Steller Sea Lion (Eumetopias jubatus) Decline in Alaska: What are the Main Causes?

Defense Date


Document Type


Degree Name

M.S. Marine Biology

Second Degree Name

M.S. Coastal Zone Management

First Advisor

Edward Keith

Second Advisor

Curtis Burney


Steller sea lions (Eumetopias jubatus) are the largest member of the family Otariidae. They range from the central coast of California northward to the Gulf of Alaska, and westward into Russia and Japan, with the majority of breeding occurring in the central Gulf of Alaska and the western Aleutian Islands. There are two geographically defined stocks of Steller sea lions (SSL) in the Gulf of Alaska: the western stock and the eastern stock, with 144°W longitude acting as the dividing line. Both stocks were listed as threatened in 1990, and the western stock, which began declining in the 1970s, with counts at about 282,000 individuals, and is still declining today, with population estimates of about 40,000, was reclassified as endangered in 1997. The reasons for the decline have been the focus of much study, and it is widely agreed upon that there is no one cause, but a combination of factors, such as increased predation pressure from killer whales (Orcinus orca) and Pacific sleeper sharks (Somniosus pacificus), environmental changes and impacts, such as contaminant exposure, infectious disease, conflict with commercial fisheries, and oceanic regime shifts, and nutritional stress, which is the leading theory for the decline and subsequent lack of recovery. There are many potential management plans that are being studied and tested for effectiveness. It is believed that these direct management plans, coupled with fishery management plans, have helped to slow the decline of SSL populations. Since there was little work done during the period of initial decline, there is a gap in the available data as to what caused the initial drop. The one thing all researchers can agree on is that more study needs to be done.

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