Fur seal investigations, 2000-2001

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 size of the Pribilof stock of northern fur seals in 2000 was estimated to be 919,000 individuals and the total population of fur seals in U. S. rookeries was estimated to be 947,000 individuals.

In 2000 and 2001, 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 2000 yielded totals of 3,646 harem and 6,998 idle adult male seals; counts in 2001 were 3,388 and 7,174, respectively. On St. George Island in 2000, a total of 871 harem and 1,300 idle adult male seals were counted. In 2001, 842 harem males and 1,596 idle males were counted on St. George Island. There were decreases in the counts of territorial males with females on St. Paul Island both between 1999 and 2000 (-3.3%) and between 2000 and 2001 (-7.0%). The count of these males on St. George Island also decreased between 1999 and 2000 (-17.2 %) and between 2000 and 2001 (3.2%). The Pribilof Islands total for these males decreased by about 9.4% between 1999 and 2000 and by 6.3% between 2000 and 2001. On St. Paul Island, 754 and 597 sub-adult male seals were harvested in 2000 and 2001, respectively. On St. George Island, 121 and 184 sub-adult male seals were taken in the two harvest years, respectively.

In 2000, the number of pups born and the mortality rates of fur seals on both islands were assessed. The estimate for the total number of pups born in 2000 was 158,736 (SE = 17,284) on St. Paul Island and 20,176 (SE = 271) on St. George Island. On St. Paul Island, pup numbers declined 11.4% since the census in 1998. This decline was not statistically significant as the standard error (SE) for the 2000 estimate was quite high. The point estimate in 2000 is the smallest recorded since 1921. On St. George Island, pup numbers declined 8.7% from the census in 1998; the decline was significant (P < 0.001). The estimate for 2000 is the smallest since 1917. When data are combined for the two islands, there is evidence of a small, but statistically significant decline since 1990 of 1.8% (SE = 0.45%, P < 0.01).

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. As in previous years, the size of pups varied by sex: male pups were heavier and longer than female pups. In 2000, both male and female pups on St. George Island were longer than those on St. Paul Island. The sex ratio in 2000 was skewed toward males on St. George Island (54:46) but not on St. Paul Island (50:50). In 2001, the sex ratio was skewed toward males on St. Paul Island (53:47) but not on St. George Island (50:50).

In 2000 and 2001, the northern fur seal population at San Miguel Island continued its recovery from a decline caused by the 1998 El Niño. Pup production increased at both colonies over the 2 years, however in 2001, production at both colonies remained below the 1997 production levels by more than 24%. Other signs of population recovery included good condition of 4-month-old pups and reduced late-season pup mortality in 2000 and 2001.


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