Fur seal investigations, 1995

Abstract

Counts of adult male northern fur seals (Callorhinus ursinus) are obtained each year on St. Paul Island and semi-annually on St. George Island as a factor of population monitoring. In 1995, counts were obtained from 11 to 16 July. A total of 5,154 harem and 8,459 idle adult male seals were counted on St. Paul Island. On St. George Island a total of 1,242 harem and 1,054 idle adult males were counted. Overall, these numbers represent a decrease of 13.2% in adult male seals on the Pribilof Islands since 1994. A similar pattern occurred between 1993 and 1994.

Northern fur seal entanglement in marine debris has been studied cooperatively since the early 1980s by the National Marine Mammal Laboratory (NMML), the National Research Institute of Far Seas Fisheries of Japan, and the Aleut community of St. Paul Island. Studies conducted since 1988 indicate a decline in the rate of entanglement. There is variation in the degree of entanglement by debris type, seal age, and island. For instance, the 1995 rate of entanglement was significantly different among juvenile males on St. Paul (0.22%) and St. George Islands (0.39%), however there was no significant difference in adult male entanglement rates between the two islands. Differences are probably due to a higher incidence of entanglement in trawl net debris among juveniles on St. George Island. The overall rate of entanglement for all males was calculated as 0.18% and 0.22% for St. Paul and St. George Islands, respectively (Robson et al., this volume).

Trends in the mass and length of fur seal pups serve as indicators of population health and have been monitored semi-annually since 1989. Consistent with earlier years, studies in 1995 demonstrated that male pups are heavier and longer on average than female pups, and pups tend to be both heavier and longer on St. George Island relative to St. Paul Island. The mean mass of both male and female pups was significantly less in 1995 than in previous years on both Islands (Towell et al., this volume).

The shearing-sampling method has been used to estimate the number of pups born on the Pribilof Islands since 1961. The technique is evaluated regularly for accuracy as new techniques or interpretations become available. Some aspects of potential biases inherent in this method; such as observer variability, subsampling, and declining probability of relighting with time; are discussed (York and Towell, p. 65-75; York and Towell, p. 77-98).

Diet records from tagged seals collected during 1960-74 were reanalyzed to determine whether prey composition differs between fur seals from St. George and St. Paul Islands. Findings support earlier research based on scat remains, that seals from St. Paul Island eat a greater percentage of fish and a lower percentage of squid in their diet than St. George Island seals, but sample sizes were too small for a rigorous statistical treatment (Perez, this volume).

Northern fur seal pups were counted (n = 1,272) on Bogoslof Island on 25 September. For the first time in the 12 years (1980-95) since this new colony established itself, the numbers of pups counted decreased. The 1995 counts were down 13.5% from 1994 (Towell and Strick, this volume).

Studies on San Miguel Island indicate that population growth slowed in 1995. The observed pup production was 2,509 which represents a 4.7% decline from 1994. Continued monitoring will determine whether the decrease in numbers is a temporary fluctuation or a long-term trend in population growth on San Miguel Island (Melin and DeLong, this volume).

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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