Bottom-Up Forcing and the Decline of Steller Sea Lions in

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Alaska: Assessing the Ocean Climate Hypothesis

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stron

6611

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University of British Columbia

2005

rok

ABSTRACT

Declines of Steller sea lion (Eumetopias jubatus) populations in the Aleutian Islands and Gulf of

Alaska could be a consequence of physical oceanographic changes associated with the 1976-77

climate regime shift. Changes in ocean climate are hypothesized to have affected the quantity,

quality and accessibility of prey, which in turn may have affected the rates of birth and death of

sea lions. Recent studies of the spatial and temporal variations in the ocean climate system of the

North Pacific support this hypothesis. Ocean climate changes appear to have created adaptive

opportunities for various species that are preyed upon by Steller sea lions at mid-trophic levels. The

eastwest asymmetry of the oceanic response to climate forcing after 1976-77 is consistent with

both the temporal aspect (populations decreased after the late 1970’s) and the spatial aspect of the

decline (western, but not eastern, sea lion populations decreased). These broad-scale climate

variations appear to be modulated by regionally sensit ive biogeographic structures along the

Aleutian Islands and Gulf of Alaska, which include a transition point from coastal to open-ocean

conditions at Samalga Pass westward along the Aleutian Islands. These transition points delineate

distinct clusterings of different combinations of prey species, which are in turn correlated with

differential population sizes and trajectories of Steller sea lions. Archaeological records spanning

4000 years further indicate that sea lion populations have experienced major shifts in abundance in

the past. Shifts in ocean climate are the most parsimonious underlying explanation for the broad

suite of ecosystem changes that have been observed in the North Pacific Ocean in recent decades.