ASFB Home > Potential Changes in Prey Population Structure Following Removal of Predators by Fishing

Retrospective Assessment of the Effects of Marine Protected Areas on Relative Abundance and Size Structure of Populations of Harvested Reef Fish on the Great Barrier Reef

Campbell Davies and Bruce Mapstone

National Oceans Office, HOBART, AUSTRALIA

Theme: TH4

Marine Protected Areas are widely used as a conservation management strategy. Recent failure of conventional fisheries management controls in many parts of the world and growing evidence of the impacts of human activities on marine biodiversity has seen increased interest in the use of spatial closures as: i) a conservation management strategy for biodiversity, and ii) a harvest management strategy for fisheries. The Great Barrier Reef Marine Park includes over 3000 individual coral reefs that are assigned zoning categories which range from managed extractive activities (including fishing) to completely protected/no access. This system of replicated management units has provided a unique opportunity to assess, retrospectively and experimentally, the effects of MPAs through a large-scale field experiment, the Effects of Line Fishing (ELF) Experiment. The ELF Experiment involved four clusters of six reefs with four closed reefs and two open reefs per cluster. Closed reefs had been closed to all forms of fishing for 10-12 years at the commencement of the study. Clusters were dispersed over seven degrees of latitude along the Great Barrier Reef. All reefs were surveyed by structured line fishing on an annual basis from 1995-2000, with additional within-year surveys done on a subset of reefs in “manipulation” years. Analyses demonstrated strong regional and temporal (inter-annual) effects on both relative abundance (CPUE) and size structure of primary target species. There were significant effects of management zone (closed vs open) with higher abundance and larger average size on closed reefs. These effects, however, were not consistent among regions or over all years. These results demonstrate the complexity of potential responses of populations to closed areas and the need for a combination of multi-survey, experimental and systems modelling approaches to resolve most likely causal mechanisms.