Predicting prey recovery from scats of California sea lions (Zalophus californianus) for novel prey species and sizes

Diet studies of marine top predators increasingly inform assessments of prey stocks and ecosystems, as well as predator ecology. Prey hard parts provide quantitative information on diet composition. However, species and size variation in digestion of hard parts, and thus in recovery rates, introduces bias in diet reconstruction. Captive feeding studies estimate species- and size-specific recovery rates of prey hard parts in pinnipeds, but such studies can only include a small subset of prey species and sizes consumed by these commonly generalist predators. We developed a generalized linear mixed effects model that estimates recovery rates of otoliths in California sea lion (Zalophus californianus) scats from otolith morphometrics, based on previously published captive feeding studies. The resulting model explains 85% of the variance in recovery rates among common teleost prey species of California sea lions. We also reestimated cephalopod beak recovery rate in California sea lion scats using an intercept-only mixed effects model. The resulting models can estimate recovery rates—and numerical correction factors—for novel teleost and cephalopod prey species and sizes in California sea lion scats, supporting more accurate reconstruction of diet composition in the wild and suggesting a way forward for other pinniped species.