DIET DIVERSITY OF STELLER SEA LIONS (EUMETOPIAS-JUBATUS) AND THEIR POPULATION DECLINE IN ALASKA - A POTENTIAL RELATIONSHIP

We examined the diet of Steller sea lions (Eumetopias jubatus) during June-August 1990-1993 from six areas in the Aleutian Islands and Gulf of Alaska and related these diets to sea lion population changes that occurred during the period. Seven general prey categories were identif ied, but either walleye pollock (Theragra chalcogramma) or Atka macker el (Pleurogrammus monopterygius) dominated in every area. The diversit y of prey consumed varied among sites. Only the eastern Aleutian Islan ds area had all seven categories in the diet, and there, walleye pollo ck and Atka mackerel each made up around 30% of the diet. The remainde r was composed mostly of small schooling fish (e.g., Pacific herring ( Clupea pallasi) and salmon (Oncorhynchus spp.)). The diet in the Gulf of Alaska included mostly walleye pollock whereas the central and west ern Aleutian diet was composed mostly of Atka mackerel. Populations in the six areas decreased up to 49% during 1990-1994. A strong positive correlation (r = 0.949, P = 0.004) was found between diet diversity a nd the amount of decline in an area: as diet diversity decreased, popu lations decreased. This suggests that sea lions need a variety of prey available, perhaps to buffer significant changes in abundance of any single prey.