Net contribution of spillover from a marine reserve to fishery catches

Benefits for fisheries from marine protected areas (MPAs) are expected from recruitment of exported eggs and larvae as well as from spillover of adults to adjacent fishing grounds. Because the recruitment effect is difficult to detect, spillover is presently the only tangible potential fishery benefit of MPAs. Despite abundant evidence of spillover in the literature, this is the first study to quantify the number and biomass of individuals annually spilling over from an MPA and their contribution to the local fishery catches. Using a decade (1997–2007) of tag-recapture data for the lobster Palinurus elephas from the Columbretes Islands Marine Reserve (CIMR), we estimated annual emigration probabilities of 3.7% (female) and 6.7% (male), and quantified the resulting spillover to adjacent fished areas. We showed that during an 8 to 17 yr protection period, harvested spillover offset the loss of yield resulting from the reduction of fishing grounds set aside in the CIMR, producing a mean annual net benefit of 10% of the catch in weight. Although the number of lobsters spilling over annually did not quite make up for the loss of fishing grounds in the CIMR, it did in weight because the mean size of the lobsters emigrating from the reserve was larger than that of those outside. We propose that nomadic or home range movements of individuals near MPA boundaries, seasonal migrations and migrations forced by extraordinary meteorological events, and density-dependent movements facilitated spillover from the CIMR. Fishing effort concentration along its boundaries and high exploitation rates in the local fishery limited the spatial extent of spillover.