTERN NATURALIST

) : X CALIFORNIA

. I , cl ,

FIGURE 1. Thirty-two sites along the Oregon coast where harbor seals were observed on land.

Trends in abundance were assessed USixIg counts of harbor seals at 14 haulout sites (Table 1). Only 14 sites were used because these sites were observed most years, nearly 90% of all sightings came from these sites, and equal numbers of ocean and bay haulout substrates (7 each) were represented in this subsample. Instantaneous rate of increase (r) was calculated as the slope of ln count of seals regressed on time (Caughley and Birch 1971) and then converted to a finite annual rate

Nine annual surveys of harbor seals along the entire coast provided information for year-to-year comparisons. The July 1981 count, however, was not flown during the 3-4 hour period surrounding low tide, which resulted in a low count for that year. Therefore, in discussions of trends in harbor seal abundancet we excluded data fronl 1981. Two surveys conducted in June 1983 were used to examine within-year variability

RESULTS

Harbor seals were counted on 32 haulout sites along the Oregon coast (Fig. 1). Sites were often composed of many small aggregations of seals. Three types of substrate were used as haulout sites: estuarine sand and mud flats exposed during low tides; remote, mainland shoreline; and offshore rocks and islands. Some unique sites included a grassy



WINTER 1990 HARVEY ET AL.: HARBOR SEALS IN OREGON 67

TABLE 1. Counts of harbor seals on 32 haulout sites along the Oregon coast (north to south), based on aerial censuses during one summer month each year from 1975 to 1983. Hyphens designate no count. Each haulout site is designated as offshore (O), shoreline (S), or estuarine (E). Fourteen haulout sites were surveyed on most flights, and are designated with an asterisk.

Haulout site MoIlth and year of count Hab- July July June June July July July August June

Location itat 1975 1976 1977 1978 1979 1980 1981 1982 1983 Columbia River* E - 145 152 307 4 356 194 65 153 Tillamook Head S - 8 25 49 110 0 - 115 112 Ecola Point S - - 4 0 0 - - 0 Cape Falcon S - 60 102 0 0 0 31 55 15 Tillamook BayX E 312 85 247 367 385 279 203 545 527 Netarts Bay E 24 - 23 44 54 16 48 207 167 Cape Lookout S 23 - - 0 20 15 10 11 0 Cascade Head S - - 31 27 18 28 3 0 0 Siletz Bay E - - 43 0 0 0 0 0 0 Boiler Bay S - - - - - - - - 8

Whale Cove S - - 0 - 19

Cape Foulweather O 50 25 18 0 38 64 49 11 30 Yaquina Head S - - 11 13 55 4 0 54 50 Yaquina Bay E 0 - 0 0 - 19 0 0 0 Seal Rock S - 0 0 0 - - 0 6 0 Alsea Bay* E 71 0 1 0 0 119 148 251 193 Strawberry Hill S 10 0 46 33 75 52 70 75 86 Siuslaw River E 0 0 10 4 0 20 0 121 60

Siltcoos outlet S 53 - 25 0

Umpqua River* E 454 443 596 464 382 549 210 280 774 Coos Bay E 113 35 - 85 95 35 117 222 233 Cape Arago S 670 125 425 530 483 440 104 985 683 Coquille Point$ O 91 106 61 81 79 6Z 55 75 120

Blacklock Point S B - - 9

Gull Rock O 0 55 - 83 88 - 78 80 75 Blanco Reef O 0 0 - 49 37 148 23 24 0 Orford Reef O 60 0 0 0 15 - 19 75 0 The Heads O - - 0 0 14 24 - - 35 Island Rock O 0 - 6 0 - - - 11 Hubbard Md. ReefX O - 90 126 182 0 98 100 20 166 Rogue Reef O 133 236 201 211 6 189 0 184 212 Hunter's Island O 180 151 148 49 106 - 84 30 15 Totals for 14 haulout

sites designated with *: 2108 1441 2054 2357 1707 2279 1382 3071 3419 Percent of total count: (93.9) (92.1) (89.3) (91.4) (82.4) (90.5) (89.4) (87.5) (89.8)

plateau in Alsea Bay accessible only during high tides, a seldom used boat dock in Siletz Bay, and a shoreline location (Strawberry Hill) where seals across a 20 m surge channel were approached closely. Nine haulout substrates were estuarine, 13 were shoreline, and 10 were offshore. Ninety percent of offshore haulout substrates were located in the southern portion of the state (south of Strawberry Hill), whereas the other haulout substrates were primarily found in the northern portion of the state (66.7% of estuarine and 69% of shoreline sites were north of Strawberry Hill). Most seals (x = 89.8%/year, SD = 3.7) were counted at 14 sites (Table 1). Numbers of

seals at Cape Arago, Umpqua River, and Tillamook Bay averaged 22.1, 20.6, and 14.7S, respectively, of the total state-wide count. The greatest number of seals at any area was 985, recorded at Cape Arago in July 1982. The second largest concentration was 774 individuals in the Umpqua River in June 1983. Counts at specific haulout sites varied dramatically from year to year (Fig. 2). Counts

on offshore and shoreline haulout sites had the greatest year-to-year variability, partic-



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68 NORTHWESTERN NATURALIST 71(3)

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FIGURE 2. Numbers of harbor seals on land at 12 locations in Oregon from 1975 to 1983.

ularly at Cape Arago and Rogue Reef, although counts also fluctuated widely in the Columbia River (Fig. 2). Few offshore or shoreline sites had a detectable trend in seal abundance; an exception was Hunters Island, where counts declined. Counts at 83% of bay haulout sites rose dramatically in 1982 and 1983 (Fig. 2). Based on counts at the 14 haulout sites observed most frequently (Table 1), the proportion of harbor seals counted in bays increased from 47% of the total in 1975 to 61% in 1983 (Fig. 3). Between 1975 and 1978, harbor seals used an average 16.3 haulout sites in Oregon, compared with an average 30.3 haulout sites from 1980 to 1983.

Between 1975 and 1983 counts increased, mainly because of greater counts in 1982 and 1983 (Fig.4). Counts on 14 haulout sites over the 8 year period increased significantly at a mean rate of 8.1% per year (r2 = 0.60, p < 0.01; Fig. 4). Counts were not made at two sites in 1975, nor at one site in 1976, 1977, and 1980, because of weather problems. Two counts (3739 and 3565) were made in June 1983. These surveys were conducted 11 days apart, and the counts were different by only 174 seals (4.7%).

DISCUSSION

Aerial surveys for assessment of pinniped populations pose inherent problems. Some problems are undercounting (i.e., some animals are missed and those in the water are not counted), aircraft disturbance of pinniped behavior, and weather restrictions on time of counts (Eberhardt et al. 1979; Estes 1976; Norton-Griffiths 1974; Packard et al. 1985).



69 WINTER 1990 HARVEY ET AL.: HARBOR SEALS IN OREGON

70

cn

n 60- n

cn

-

Z 50- @

r

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75 76 77 78 79 80 81 82 83

Y E A R FIGURE 3. Changes in percentage of harbor seals counted in Oregon bays from 1975 to 1983.

Aerial counts in Oregon were conducted during summer months under relatively consistent environmerltal conditions (i.e., weather safe for aircraft operations and clear enough to photograph seals on land). Although portions of the 1981 survey were conducted during both high and low tides, all surveys during other years were performed during low tides. Therefore, a first-order assessment of trends in harbor seal abundance can be made from the latter counts.

Conclusive evidence of increasing abundance of harbor seals in Oregon is lacking because within-year variability was not determined. Two counts made in June 1983 were similar, but there were no data to compare variability in counts over an entire summer during any one year. Bayer (1985), however, reported no significant difference in counts of harbor seals on haulout sites in Yaquina Bay throughout the summers of 1977 to 1982. Counts of seals on land, however, may fluctuate dramatically from day to day especially in response to disturbance and environmental factors (Ling et al. 1974; Harvey 1987). The observed trends in annual abundance may be caused by: random chance fewer harbor seals ashore during counts in early years and more in later years; immigration harbor seals moving north from California or south from Washington; or an actual increase of harbor seals born in Oregon.

Increasing counts of harbor seals in Oregon from 1975 to 1983 may represent an increase in abundance because: counts made before 1975 were smaller than those made from 1975

c,, 4000 8

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FIGURE 4. Total number of harbor seals counted on land during summer surveys in Oregon from 1975 to 1983, excluding 1981 (O). Number of harbor seals on land at 14 sites most frequently surveyed (O); line is fitted to the counts from the 14 sites.



70 NORTHWESTERN NATURALIST 71(3)

to 1983, seals are using new haulout sites, and counts made since 1983 (Brown 1986) are greater than counts made in 1982 and 1983. Estimates of 250 harbor seals on offshore rocks of Oregon during 1967 (surveys conducted by U.S. Bureau of Sport Fisheries and Wildlife) and 500 during 1968 (Pearson and Verts 1970) are well below counts made from 1975 to 1983. Although these earlier estimates were based on fewer counts and probably did not cover all haulout sites, the numbers of harbor seals counted at particular sites were fewer than recent counts at the same locations. Since 1983, counts of harbor seals in Oregon have continued to increase at a rate similar to that reported for 1975- 1983 (Brown 1986). This increase principally occurred in bays and estuaries, and may be a result of immigration (a consequence of reduced harassment) and recruitment. Use of new haulout sites may have resulted from increased numbers of harbor seals forcing some individuals to inhabit other areas for rest and food.

Harbor seal populations were kept low before 1972 because many animals were harassed and driven from protected bays and estuaries (Newby 1973). Pregnant females in Oregon and Washington use protected areas of estuaries for birthing and caring for pups (Brown and Mate 1983; Beach et al. 1985). If these areas are important in the reproductive cycle, harassment in bays may have effectively lowered the reproductive rate and pup survival.

An 8.1So annual increase since 1975 is consistent with recovering populations of other pinniped species (Eberhardt and Siniff 1977) and with harbor seals in other regions. Harbor seals near Humboldt Bay, California, increased from 6 in 1966 to 311 in 1973 (Loughlin 1978). Payne and Schneider (1984) reported an average rate of increase of 11.9% per year for harbor seals using a Massachusetts haulout site between 1972 and 1983. Harbor seals counted on haulout sites on San Miguel Island, California, during the summers from 1958 to 1976 increased an average 22% per year, although the rate was much lower from 1976 to 1986 (Stewart et al. 1988). In contrast, little increase in harbor seal abundance was reported in San Luis Obispo County, California (1972-1976), San Francisco Bay, California (1972-1980) Point Reyes, California (1976-1984), or in Yaquina Bay, Oregon (1977-1983) (Wade 1981; Fancher and Alcorn 1982; Allen and Huber 1984; Bayer 1985).

The information presented here indicates an increase in harbor seal abundance in Oregon since 1973, especially after 1981 Our counts represent trends in abundance, and are estimates of the minimum population size.

ACKNOWLEDGMENTS

We thank G. Antonelis, Jr., H. Braham, R. Ferrero, T. Loughlin, W. Pearcy, B. Turnock, B. J. Verts, and M. Yoklavich for constructive comments on an earlier version of this paper. Funding of aerial surveys was provided by the Oregon Department of Fish and Wildlife. The National Marine Mammal Laboratoryr Alaska Fisheries Science Center, Seattle, WA, provided technical support and funds for publication.

LITERATURE CITED

ALLEN, S G., AND H. R. HUBER. 1984. Pinniped assessment in Point Reyes, CA 1983-1984. Point Reyes Bird Observatory, Stinson Beach, CA, Final Rep. 71 pp.

BAYERj R. D. 1985. Six years of harbor seal counts at Yaquina estuary, Oregon. Murrelet 66:44-49. BEACH, R. J., A. C. GEIGER, S. J. JEFFRIES, S. D. TREACY, AND B. L. TROLrrMAN. 1985. Marine mammals

and their interactions with fisheries of the Columbia River and adjacent waters, 1980-1982. NWAFC Processed Rep. 85-04, 316 pp. Northwest and Alaska Fish. Cent., Natl. Mar. Fish. Serv., NOAA, Seattle, WA.

BROWN, R. F. 1986. Assessment of pinniped populations in Oregon, April 1984 to April 1985. NWAFC Processed Rep. 85-04J 316 pp. Northwest and Alaska Fish. Cent., Natl. Mar. Fish. Serv., NOAA, Seattle, WA.

BROWN, R. F., AND B. R. MATE. 1983. Abundance, movements, and feeding habits of harbor seals, Phoca vitulina, at Netarts and Tillamook Bays, Oregon. Fish. Bull. U.S. 81:291-301.



WINTER 1990 HARVEY ET AL.: HARBOR SEALS IN OREGON 71

CAUGHLEY, G., AND L. C. BIRCH. 1971. Rate of increase. J. Wildl. Manage. 35:658-663. EBERHARDT, L. L., D. G. CHAPMAN, AND J. R. GILBERT. 1979. A review of marine mammal census

methods. Wildl. Monogr. 63:1-46. EsERHARDT, L. L., AND D. B. SINIFF. 1977. Population dynamics and marine mammal policies. J.

Fish. Res. Board Can. 34:183-190. ESTES, J. 1976. The reliability of aerial surveys to estimate population abundance of marine mam-

mals: considerations from a survey of Pacific walruses. Advisory Com. Mar. Resources Res., Food and Agric. Org. United Nations, ACMRR/MM/SC/80. 13 pp.

FANCHER, L. E., AND D. J. ALCORN. 1982. Harbor seal census in south San Francisco Bay (1972-1977 and 1979-1980). Calif. Fish Game 68:118-124.

GRAYBILL, M. R. 1981. Haul out patterns and diet of harbor seals, Phoca vitulina, in Coos County, Oregon. Unpubl. M.S. thesis, Univ. Oregon, Eugene. 55 pp.

HARVEY, J. T. 1987. Population dynamics, annual food consumption, movements, and dive behav- iors of harbor seals, Phoca vitulina richardsi, in Oregon. Ph.D. dissertation, Oregon State Univ., Corvallis. 177 pp.

LING, J. K., C. E. BUrrON, AND B. A. EBSARY. 1974. A preliminary account of gray seals and harbor seals at Saint-Pierre and Miquelon. Can. Field-Nat. 88:127-132.

LOUGHLIN, T. R. 1978. Harbor seals in and adjacent to Humboldt Bay, California. Calif. Fish Game 64:127-132.

NEWBY, T. C. 1973. Changes in the Washington state harbor seal population. Murrelet 54:4-6. NoRToN-GRIFFrrHs, M. 1974. Reducing counting bias in aerial censuses by photography. J. East

Afr. Wildl. 12:245-248. PACKARD, J. M., R. C. SUMMERS, AND L. B. BARNES. 1985. Variation of visibility bias during aerial

surveys of manatees. J. Wildl. Manage. 49:347-351. PAYNE, P. M., AND D. C. SCHNEIDER. 1984. Yearly changes in abundance of harbor seals, Phoca

vitulina, at a winter site in Massachusetts. Fish. Bull., U.S. 82:440-442. PEARSON, J. P. 1969. The abundance and distribution of harbor seals and Steller sea lions in Oregon.

M.S. thesis, Oregon State Univ., Corvallis. 23 pp. PEARSON, J. P., AND B. J. VERTS. 1970. Abundance and distribution of harbor seals and northern

sea lions in Oregon. Murrelet 51:1-5. 1{OFFE, T. J., AND B. R. MATE. 1984. Abundances and feeding habits of pinnipeds in the Rogue

River, Oregon. J. Wildl. Manage. 48:1262-1274. SCHNEIDER, D. C., AND P. M. PAYNE. 1983. Factors affecting haul-out of harbor seals at a site in

southeastern Massachusetts. J. Mammal. 64:518-520. STEWART, B. S., G. A. ANrONELIS, JR., R. L. DELONG, AND P. K. YOCHEM. 1988. Abundance of harbor

seals on San Miguel Island, California, 1927 through 1986. BU11. South Calif. Acad. Sci. 87: 39-43.

TERHUNE, J. M., AND M. ALMON. 1983. Variability of harbor seal numbers on haul-out sites. Aquat. Mamm. 10:71-78.

WADE, L. S. 1981. Census of harbor seals, Phoca vitulina, in San Luis Obispo County, California, 1972-1973. Calif. Fish Game 67:126-129.

College of Oceanography, Oregon State University, Hatfield Marine Science Center, Newport, OR 97365 (JTH and BRM); and Oregon Department of Fish and Wildlife, Marine Region, Marine Science Center Drive, Bldg. #3, Newport, OR 97365 (RFB). Present address (JTH): Moss Landing Marine Laboratories, P.O. Box 450, Moss Landing, CA 95039. Received 8 February 1989, accepted 9 April 1990.



abundance and distribution of harbor seals (phoca vitulina) in oregon, 1975-1983

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