Has Steller Sea Lion Predation Impacted Survival of Fraser River Sockeye Salmon?

Abstract

The commercially, recreationally, and culturally important Fraser River Sockeye Salmon Oncorhynchus nerka has experienced a productivity decline over the past 3 decades, which—along with greater temporal variation in annual abundance (i.e., cyclic dominance)—may at least partly be due to Steller sea lion Eumetopias jubatus (SSL) predation on returning adult salmon. This assumes that SSLs residing around northern Vancouver Island (British Columbia, Canada) target Sockeye Salmon for just a few weeks during the peak of their run. It is a reasonable enough assumption to warrant immediate priority for field research on SSL behavior and diets during the migration period. We evaluated the plausibility of the assumption with a variety of approaches ranging from simple estimates of maximum SSL consumption to partitioning of observed marine mortality rates and analysis of SSL foraging behavior to show that SSLs could have caused the decline in productivity and abundance of Fraser River Sockeye Salmon.