Fur seal investigations, 1998

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 1998 on the Pribilof Islands included the size of the subsistence harvest, the number of adult males, the number of pups born and the mortality rates of fur seals on both islands. A total of 4,762 harem and 8,396 idle adult male seals were counted on St. Paul Island and 1,116 harem and 1,084 idle adult males were counted on St. George Island. From 1997 to 1998, counts of harem males decreased by 6.0% on St. Paul Island, and increased by 22.6% on St. George Island. During this same period, the total number of adult males on the Pribilof Islands decreased by 1.6%.

The estimate for the total number of pups born in 1998 was 179,149 (SE = 6,193) on St. Paul Island and 21,638 (SE = 222) on St. George Island. The mortality rate of pups in 1998 was estimated to be 2.8% on St. Paul Island and 2.1% on St. George Island. The estimated number of pups born in 1998 was not significantly different (P = 0.82) from 1996, but was significantly less than in 1994 (P < 0.01). The 1998 estimate of pups born on St. George Island was significantly less (P < 0.01) than the number of pups born in 1996, but the estimate was not significantly different (P = 0.22) from the estimate of the number of pups born in 1994.

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 mass of male pups on St. George Island was significantly greater than male pups on St. Paul Island in 1998. Both male and female pups were significantly longer (P < 0.001) on St. Paul Island than on St. George Island in 1998. The proportion of females was significantly different (P < 0.05) than 50%, (46.0% on St. Paul Island and 46.2% on St. George Island, Table 21) for both islands in 1998.

The 1997-98 El Niño had a significant impact on several population parameters of northern fur seals on San Miguel Island. The number of territorial males with females in Adams Cove decreased from 142 bulls in 1996 to 74 in 1997. 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.

The Tribal Government of St. Paul Island continued to monitor the rate of entanglement of northern fur seal males in marine debris in conjunction with the subsistence harvest. The rate of entanglement for male seals was 0.2% (13/6,603) on St. Paul Island. Female entanglement surveys were also conducted on St. Paul Island by researchers from the National Research Institute of Far Seas Fisheries, Japan. The rate of entanglement among females was calculated at 0.0105% for entangled females, 0.0281% for scarred females and 0.0386% for the two categories combined.

Researchers observed large numbers of northern fur seals with fungal infections of the pelage during the 1998 breeding season on both the Pribilof Islands and at San Miguel Island. The average incidence of fungal infections was 2.1% (332/16,116) among females on St. Paul Island rookeries and may have reached as high as 7.0% depending on the date and location of the survey. A lower incidence of fungal infections, 0.5% (17/3,292) was observed among adult male fur seals at seven rookeries on St. Paul Island . On San Miguel Island the incidence of fungal infections was dramatically higher than that observed on the Pribilof Islands. Sixty-five percent of the tagged females (n = 20) and 60% of the tagged males (n = 20) had fungal infections.


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