Fur seal investigations, 2006-2007

Abstract

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 2006 was ~678,000. The approximate total stock size for the United States was 744,000 northern fur seals.

In 2006 and 2007, 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 yielded totals of 3,669 and 3,568 territorial male seals with females in 2006 and 2007, respectively. On St. George Island comparable numbers were 720 and 744. On St. Paul Island, 392 and 269 sub-adult male seals were harvested in 2006 and 2007, respectively. On St. George Island, 212 and 208 sub-adult male seals were taken in the two harvest years, respectively.

In 2006, 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 109,961 (SE = 1,520) on St. Paul Island (excluding Sea Lion Rock) and 17,072 (SE = 143) on St. George Island. Pup mortality from birth to late August was 4.5% on St. Paul Island and 4.2% on St. George Island. The number of pups on St. Paul Island was 10.5% less than in 2004 (P < 0.01), but on St. George Island there was a 1.1% increase in the same period. The annual rate of decline on the Pribilof Islands (excluding Sea Lion Rock) was 5.75% (SE = 0.34%, P < 0.01) from 1998 to 2006.

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. In 2006, both male and female pups on St. George Island were significantly longer (P < 0.01), but not heavier than those on St. Paul Island, though the difference in length may be attributable to measurement biases. The sex ratio in 2002 was significantly skewed toward males on St. Paul Island (46.2% female, P = 0.002), but not St. George Island (47.5%, P = 0.22).

Pup production on San Miguel Island, California, was estimated at 2,390 and 2,465 in 2006 and 2007, respectively. There was no evidence of a trend in recent production estimates, but pup mortality was 49.0% and 64.6% in the rookery at Adams Cove in 2006 and 2007, continuing the high levels of pup mortality begun in 2004 that were likely caused by the high incidence of hookworm disease. Pup weights suggest that pups were in poor condition in 2006, but weights conformed to the long-term average at San Miguel Island in 2007. Pup production remained below the 1997 production levels by more than 22% in Adams Cove and by more than 24% at Castle Rock in 2007.

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