2012 Northern Fur Seal Pup Production and Adult Male Counts on the Pribilof Islands, Alaska
Pup production and adult male northern fur seal count
Numbers of northern fur seal, Callorhinus ursinus, pups were estimated using a mark-recapture method, shearsampling, on the Pribilof Islands during August 2012. We estimated 96,828 (SE = 1,260, Table 1) pups were born on St. Paul Island and 16,184 (SE = 155, Table 1) pups were born on St. George Island. To minimize disturbance, dead pup counts were conducted at 4 sample rookeries on St. Paul Island and 3 sample rookeries on St. George Island. Since 2006 this dead pup counting procedure resulted in a ratio estimation protocol for the calculation of the estimate of the total number of pups born. The observed pup mortality rates were 3.7% on St. Paul Island and 3.1% on St. George Island. Pup production was not estimated on Sea Lion Rock, a small island approximately 500 m from St. Paul Island in 2012. The 2012 pup production estimate for St. Paul Island is 2.5% greater than the estimate in 2010 (Figure 1). The 2012 pup production estimate for St. George Island is 9.9% less than the estimate in 2010. Overall pup production for the Pribilof Islands increased approximately 0.5% from 2010 to 2012, marking the first year since 1998 that estimated pup production did not decline (overall or on St. Paul Island). During this time, however, pup production on St. Paul Island declined 46.0%, or at an annual rate of 4.8% (SE = 0.49), while pup production on the Pribilof Islands (St. Paul and St. George Islands combined) declined 43.8%, or at an annual rate of 4.3% (SE = 0.46).
Adult male northern fur seals on St. Paul and St. George Islands (Table 2) were counted over the period July 9 to 15, 2012. Counts of territorial males with females ("harem" males) on St. George Island increased by 1.2% compared to 2011. Idle males on St. George Island decreased in comparison to 2011 by 5.1%. On St. Paul Island the "harem" males decreased by 12.9% and the idle males decreased by 28.8% (Figure 2). Overall, the total number of adult males counted on the Pribilof Islands in 2012 decreased by 18.5% from 2011 to 8,900. The total number of “harem” males decreased by 10.3% on the Pribilof Islands.