Temporal variability in ocean climate and California sea lion diet and biomass consumption: implications for fisheries management

Key to understanding ecosystem structure and function in the California Current System is quantitative modeling of trophic interactions of California sea lions Zalophus californianus, one of the largest and most abundant predators in this system, and how these interactions are affected by climate variability. Because sea lions consume almost exclusively commercially important prey species, we hypothesized that the potential for competition for specific prey resources would vary with environmental conditions. We evaluated seasonal and annual variation in sea lion diet in Monterey Bay during the strong 1997-98 El Nino and subsequent 1999 La Nina through the examination and identification of prey hard parts found in fecal samples. Annual consumption was modeled using population size, dietary data, sea lion energetics, and prey energy content. Sea lions were plastic specialists, feeding on seasonally abundant aggregating prey, exploiting several species at. a time. Short-term seasonal changes in diet corresponded with prey movement and life history patterns, whereas long-term annual changes corresponded with large-scale ocean climate shifts, namely the large 1997-98 El Nino and 1999 La Nina. Annual sea lion consumption of specific prey varied as a function of ocean climate, with an estimated 17 154 t prey consumed in 1998 and 20 229 t in 1999, Sea lion consumption was similar in magnitude to fisheries landings for several prey species and has the potential to adversely impact specific prey resources and directly compete with these fisheries; however, the degree of competition depends on spatial and temporal overlap of fisheries and sea lion foraging efforts.