Dietary niche overlap and prey consumption for the Steller sea lion (Eumetopias jubatus) and California sea lion (Zalophus californianus) in northwest Washington during 2010–2013

Abstract

The diets of Steller (Eume-topias jubatus) and California (Zalophus californianus) sea lions in northwest Washington are poorly documented. We hypothesized that these species exploit the same prey in Washington because they are both generalist predators that utilize the same haul-out sites and are similar in behavior and body size. We analyzed 776 samples of scat from Steller sea lions and 263 samples of scat from California sea lions collected throughout each year during 2010–2013. The aim of this analysis was to charac-terize seasonal and annual diets, esti-mate biomass of prey consumed, and evaluate dietary niche overlap. Steller and California sea lions ate diverse diets that varied seasonally and annu-ally. Primary prey groups for both sea lion species were Clupeidae, Salmoni-dae, Sebastidae, Rajidae, Pleuronecti-formes, Squalidae, and Merlucciidae. We estimated that Steller sea lions ate 11,327 metric tons (t) (standard devia-tion [SD] 1600) and that California sea lions ate 9063 t (SD 4098) of prey per year during our study. We found sig-nificant dietary niche overlap between California and Steller sea lions that feed in northwest Washington.