Differences in Trophic and Community Structure of Kelp Forest Fishes Inside and Outside of Three Long-standing MPAs in the Southern California Bight

Abstract

In many marine ecosystems worldwide, overfishing is a prominent cause in removing large predatory fishes from ecological communities. Fluctuation in the abundance of higher trophic level species can transform an ecosystem's structure and function by altering trophic interactions through density-mediated top-down control. Accordingly, understanding the extent to which humans indirectly influence a community through altering predator abundance is of critical importance. Thus, during the summer of 2013 and 2014 the impacts of fishing on the trophic structure of kelp forest fishes were examined within the Southern California Bight. In 2013, we tested whether decreased abundance through fishing for higher trophic level predators relieves predation pressure on lower trophic level prey. Using a combination of underwater survey techniques, density (no. fish/100 m2) and biomass (g/100 m2) of conspicuous fish species were sampled inside and outside of three long-standing Marine Protected Areas (MPAs) off La Jolla, Santa Catalina Island, and Anacapa Island, California. We found that the secondary carnivore and herbivore/omnivore trophic levels were significantly lower in density and biomass outside of MPAs. Inversely, the primary carnivore trophic level biomass was higher outside of MPAs. At the species level, we observed a lower abundance outside MPAs of large kelp bass (>25 cm) and higher densities of potential prey, kelp perch. Other primary carnivore fishes (blacksmith and señorita) showed a non-significant trend of higher biomass outside MPAs. Our results provide evidence of trophic level changes due to fishing pressure and provide support for a possible weakening of top-down control on specifically the kelp perch population through the removal of large kelp bass outside MPAs. The removal or recovery of predators can greatly influence an ecosystem. As more recent studies suggest that indirect community effects of fishing and protection can take up to decades to detect, it is necessary to document the continued changes on the structure, function, and dynamics of the kelp forests and rocky reefs off southern California.