Gradients of abundance of sea breams across the boundaries of a Mediterranean marine protected area

In this study we assess the existence of gradients of abundance (density and biomass) of the sea breams Diplodus puntazzo, Diplodus sargus and Diplodus vulgaris across the boundaries of the Portofino MPA (NW Mediterranean) in order to evaluate reserve effect and to provide evidence of spillover. Fish were visually censused in shallow rocky habitat (4–7 m depth) at replicate sites within the different MPA zones and in adjacent unprotected areas. Higher values of sea breams size, density and biomass were usually observed inside MPA compared to unprotected areas, likely due to the enforcement of protection. We found evidence of negative gradients of sea bream density and biomass across the MPA boundaries, although shape and steepness of gradients changed depending on species, sampling time and boundary orientation. Fish density and biomass decreased abruptly just 100 m outside the MPA and remained close to zero off the western boundary. A partial recovery in fish density was recorded eastward, about 1 km apart from the MPA boundary. Overall, our results were consistent with the presence of a moderate fish spillover, considering that a high fishing effort close to the MPA boundaries could have prevented any potential increase in abundance just outside the MPA.

► We assess the presence of abundance gradients of sea breams across the boundaries of an MPA.
► Fish size, density and biomass were higher inside than outside MPA.
► We suggest the presence of a moderate fish spillover, especially across the eastern MPA boundary.