Food limitation of sea lion pups and the decline of forage off central and southern California

California sea lions increased from approximately 50,000 to 340,000 animals in the last 40 years, and their pups are starving and stranding on beaches in southern California, raising questions about the adequacy of their food supply. We investigated whether the declining sea lion pup weight at San Miguel rookery was associated with changes in abundance and quality of sardine, anchovy, rockfish and market squid forage. In the last decade off central California, where breeding female sea lions from San Miguel rookery feed, sardine and anchovy greatly decreased in biomass, while market squid and rockfish abundance increased. Pup weights fell as forage food quality declined associated with changes in the relative abundances of forage species. We were able to predict pup weights using a model based on forage variables. A shift from high to poor quality forage for breeding females results in food limitation of the pups, ultimately flooding animal rescue centers with starving sea lion pups. Our study is unusual in using a long-term, fishery-independent data set to directly address an important consequence of forage decline on the productivity of a large marine predator. Previous studies relied upon proxies for forage such as temperature and ENSO indices, while we were able to predict sea lion pup weights using a direct estimate of forage abundance derived from research surveys. Whether forage declines are environmentally driven, a combination of environmental drivers and fishing removals, or are due to density-dependent interactions between forage and sea lions is uncertain. However, declining forage abundance and quality was coherent over a large area (32.5-38 oN) for a decade, suggesting that trends in forage are environmentally driven.JRC.D.2-Water and Marine Resource