Forage Abundance, Quality, and Distribution Drive Food Limitation in an Expanding California Sea Lion Population

The California sea lion population increased after the Marine Mammal Protection Act (1972), but carrying capacity is currently unknown. An indicator of population status suggests that the sea lion population passed its maximum productivity level in the 1990s. In the last decade, key components of sea lion diet have either almost disappeared (anchovy), or decreased in biomass (sardine), while market squid abundance increased. During this time, sea lion abundance steadily increased, except for intermittent “unusual mortality events” (UMEs) that were often, but not always, associated with El Niños. Increasing squid abundance and declining sardine and anchovy abundance, reduced the caloric value of available forage in the last 10 years. Sardine have spawned progressively further offshore associated with increased offshore transport, likely affecting forage availability for nursing females. Population of sea lions was positively correlated with sardine and squid catches indicating that fishery take had little or no influence on sea lion population trajectory. Large population size, limited foraging range of nursing females, decreasing abundance of high calorie prey, and a trend to more offshore sardine distribution all contributed to the 2013 UME. Occurrence of numerous malnourished pups may now become frequent as the sea lion population experiences food limitation.