Thesis

Direct and indirect impacts of fishing on the trophic structure of kelp forest fishes off southern California

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 and community assemblage of kelp forest fishes were examined within the Southern California Bight. In 2013, I 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. I found that secondary carnivore and herbivore/omnivore trophic levels significantly decreased outside of MPAs. Inversely, the primary carnivore trophic level biomass increased outside of MPAs. Species-level results revealed a lower abundance outside MPAs of large kelp bass (> 25 cm) and higher densities of its prey, kelp perch. My results show overall fish trophic level changes due to fishing pressure, and provide support for a weakening of top-down control on the kelp perch population through the removal of predatory fishes outside MPAs. To investigate the possible return of the historically overfished apex predator of the kelp forest fish community, I censused the giant sea bass (Stereolepis gigas) population at eight sites off Santa Catalina Island from mid-June through mid-August, 2014. Three possible spawning aggregations were identified at the sites Twin/Goat, The V's, and Little Harbor. The giant sea bass population at these sites primarily consisted of individuals 1.2 - 1.3 m long (total length, TL) with small and probably newly mature fish (estimated to be 10 - 11 years old) observed in aggregations. However, larger individuals 1.8 - 1.9 m TL accounted for the majority of the population biomass. Overall, mean spawning stock biomass of giant sea bass was 36.3 kg/1000 m2. Providing a general comparison of mean biomass among the trophic levels of kelp forest fishes off Santa Catalina Island revealed a nearly top-heavy biomass pyramid. The relatively high abundance of giant sea bass provides evidence that this species is recovering at kelp forests off Santa Catalina Island, and possibly throughout the Southern California Bight. 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.

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