Masters Thesis

Evidence for Food Limitation in Rebuilding Populations of a Fish in Marine Protected Areas

Marine protected areas (MPAs) are used to allow organisms and habitats to recover from anthropogenic impacts such as overfishing, which has significantly diminished populations of many exploited marine species. MPAs typically have higher biomass of harvested species within their borders than in fished area outside, which may cause intraspecific competition for limited resources upon population recovery. I evaluated evidence for whether a heavily fished species, California sheephead (Semicossyphus pulcher), becomes prey limited within MPAs given increasing duration since the cessation of fishing. I (1) verified that biomass of California sheephead was higher inside the MPAs than in nearby reference areas open to fishing; (2) tested the prediction that prey density would be lower in MPAs than reference sites outside; and (3) made foraging observations, to test whether foraging behavior of California sheephead differed inside versus outside of MPAs. I used the rock wrasse (Halichoeres semicinctus), an unfished, but taxonomically related species to test whether any differences in foraging behavior inside vs. outside of MPAs for California sheephead might not actually be caused by protection in MPAs. I studied three new MPAs in southern California established 2012 and three old MPAs established between 1971 and 1988. Underwater visual transect surveys conducted by scuba divers provided data on fish biomass and prey density. Foraging observations by divers quantified fish foraging behavior (foraging rate and time spent searching). Baited remote underwater videos (BRUVS) were used to quantify fish abundance (MaxN), foraging rate, and time to attack bait. As expected, there were no apparent effects of MPAs on rock wrasse biomass or foraging behavior. In contrast, biomass of California sheephead was higher within MPAs than at reference sites outside and the difference in biomass inside vs. outside MPAs was higher in older MPAs than younger ones. Prey densities were lower for certain prey species within MPAs and generally correlated with California sheephead biomass and MPA age. California sheephead foraging behavior was different inside vs. outside of MPAs, although the magnitude of the behavioral differences depended on the MPA. At 5 of the 6 MPAs, California sheephead spent more time searching for food and foraged at a faster rate within MPAs. Higher foraging rates within MPAs were likely the result of consuming smaller, less valuable prey than at fished sites outside MPAs. BRUV observations revealed higher numbers of California sheephead attracted to bait stations within MPAs, despite diver transects revealing that sheephead densities were not higher in MPAs. BRUV observations also recorded higher foraging rates and faster attack times of bait in MPAs than at references sites outside. These results suggest that California sheephead may be food limited within MPAs, implying that as populations of harvested species rebuild within MPAs, they may become resource limited. This research documents unintended effects of MPAs.

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