Fur seal investigations, 1990

Abstract

During 1990, the National Marine Mammal Laboratory's program on the northern fur seal (Callorhinus ursinus) included investigations on the Pribilof Islands, Bogoslof Island, San Miguel Island and nearby Castle Rock off the southern California coast, and Medny Island (U.S.S.R.). This report is a collection of papers resulting from 1990 fur seal research.

Northern fur seal research on the Pribilof Islands was conducted from 1 July through 1 September 1990. Censuses of adult males were conducted on all rookeries of St. Paul and St. George Islands and Sea Lion Rock in mid-July of 1990. Harem bull counts on St. Paul Island were 3.1% higher in 1990 than in 1989. On St. George Island, harem bull counts decreased 3.6% between 1989 and 1990. The number of animals on St. George Island comprise approximately 10% of the Pribilof Island population. Pup abundance was estimated by the shearing-sampling method during August and dead pups were counted on all rookeries (except Little Polovina) on St. Paul and St. George Islands and for the first time on Sea Lion Rock.

Approximately 201,305 (SD = 3,724) pups were born on St. Paul Island in 1990, a number not significantly different from the estimates of the 1988 and 1989 year classes. The estimate of the number of pups born on St. George Island in 1990 is 23,397 (SD = 2,054); this estimate is not significantly different from the number of pups born on St. George Island in 1988 (24,820), nor is it significantly different from the predicted number of pups to be born based on the 1973-88 data, which indicated a 5.9% decline. On Sea Lion Rock, we estimated that 10,217 (SD = 568) pups were born.

A preliminary assessment of potential noise impacts on northern fur seals was conducted. Of primary concern were direct noise sources, such as airplanes, off-road vehicles, ship traffic, and construction activities.

Information on tag resights and weights of 2-and 3-year-old male northern fur seals was collected on St. Paul Island during roundups in July and August 1990. A total of 56 and 319 tag resights were obtained for males 2 and 3 years of age, respectively. Males tended to return to their rookeries of birth. Within a cohort, weights of tagged males had no influence on haul-out patterns of individuals. Although 2-and 3-year-old males lost weight during their time on land, intermittent foraging trips throughout the summer probably resulted in a net increase in growth. Individual male weight data also indicated that weight as pups was correlated to weight at 2 and 3 years of age.

Primary enteric pathogens do not appear to infect significant numbers of pups on the Pribilof (St. Paul and St. George) Islands or Medny Island. Opportunistic pathogens were found in all pups tested from each island, and the most frequently isolated organisms were nonhemolytic Escherichia coli, and Citrobacter, Proteus, and Enterobacter spp.

Beach surveys were conducted on two sites on St. Paul Island during 1990 to determine the accumulation of beach debris. The accumulation of beach debris on the Pribilof Islands has decreased among most categories of debris found since 1983. However, plastic banding material increased at Northeast Point during 1990.

A census conducted on Bogoslof Island on 24 July 1990 counted a total of 1,473 northern fur seals including 44 territorial males, 951 nonterritorial males, 295 females, 181 live pups, and 2 dead pups.

Research was carried out on San Miguel Island and nearby Castle Rock off the southern California coast intermittently from June through December 1990. At Adams Cove, San Miguel Island, there were 68 adult northern fur seal males defending territories that contained females and pups and an additional 8 solitary adult males defending territories. This is the largest number of territorial males recorded during a breeding season. Pup abundance at Adams Cove on 27 July was 1,137 live and 12 dead and counts on Castle Rock on 29 July yielded 634 live and 14 dead pups. Two hundred northern fur seal pups were tagged on 23 September in Adams Cove. California sea lion (Zalophus californianus) × northern fur seal hybrid pups were again observed on San Miguel Island.

Three U.S. biologists conducted northern fur seal research on Medny Island (U.S.S.R.) from 19 July through 28 August 1990 to examine the possible relationship between the continental shelf and the 1956-81 decline in population abundance in the Pribilof Islands. Data collected included frequency and duration of trips by female fur seals to sea, depth and location of diving, milk fat content, quantity of milk delivered to the pup, attendance patterns of females, pup growth rates, diet, and bacterial pathogens of fur seal pups.

Juvenile fur seals from 122 roundups were sources of data for estimates of entanglement-caused mortality and entanglement rates. The proportion of juvenile males observed entangled in 1990 was 0.33%. This rate reflects the continued reduction in the numbers of animals entangled in fragments of trawl webbing. The frequency of occurrence of trawl webbing among the entangling debris remains about one-half that of the former levels, whereas the proportion of seals entangled in other types of debris did not change. The observed proportion of fur seals entangled in 1990 was similar to that observed during 1988 and 1989, continuing at a rate that is lower than the rate recorded for the last few years of the commercial harvest and roundups through 1986.

These entanglement studies confirm earlier estimates indicating that after 1 year, seals entangled in small debris (light enough to permit the animals to return to land) are reduced in numbers to about one-half the comparable numbers for unentangled seals. Rates at which entangled animals are resighted indicate that mortality of entangled seals increases with the size (weight) of debris. Data collected on the extent of wounds caused by entangling debris show that wounds tend to grow, some encompassing the entire neck within 1 year's time, and contribute to the sources of mortality for seals entangled in small debris.

The 1990 studies suggest the rate of return of tagged seals from which debris is removed is significantly higher than for tagged seals on which entangling debris was left.


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