Examining the evidence for killer whale predation on Steller sea lions in British Columbia and Alaska

The discovery of e ipper tags from 14 Steller sea lions (Eumetopias jubatus ) in the stomach of a dead killer whale ( Orcinus orca ) in 1992 focused attention on the possible role of killer whale predation in the decline of Steller sea lions in western Alaska. In this study, mariners in British Columbia and Alaska were surveyed to determine the frequency and outcome of observed attacks on sea lions, the age classes of sea lions taken, and the areas where predatory attacks occurred. The 126 survey respondents described 492 killer whale/sea lion interactions, of which at least 32 were fatal attacks on the sea lion. The greatest rate of observed predation occurred in the Aleutian Islands. The stomach contents of dead and stranded whales also were examined. Stomachs that were not empty contained only e sh or marine mammal remains, but not both. This supports earlier evidence of dietary segregation between e sh-eating resident and marine mammal-eating transient killer whales in Alaska. Steller sea lion remains were found in two of 12 killer whale stomachs examined from Alaska between 1990 and 2001. Stomach contents from two oVshore killer whales provided the e rst direct evidence that this third form of killer whale feeds on esh.