Fur seal investigations, 1991

Abstract

This report is a collection of papers resulting from northern fur seal (Callorhinus ursinus) research on the Pribilof Islands (St. Paul and St. George) in the eastern Bering Sea (30 June -2 August), and San Miguel Island off southern California (30 April -22 August) in 1991. Research conducted on St. Paul Island in 1991 included assessment of the numbers of adult males (bulls), resights and weights of juvenile males tagged as pups, and disease and entanglement studies. A pilot study on age determination of females based on physical characteristics and comparison of age structure between St. Paul and St. George Islands was conducted in 1991. Bull counts and disease studies were also conducted on St. George Island, and pup counts and tag resights were recorded on San Miguel Island. Results of a pilot study on the weights and sex ratio of pups born in 1990 are also presented in this report. Pribilof Islands research conducted by National Marine Mammal Laboratory (NMML) personnel in 1991 included cooperative studies with three guest researchers from the All-Russia Research Institute for Fisheries and Oceanography (VNIRO) and the Pacific Research Institute of Fisheries and Oceanography (TINRO) marine mammal research agencies of Moscow and Kamchatka, Russia.

A total of 4,729 harem and 9,543 idle bulls were counted on St. Paul Island. Counts on St. George Island totaled 736 harem and 1,271 idle bulls. Tag resights were obtained for males 2 (n = 52), 3 (n = 270), and 4 (n = 281) years of age on St. Paul Island. Weight data from tagged 3- and 4-year-old males indicated that neither size nor age influenced the proportion of time spent ashore for those age classes. Weights of 3-year-old individuals were significantly correlated with their weights as pups, but weights at ages 2 and 4 were not significantly correlated with pup weight. Weight of individuals at age 3 were significantly correlated with weight at age 2. Likewise, weight at age 4 was significantly correlated with weight at age 3.

The weights of pups from all but two rookeries on St. Paul Island 25 August to 30 August, and from all rookeries on St George Island 19-20 August in 1990 indicated there are significant differences in average weights by rookery for male and female pups, males outweigh females, and the average weight of sheared pups is less than that of nonsheared pups but not consistently and not always significantly.

Preliminary results of cooperative research between Russian and U.S. scientists indicate that older females (age 12+ years) arrive earliest in the season and younger females (age 3-6 years) arrive late in the season on both St. Paul and St. George Islands, but that proportionally more young females are present on St. Paul Island. These results indicate the importance of simultaneous surveys in inter-island comparisons. Two hundred and fifty-three tags were deployed in 1991 to assess neonatal survival.

Emaciation was the most common cause of mortality accounting for 43% of deaths among 332 pups (full-term fetuses to 8 weeks of age) on St. Paul and St. George Islands from 6 July to 7 August 1991.

The observed proportion of juvenile male seals entangled in 1991 (0.21%) was less than that observed during the last several years and lower than that recorded during the commercial harvest and roundups from 1967 to 1986. This reflects a continued reduction in the incidence of entanglement in trawl webbing fragments. The frequency of occurrence of trawl webbing among the entangling debris in 1991 was about one-half that observed for 1990, which itself was about one-half the frequency observed in earlier years. In contrast, the rate of entanglement did not change. These 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 to about one-half the number expected had they not been entangled. Studies in 1991 also confirm survival rates of tagged seals from which debris is removed is significantly higher than for tagged seals which were left entangled.

On San Miguel Island, a maximum of 64 adult males maintained territories during the breeding season. A minimum of 1,011 pups were born in Adams Cove and 520 on Castle Rock based on direct counts of pups conducted 7 August and 14 August, respectively. Three hundred pups were tagged in September and October of 1991. Tag resights totaled 210 throughout 1991 representing 80 individuals. No pups tagged in 1990 were resighted.


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