A critical review of the regime shift-Junk Food-nutritional stress hypothesis for the decline of the western stock of Steller sea lion

Steller sea lions (Eumetopias jubatus) in the central and western Gulf of Alaska, Aleutian Islands, and Bering Sea have declined by 80% in the last 30 yr. One hypothesis for the decline in this western Steller sea lion population is that a climate regime shift in 1976-1977 changed the species composition of the fish community and reduced the nutritional quality (energy density) of the sea lion prey field. This in turn led to nutritional stress and reduced individual fitness, survival, and reproduction of sea lions. Implications of this regime shift - junk food hypothesis are that (1) the recruitment and abundance of supposed high quality species (e.g., Pacific herring, Clupea pallasi) decreased while those of supposed low quality (e.g., species in the family Gadidae) increased following the regime shift, (2) Steller sea lion diets shifted in response to this change in fish community structure, and (3) a diet composed principally of gadids (e.g., walleye pollock, Theragra chalcogramma) is detrimental to sea lion fitness and survival. We examine data relating to each of these implications and find little support for the hypothesis that increases in the availability and consumption of gadids following the regime shift are primarily responsible for the decline of the western population of Steller sea lion.