Fur seal investigations, 2008-2009

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) on the Pribilof Islands, Bogoslof Island in the eastern Bering Sea, and on San Miguel Island off the coast of California. This report summarizes these monitoring efforts.

In 2008 and 2009, 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 4,119 and 4,121 territorial male seals with females in 2008 and 2009, respectively. On St. George Island the respective totals were 805 and 873. On St. Paul Island, 322 and 341 subadult male seals were harvested in 2008 and 2009, respectively. On St. George Island the respective totals were 170 and 113 subadult male seals.  The estimate for the total number of pups born in 2008 was 102,674 (SE = 1,084) on St. Paul Island (not including 6,741 pups born on Sea Lion Rock) and 18,160 (SE = 288) on St. George Island. Pup mortality from birth to late August was 5.3% on St. Paul Island and 5.4% on St. George Island. The number of pups born on St. Paul Island in 2008 was 6.6% less than in 2006 (P < 0.01), while at St. George the number of pups increased 1.2% from 2004 to 2006 and 6.4% from 2006 to 2008. Both male and female pups on St. George Island were significantly heavier than those on St. Paul Island (P < 0.01), but only males were significantly longer (P < 0.01).

Index counts of territorial bulls on San Miguel Island, California, in 2008 and 2009 were 154 and 116, respectively. Estimates of pup production in recent years have been revised to incorporate more realistic mortality estimates. In 2008 and 2009, total numbers of pups born at San Miguel Island and nearby Castle Rock were 2,914 (SE = 58) and 3,117 (SE = 20), respectively. Pup mortality in recent years has been high, and remained so in 2008 (38%) and 2009 (82%), and appears to be driven by a high incidence of hookworm. Pup weights in both 2008 and 2009 were lower than the long-term average seen at San Miguel Island.  Pup production and bull counts remain below the peaks recorded in 1997, just prior to a strong El Niño event, and evidence of recovery since that time has been sporadic.            

The estimated stock size for all fur seals breeding in the United States was ~687,000, with the Pribilof Islands population accounting for ~608,000.  

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