Fur Seal Investigations, 1999

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 and on Bogoslof Island in the eastern Bering Sea and on San Miguel Island located off the coast of California. The size of the Pribilof stock of northern fur seals in 1998 was estimated to be 973,000 individuals and the total population of fur seals in U. S. rookeries was estimated to be 1,004,000 individuals.

Population parameters monitored in 1999 on the Pribilof Islands included the size of the subsistence harvest and the number of adult male fur seals on both islands. A total of 3,801 harem and 7,589 idle adult male seals were counted on St. Paul Island and 1,052 harem and 916 idle adult males were counted on St. George Island. The general decrease in the count of adult males on the Pribilof Islands noted in recent years continued in 1999. The count of territorial males with females (Class 3) on St. Paul Island has declined at a rate of about 3% per year for the last 6 years. The decline between 1998 and 1999 exceeds previous declines, being about 20% for Class 3 males on St. Paul and 17% for Class 3 males on St. George.

A total of 997 sub-adult male seals were killed in the 1999 subsistence harvest on St. Paul Island between 2 July and 7 August. On St. George Island, 193 sub-adult male seals were taken in the subsistence harvest, between 6 July and 8 August.

Trends in the mass and length of fur seal pups are used as indicators of population health and have been monitored semi annually since 1989. Consistent with earlier evaluations of pup mass data, the strongest pattern was that the size of pups varied by sex: male pups were heavier and longer than female pups. The proportion of females (42.7%) was significantly different (P < 0.001) than 50% (46.0% on St. Paul Island in 1999).

The northern fur seal population at San Miguel Island began its recovery in 1999 from the 1998 decline. Although pup production increased at both San Miguel colonies, they remained below the pre 1997-98 El Nino event production levels by more than 60%. The number of territorial males with females in Adams Cove increased from 74 bulls in 1998 to 106 in 1999. The total observed pup production during 1998 was 424 in Adams Cove and 194 on Castle Rock, a decline of 79.6% and 79.5%, respectively, from 1997 to 1998. The 79.6% decline in the number of northern fur seal pup births at San Miguel Island in 1998 represents the greatest decline in population growth since studies began in 1968.


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