Fish assemblages associated with coastal defence structures: Does the surrounding habitat matter?

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Artificial structures are sprawling along seashores due to growing populations, coastal use and greater need for coastal erosion and flood protection. The impacts of these anthropogenic modifications of coastlines on fish populations are poorly described, and little is known about factors affecting the ecological performance of these novel habitats for fish. We compared the abundance and community structure of fish assemblages associated with artificial coastal defence structures built in either rocky or sedimentary contexts at two locations along the coast of Sicily, to explore whether the performance of the artificial habitats for fish changed in relation to the characteristics of the surrounding natural habitat. We also quantified how fish associated with artificial structures in either habitat contexts differed from that at nearby natural rocky reefs. We found that fish assemblages differed in abundance and structure between artificial coastal defences deployed in sandy contexts vs natural rocky reefs, with different ecological taxa showing greater (i.e. benthic carnivorous) or lower (i.e. serranids) abundances in the artificial habitat. We also found that fish assemblages associated to artificial structures built on rocky bottoms had inconsistent outcomes, more frequently grouping with artificial sandy habitats at one location and with natural rocky reefs at the other. These results suggest that the surrounding environmental setting could have an overriding, and so far largely overlooked, importance in affecting the performance of artificial structures as habitats and in facilitating the development of fish assemblage with distinctive composition and structure.