SEASONAL AND SPATIAL DIFFERENCES IN DIET IN THE WESTERN STOCK OF STELLER SEA LIONS (EUMETOPIAS JUBATUS)

Abstract

We identified prey remains from 3,762 scats (feces) of Steller sea lions (Eumetopias jubatus). Scats were collected from 1990‐1998 on island sites across most of the range of the United States western stock of the species. Walleye pollock (Theragra chalcogramma ) and Atka mackerel (Pleurogrammus monopterygius ) were the 2 most common species of prey, followed by salmonids (Oncorhynchus) and Pacific cod (Gadus macrocephalus ). An additional 16 species of fish and unidentified cephalopods were considered primary in the diet, either because they occurred in .5% of scats collected across the range in winter and summer or because they consistently occurred among the top 3 prey items in particular islands or island groups. Capelin (Mallotus villosus) occurred at very low frequencies despite their predominance in the diet of Steller sea lions before the 1980s. Regions of diet similarity suggest area-specific foraging strategies, with strong seasonal patterns in consumption of most species of prey. Patterns in prey consumption and characteristics of prey indicate that Steller sea lions target prey that are densely schooled in spawning or migratory aggregations at the continental shelf or along oceanographic boundary zones. We suggest that regional diet patterns among the western stock reflect regional foraging strategies of females learned at islands near the natal rookery site.