Fur seal investigations, 2002-2003

Abstract

Researchers from the National Marine Mammal Laboratory conduct field investigations on the population status of northern fur seals (Callorhinus ursinus) annually on the Pribilof Islands, Bogoslof Island in the eastern Bering Sea, and on San Miguel Island off the coast of California. The estimate of the total stock for the Pribilof Islands population in 2002 is about 848,000 fur seals. The approximate total stock size for the United States, which includes the Pribilof, Bogoslof, and San Miguel Island populations, is about 880,000 fur seals.

In 2002 and 2003, population parameters monitored on the Pribilof Islands included the size of the subsistence harvest and the number of adult male fur seals. Counts on St. Paul Island in 2002 yielded totals of 3,669 territorial male seals with females and 7,887 idle adult male seals; in 2003, 3,652 and 7,572 were counted in the those categories, respectively. On St. George Island in 2002, a total of 899 territorial males with females and 1,265 idle adult males were counted; 716 and 1,158 were counted in those categories in 2003. On St. Paul Island, 647 and 522 sub-adult male seals were harvested in 2002 and 2003, respectively. On St. George Island, 202 and 132 sub-adult male seals were taken in the two harvest years, respectively.

In 2002, the number of pups born and the mortality rates of fur seals were assessed on St. Paul and St. George Islands. The estimate for the total number of pups born in 2002 was 145,716 (SE = 1,629) on St. Paul Island (excluding Sea Lion Rock) and 17,593 (SE = 527) on St. George Island. Pup mortality to late August was 3.3% on St. Paul Island and 3.0% on St. George Island. During 1998-2002, the annual rate of decline in numbers of pups born was 5.12% (SE = .26, P = 0.03) on St. Paul Island and 5.44% (SE = 0.72, P = 0.08) on St. George Island. The rate of decline at both islands combined was 5.2% (SE = 0.19, P = 0.02). Pup production in 2002 was the lowest since 1919 on St. Paul and the lowest since 1916 on St. George, when both populations were rapidly increasing.

The mass and length of fur seal pups on the Pribilof Islands are used as indicators of population health and have been monitored semi-annually since 1989. Both male and female pups on St. George Island were significantly (P < 0.05) longer and heavier than those on St. Paul Island. The sex ratio in 2002 was significantly (P < 0.05) skewed toward males on St. Paul Island (58:42) but not on St. George Island (52:48).

After 3 years of increasing pup production on San Miguel Island, California, following the 1998 decline, production declined slightly in 2002. This may have resulted from the 2002-2003 El Niño or from poor recruitment from the 1997 cohort. The combination of a decrease in pup births and a significant decrease in pup growth in 2002 suggests that the mild El Niño conditions had a moderate impact on northern fur seals at San Miguel Island. In 2003, pup production increased at Adams Cove rookery (Castle Rock rookery was not counted) and pup weights returned to the long-term average, indicating that foraging conditions for adults in the population had improved. Pup production remained below the 1997 production levels by more than 39% in Adams Cove in 2003, and by more than 25% at Castle Rock in 2002.


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