Abstract
Groupers species are extremely vulnerable to overfishing and many species are threatened worldwide. In recent decades, Mediterranean groupers experienced dramatic population declines. Marine protected areas (MPAs) can protect populations inside their boundaries and provide individuals to adjacent fishing areas through the process of spillover and larval export. This study aims to evaluate the effectiveness of six marine reserves in the Western Mediterranean Sea to protect the populations of three species of grouper, Epinephelus marginatus, Epinephelus costae and Mycteroperca rubra, and to understand in which circumstances MPAs are able to export biomass to neighbouring areas. All the studied MPAs, except one where no grouper was observed, were able to maintain high abundance, biomass and mean weight of groupers. Size classes were more evenly distributed inside than outside MPAs. In two reserves, biomass gradients could be detected through the boundaries of the reserve as an indication of spillover. In some cases, habitat structure appeared to exert a great influence on grouper abundance, biomass and mean individual weight, influencing the gradient shape. Because groupers are generally sedentary animals with a small home range, we suggest that biomass gradients could only occur where groupers attain sufficient abundance inside MPA limits, indicating a strongly density-dependent process.
Highlights
Several marine species are seriously threatened by an array of anthropogenic actions [1], from which fishing is likely the main human activity seriously affecting fish population abundance and size structure, and causing marine biodiversity loss
The notable effects of marine reserves are an increase in abundance and an enlargement of the average size of individuals of the target species inside the boundaries of the protected area, so that greater abundance and size theoretically imply an increase in reproductive potential [3,4,5,7,8]
Study Area The work was conducted from July to October of 2003 and 2004 in six Marine protected areas (MPAs) spread over the Western Mediterranean Sea (Fig. 1): the natural marine reserve of Cerbere-Banyuls and the Carry-le-Rouet (Carry) marine park in France, and the National Park of Cabrera (Cabrera), and the marine reserves of Medes islands (Medes), Tabarca island (Tabarca) and Cabo de Palos – Hormigas islands (Cabo de Palos) in Spain
Summary
Several marine species are seriously threatened by an array of anthropogenic actions [1], from which fishing is likely the main human activity seriously affecting fish population abundance and size structure, and causing marine biodiversity loss. Eggs and larvae from restored spawning stocks inside MPAs could be exported by currents to adjacent fishing areas ([11], but see [12]). Because of increased density inside the MPA, adults and juveniles fishes from target species may emigrate from inside the protected locations to outside where the density is lower (‘‘spillover’’, [13]). An indirect method to estimate the magnitude and importance of such export of larvae, juveniles and adults fishes from MPA to neighbouring areas is to look for the likely existence of gradients of biomass of target species across MPA limits, under the rationale that, if spillover occurs, there would be more fishes near than far away from the MPA [6,14]
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