Context-dependent effects of marine protected areas on predatory interactions

We studied the effects of marine protected areas (MPAs) on predatory interactions at a regional scale of 3 different islands across the geographical gradient of the Canary Islands. Protection measures positively affected predatory fish assemblages, enhancing the intensity of predatory interactions in comparison to equivalent unprotected areas (UAs), and causing indirect effects on populations of the key herbivorous sea urchin Diadema aff. antillarum. Results of tethering experiments and the strong negative correlation found between predation rate and prey density strongly suggest that predation controls sea urchin populations. Overall urchin density was lower in MPAs than in UAs; however, species composition of predatory fish assemblages and the size of sea urchin effectively preyed upon differed significantly between studied islands, as did urchin size-frequency distributions. Only in locations where key predatory species (balistids, diodontids and large labrid species) were present was there the potential to promote top-down control on sea urchins. The present study shows the importance of both protection measures and environmental context to enhance the strength of predatory fish activity. Protection under different contexts can produce different consequences of predation interactions, even over relatively small spatial scales. Therefore, regional and local-scale environmental gradients should not be overlooked as a factor affecting the occurrence and magnitude of predatory interactions in benthic marine communities, especially in systems that are markedly variable at small scales.