Fur seal investigations, 1997

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 and on Bogoslof Island in the eastern Bering Sea and on San Miguel Island located off the coast of California. The Pribilof Islands (St. Paul and St. George Islands) are home to approximately 1,000,000 northern fur seals - 90% of the world population.

Population parameters monitored in 1997 on the Pribilof Islands consisted of direct counts of adult males. A total of 5,064 harem and 8,560 idle adult male seals were counted on St. Paul Island and 910 harem and 1,474 idle adult males were counted on St. George Island. From 1996 to 1997, counts of harem males decreased by 10.3% on St. Paul Island, and decreased by 27. 1% on St. George Island. During this same period, the total number of adult males on the Pribilof Islands decreased by 5.4%.

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 we're heavier and longer than female pups. The mass of female and male pups on St. George Island was greater than pups on St. Paul Island in 1997 and both male and female pups were longer on St. George Island. The proportion of females was not significantly different than 50% for both islands in 1997.

Surveys of northern fur seal entanglement in marine debris indicated that incidence of entanglement among juvenile males on St. Paul (0. 19 %) and St. George (0.23 %) Islands during 1997 was close to the rate of entanglement observed on St. Paul Island from 1988 to 1992 and in 1995-96. Packing bands comprised the largest proportion of entangling debris among juveniles on St. Paul Island followed by trawl net. Conversely, trawl net was the most frequent debris type observed on St. George Island followed by packing bands and twine.

An analysis conducted to investigate the feasibility of using ratio estimates based on the June bull counts to estimate missing July bull and pup production numbers concluded that estimating the number of breeding males in July from the count of breeding males in June is not likely to produce a reliable estimate. The relationship between the June and July harem male counts is not stable enough to provide an accurate estimate of harem adult males in July. The use of July harem male counts for estimating pup production results in a less biased estimate with a smaller coefficient of variation (CV) than the use of June harem male counts.

The number of pups born on Bogoslof Island in 1997 was estimated using the "shearing sampling" method currently used in pup production estimates on the Pribilof Islands, Alaska. The estimate for the total number of pups alive at the time of marking was 5,055 (SE = 32.6) and the mortality rate for Bogoslof Island was 0.8%. The 1997 estimate represents a 300.6% increase in the number of pups over the 1995 count. The number of non-pup northern fur seals counted on Bogoslof Island in 1997 was 13,751 (SE = 541.5). This count represents a 272.6% increase over the last count of 3,691 in 1994. The total Bogoslof Island population size is estimated at 22,803 northern fur seals. Foraging studies indicated that female northern fur seals with pups on Bogoslof Island tended to feed close to the island and that foraging trips were shorter in distance and duration than those of females on the Pribilof Islands. Information on tufted puffin (Fratercula cirrhata) food habits-was collected and prey was identified as either sablefish (Anoplopoma fimbria) or rock greenling (Hexagrammos lagocephalus).

Population monitoring studies of northern fur seals on San Miguel Island were based on direct counts of adult males and live and dead pups. The number of territorial bulls increased from 162 bulls in 1996 to 250 in 1997. The total observed pup production during 1997 was 2,133 in Adams Cove and 991 on Castle Rock. Although the number of pups born reached an historical high in 1997, the observed high pup mortality indicates the 1997 El Niño will cause almost total mortality of the 1997 cohort. Additionally, samples of dead fur seal pups in July and October were examined for evidence of hookworm infestation. All dead pups examined during July had hookworm infestations in the intestines; however, by 1 October the fecal and intestine samples were free of hookworm eggs and adults.


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