Fur seal investigations, 1996

Abstract

Researchers from the National Marine Mammal Laboratory (NMML) conduct field investigations on the population health of northern fur seals (Callorhinus ursinus) annually on the Pribilof Islands in the eastern Bering Sea and San Miguel Island located off the coast of California. The Pribilof Islands (St. Paul and St. George Islands) are home to approximately 800,000 northern fur seals -90% of the world population.

Population parameters monitored in 1996 on the Pribilof Islands included direct counts of adult males (14,882 on St. Paul Island; 2,038 on St. George Island), and mark-recapture estimates of the number of pups born (170,125 on St. Paul; 27,385 on St. George). Counts of dead fur seals of all ages were incorporated into population estimates. The estimated number of pups born on St. Paul Island in 1996 was not significantly different from our 1990, 1992, or 1994 estimates. The 1996 estimate of pups born on St. George Island is significantly higher than 1992 (25,160) or 1994 (22,244) estimates. The 1996 estimate of number of pups born on St. George Island is the highest since 1985 when over 28,000 pups were born.

The patterns of fur seal entanglement in marine debris has been monitored annually on the Pribilofs. Similar to previous years, entangling debris in 1996 consisted primarily of pieces of trawl net, plastic packing bands, and loops of synthetic or natural twine. For the second year in a row, entanglement in packing bands was more prevalent on St. Paul Island (44.6%) than St. George Island (26.7%) for all age groups combined. No significant difference in overall entanglement was observed between the islands; however, juvenile male seals had a significantly higher rate of entanglement than adult male seals on both islands. The rate of entanglement for juvenile males was 0.23% on St. Paul Island and 0.21% on St. George Island relative to 0.14% and 0.06% for adult seals on St. Paul and St. George Islands, respectively. Female fur seals had very low rates of entanglement relative to males. Female entanglement on St. Paul Island in 1996 (0.013%) was comparable to rates observed in 1992, 1993, and 1995.

Population monitoring studies of northern fur seals on San Miguel Island were based on direct counts of live and dead pups. A total of 2,009 pups were counted in Adams Cove in 1996, representing a 20.6% increase from 1995 counts. The number of territorial bulls increased from 104 in 1995 to 162 in 1996, indicating that new males continue to be recruited into the breeding population.


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