The decline and fall of seals and sea lions in Alaska: Is the carrying capacity of the
North Pacific Ocean decreasing?

Andrew W. Trites, Ph.D.
Marine Mammal Research Unit
University of British Columbia
Vancouver, B. C.

Wednesday, February 25, 1998
3:30 p.m.—Pacific Forum

Significant declines have occurred in the numbers of harbor seals, northern fur seals, and Steller sea lions in Alaska. These declines have been so abrupt that the Steller sea lions were declared an endangered species, while northern fur seals were classified as depleted.

My talk will focus primarily on northern fur seals that breed on the Pribilof Islands (in the Bering Sea) and migrate as far south as California. I will also briefly present some of the recent insights we have gained from an ecosystem model of the Bering Sea, as well as discuss some aspects of our ongoing captive and field-research program into Steller sea lions.

A leading hypothesis in the decline of the Alaskan pinnipeds is that they are nutritionally stressed. If this is true, there should be noticeable changes in their physical condition.

I examined the standard body lengths of subadult males harvested on St. Paul Island that have been measured sporadically since 1911. I also collected three years of measurements (1995-1997) from subsistence-killed animals to test whether animals are physically smaller today than in the past.

The relationship I found between body length and population size suggests that the current fur seal population may be the maximum sustainable in today's Bering Sea/Gulf of Alaska ecosystem, which is half of what it was 50 years ago. Concurrent declines in numbers of harbour seals and Steller sea lions in these regions, as well as reduced body growth of Steller sea lions, all suggest that pinniped carrying capacity is significantly lower now than it was in the past.

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