Studies with Northern Fur Seals

Northern fur seals (Callorhinus ursinus) are residents of the North Pacific Ocean. Their name derives from the fact that, unlike many seals and sea lions, northern fur seals rely heavily on their thick fur coat to remain warm. They are a highly sexually dimorphic species; adult males weigh about 200 kg and females about 40 kg — a five-fold difference in body size! Pups are born in early July, and weigh approximately 5.5 kg. The pups nurse for approximately 4 months before weaning occurs in late October and early November. The pups then head off to sea to forage for themselves for up to 2 years before returning to land.

Approximately 50% of the world’s population of northern fur seals breed and give birth on the Pribilof Islands of St Paul and St George in the Bering Sea. The Pribilof population once accounted for about 80% of the world population. Historically, population levels of northern fur seals have undergone large fluctuations due to commercial hunts and management strategies. However, scientists do not understand the reasons behind most recent population declines that started in the Pribilof Islands in the mid-1970s. In 1988 northern fur seal numbers were below their optimum sustainable population level and were listed as “depleted” under the Marine Mammal Protection Act. From 1998 to 2002 the numbers of pups born declined by 5% per year on the Pribilof Islands. In contrast, the number of pups born on Bogoslof Island have increased at a rate of 59% per year (although total numbers are quite small).

Fur seals on St. Paul's Island

There is speculation that large, ecosystem-wide changes may be affecting multiple species of marine mammals and sea birds in the Bering Sea. Three hypotheses have been suggested to explain the continued decline of northern fur seals on the Pribilof Islands: climate change, competition with commercial fisheries, and increased predation. These hypotheses have been singled out from a myriad of possible explanations due to their potential power to affect multiple species over a large time and geographic frame.

To help understand the reasons behind the population changes of northern fur seals in the Bering Sea, Consortium researchers are undertaking a scientific program that studies animals in the field and in the laboratory. Results from this research will aid scientists in understanding the basis for the population changes and will help formulate science-based recovery strategies.

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