Intraspecific variation in prey quality affects the consumption rates of top predators

Prey quality, measured as energy density and energy content, is a key functional trait in predator-prey relationships. While the effects of interspecific differences in prey quality on predators have been examined previously, the consequences of intraspecific variation remain less understood. To examine how within-species variation in prey quality influences predator foraging, we modelled the effects of prey size, maturity and sampling season and region on the quality of Engraulis mordax, Sardinops sagax and Doryteuthis opalescens-three common prey species for top predators in the California Current Ecosystem (CCE). We contextualized our findings using documented energy budgets of the California sea lion (Zalophus californianus), a consumer of these species and an important ecosystem indicator in the CCE. We found significant within-species variation in prey quality related to size, maturity, season and region, with stronger effects in fish than squid. These patterns likely reflect prey life history and regional and seasonal oceanographic conditions that influence energy storage. Under static prey availability and predator energy demands, daily biomass requirements driven by intraspecific variation in prey quality were comparable to previous estimates based on interspecific differences. By integrating predator bioenergetics with prey energy content models, we found that the number of prey required can vary by tens of thousands depending on prey size-rendering smaller individuals an impractical energy source for non-filter-feeding predators. Even accounting for size, predators may need to consume up to twice as many individuals when foraging on lower-quality prey from certain regions, seasons or maturity stages compared to higher-quality conspecifics. Our findings highlight the critical importance of incorporating intraspecific variation in prey quality into bioenergetics frameworks that inform predator foraging predictions. As climate change and resource exploitation intensify, integrating functional traits and energetic trade-offs into predator-prey studies will be essential for anticipating predator responses and evaluating ecosystem resilience.