Density dependence in marine protected populations: A review

The cessation or reduction of fishing in marine protected areas (MPAs) shou ld promote an increase in abundance and mean size and age of previously exp loited populations. Thus density-dependent changes ill life-history charact eristics should occur when populations are allowed to recover in MPAs. In t his review, we synthesize the existing information on resource limitation i ll marine ecosystems, density-dependent changes in life-history traits of e xploited populations and evidence for biomass export from MPAs. Most eviden ce for compensator!, changes in biological variables has been derived from observations on populations depleted by high fishing mortality or on strong year classes, but these changes are more evident in juveniles than in adul ts and in freshwater rather than in marine systems. It is unclear if adults of exploited marine populations are resource limited. This may suggest tha t exploited populations are controlled mainly by density-independent proces ses, which could be a consequence of the depleted state of most exploited p opulations. MPAs could be a useful tool for testing these hypotheses. If we assume that resources become limiting inside MPAs, it is plausible that, i f suitable habitats exist, mobile species will search for resources outside of the MPAs, leading to export of biomass to areas which are fished. Howev er, it is not possible to establish from the available data whether this ex port will be a response to resource limitation inside the MPAs, the result of random movements across MPA boundaries or both, We discuss the implicati ons of this process for the use of MPAs as fisheries management tools.