Effects of coastal fish farms on body size and isotope composition of wild penaeid prawn

Penaeid prawn are highly valued throughout the Mediterranean Sea where they support artisanal fisheries that may be affected by coastal fish farming. We examined the effects of proximity to costal fish farms on body size and stable isotope composition of the caramote prawn Melicertus kerathurus, a species that supports a valuable artisanal fishery. Prawn sampled near fish cages were significantly larger, heavier and more depleted in 13C than those sampled further away, while laboratory trials confirmed that prawn can readily feed on fish food pellets. Proximity to fish farms had no effect on 15N isotope values, which became depleted with increasing body size, possibly as a result of organ differentiation during ontogeny. Our results indicate that fish farms can have a positive effect on the size of prawn in the vicinity of floating cages, possibly by facilitating their growth or by protecting them from the fishery. However, fish farms can also affect the isotope composition of wild prawn, and thus likely their nutritional value. These findings highlight the need to examine the potential effects of fish farms on prawn fisheries in the Mediterranean region, an area where artisanal fisheries and coastal aquaculture need to coexist in the same locations.