Fur seal investigations, 1994

Abstract

The collection of papers contained in this report describes field research conducted in 1994 on northern fur seals (Callorhinus ursinus) by researchers and research associates of the National Marine Mammal Laboratory (NMML), Seattle, Washington. This is the 51st annual or semi-annual publication on northern fur seal research conducted by the NMML and its predecessor the Marine Mammal Biological Laboratory since 1940. Population numbers and factors that influence or indicate fluctuation in population health are monitored annually by NMML on all U.S. breeding rookeries. Northern fur seal research was conducted by NMML in 1994 under Marine Mammal Permit Number 837.

Population parameters of northern fur seals monitored on St. Paul and St. George Islands (Pribilof Islands) in 1994 included direct counts of adult males (15,729, St. Paul; 2,660 St. George), and mark-recapture estimates of the number of pups born (204,995, St. Paul; 22,244, St. George) (Antonelis et al.,this volume). Counts of dead fur seals of all ages were incorporated into population estimates. Adult male counts declined overall in 1994, after 4 years of increasing male counts due to the 1984 cessation of the commercial harvest of juvenile males on St. Paul Island. Population trends, based on pup numbers, remained stable on St. Paul Island and continued to decline on St. George Island.

In addition to population assessment, a new sampling technique was tested for pup estimates on the Pribilof Islands in 1994 (York and Towell, this volume). The new design decreases disturbance to the rookeries and may increase the accuracy of counts, and will probably be employed in future population assessments.

The mean mass, lengths, and sex ratios of pups on St. Paul and St. George Islands were examined in 1994 and compared to previous years as indicators of trends in population health (Towell et al., this volume). Male pups were larger than female pups, but other comparisons between islands and years were less conclusive. Future studies will consider the combined use of length and mass to create condition indices of pups.

Weights of known-age juvenile males taken in the 1991-94 subsistence harvests on St. Paul Island were compared (Caruso and Baker, this volume). Overall, males harvested in 1992 were heavier than those in other years, but bias in size selection during the harvest may have occurred.  Entanglement of male northern fur seals in marine debris appeared to increase throughout the late 60s and early 70s, then peak in 1976. In 1994, island-wide surveys of female northern fur seals on St. Paul Island showed an increase in entanglement rates from 1992 and 1993 (Robson et al., this volume). Similar to observations in previous years, the incidence of entanglement increased as the breeding season progressed, possibly due to the higher rate of entanglement among younger seals that arrive later in the season. All entangled animals were captured and disentangled on an opportunistic basis.  

A total of 1,482 northern fur seal pups and 3,691 non-pups were counted on Bogoslof Island in 1994 (Piatt and Goley, this volume). The number of pups is 67% higher than 1993 counts and is consistent with a trend of increasing counts since the late 1980s.

Studies conducted on the San Miguel Island northern fur seal population in 1994 included pup production and condition, and the effects of the 1992-93 El Nino on population growth (Melin et al., this volume) A total of 2,452 live pups were counted representing the highest recorded counts since the San Miguel Island colony was discovered in 1968.


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