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October 27, 2003
Seattle, Washington, USA

A Critical Review of the "Regime Shift/Junk Food" Hypothesis for the Steller Sea Lion Decline

Lowell W. Fritz¹, Sarah Hinckley^2
1National Marine Mammal Laboratory, Alaska Fisheries Science Center, National Marine Fisheries Service, 7600 Sand Point Way NE, Seattle , WA, 98115, USA, Phone 206-526-4246, lowell.fritz@noaa.gov
²Resource Assessment and Conservation Engineering, Alaska Fisheries Science Center, National Marine Fisheries Service, 7600 Sand Point Way NE, Seattle, WA, 98115, USA, Phone 206-526-4109, sarah.hinckley@noaa.gov

It has been hypothesized that periodic changes in the climate of the North Pacific caused the decline of the Steller sea lion population observed in the 1980's by causing large increases in consumption of gadid (e.g., pollock) fishes with low nutritional value, and decreases in consumption of osmerid and clupeid fishes (e.g., capelin and herring) with high nutritional value. According to this regime shift-junk food hypothesis, changes in food habits of Steller sea lions stemmed from climate-induced restructuring of fish communities associated with a regime shift in 1976-77. The consequences for sea lions associated with greater and lesser consumption of gadids and forage fish, respectively, are thought to have included decreased reproductive success or survival due to nutritional stress.

We examine this hypothesis through a critical re-analysis of fishery and survey data, gadid and clupeid recruitment and biomass time series, Steller sea lion and other otariid food habits information in the North Pacific Ocean and throughout the world, and information related to the nutritional worth of gadids and other prey species, including proximate analyses of prey composition as it varies seasonally and spatially. We conclude that 1) recruitment to pollock populations was unrelated to or decreased following the 1976-77 regime shift; 2) herring populations increased in the 1980's following the regime shift; 3) it is unlikely that herring and other forage fish have ever dominated the fish community in terms of total biomass; 4) gadids have consistently been prominent parts of otariid diets in the North Pacific and other parts of the world; 5) food habits data do not support the conclusion that sea lion diet composition changed radically after the regime shift; 6) the energetic value of any particular prey item depends on the season in which it is eaten and the costs of obtaining it (at times gadids have higher energetic density than osmerids or clupeids); and 7) a diet with too high a proportion of osmerids or clupeids is known to be detrimental to many species of marine mammals and fish. While changes in the environment of Steller sea lions have certainly occurred over the last 30 years and could have contributed to the creation of sub-optimal conditions, we conclude that it is unlikely that they or the high proportion of gadids in the diet are the primary causes of the recent and ongoing decline in the western Steller sea lion population.

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