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Fur seal investigations, 2004-2005


Researchers from the Alaska Fisheries Science Center’s 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 2004 was ~756,000. The approximate total stock size for the United States was 823,000 northern fur seals.

In 2004 and 2005, 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 2004 yielded totals of 1,403 territorial male seals with females and 6,928 idle adult male seals; in 2005, 1,466 and 7,860 were counted in those categories, respectively. On St. George Island in 2004, a total of 321 territorial males with females and 1,344 idle adult males were counted; 273 and 1,266 were counted in those categories in 2005. On St. Paul Island, 493 and 479 sub-adult male seals were harvested in 2004 and 2005, respectively. On St. George Island, 123 and 139 sub-adult male seals were taken in the two harvest years, respectively.

In 2004, 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 was 122,825 (SE = 1290) on St. Paul Island (excluding Sea Lion Rock) and 16,878 (SE = 271) on St. George Island. Pup mortality from birth to late August was 3.3% on St. Paul Island and 2.5% on St. George Island. The annual rate of decline on St. Paul Island was 6.00% (SE = 0.70%, P = 0.01) and 4.56% (SE = 0.43%, P = 0.01) on St. George Island from 1998 to 2004. The rate of decline on the Pribilof Islands (excluding Sea Lion Rock) was 5.83% (SE = 0.53%, P = 0.01) since 1998.

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 < 0.01) longer and heavier than those on St. Paul Island. The sex ratio in 2002 was significantly (P < 0.016) skewed toward males on both St. Paul Island (54.2:45.8) and St. George Island (54.9:45.1).

Pup production on San Miguel Island, California, was estimated at 1,238 and 1,628 in 2004 and 2005, respectively. There was no evidence of a trend in recent estimates, but early season pup mortality in 2004 (34.5%) and 2005 (30.4%) was higher than any of the previous 7 years (4.4-27.4%). Pup weights conformed to the long-term average at San Miguel Island. Pup production remained below the 1997 production levels by more than 23% in Adams Cove and by more than 19% at Castle Rock in 2005.

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