Testing trophic indicators of fishery health in California's marine protected areas for a generalist carnivore

Fisheries have historically focused on single-species management, but there is a global movement to incorporate ecosystem-based processes. Marine protected areas (MPAs) offer an important management tool for enhancing ecosystem structure and function. This study investigated whether a non-lethal indicator of trophic position (TP) could be used to evaluate fishery health in southern California MPAs. We hypothesized that a common predator, kelp bass (Paralabrax clathratus), would occupy a higher TP and smaller niche space inside MPAs than at fished reference sites, and that these patterns would be supported by trends in abundance and gut contents. Stable isotope analysis of nitrogen (delta N-15) and carbon (delta C-13) were used to estimate the TP and niche space of kelp bass (N = 125) collected inside and outside of four MPAs during summer 2016-2017. Although all MPA sites supported a significantly higher biomass of kelp bass than reference sites, there was no consistent pattern in TP and niche space inside versus outside of MPAs. Gut contents showed that exploitation of ephemeral pelagic foods may reduce localized effects of MPAs on prey availability for this species. Our findings suggest that MPAs will not have consistent impacts on food web dynamics for higher level generalist predators and that ecosystem-level responses to protection may vary across locations within a bioregion. These results emphasize the importance of accounting for site-specific differences in food web structure before applying ecosystem-based metrics to fishery management.