Difference in diet and size‐related trophic level in two sympatric fish species, the red mullets Mullus barbatus and Mullus surmuletus, in the Gulf of Lions (north‐west Mediterranean Sea)

Stable‐isotope ratios ( δ 15 N and δ 13 C) and diet of the red mullets Mullus barbatus and Mullus surmuletus were analysed in two zones differently subjected to the Rhône River runoff in the Gulf of Lions (north‐west Mediterranean sea) in May and October 2004. δ 15 N and δ 13 C increased significantly with fish size in M. barbatus in both zones and seasons, whereas no significant trend was evidenced in M. surmuletus . A clear size‐related shift in diet was observed in M. barbatus , with an increase in polychaete and shrimp consumption with size and a decrease in small crustacean consumption. In M. surmuletus , a diet shift was observed only between medium and large individuals. Large M. surmuletus preyed on shrimps, polychaetes, bivalves, ophiurids and amphipods, and ingested prey of lower mean mass than M. barbatus of similar size. Difference in size‐related increase in δ 15 N between the two mullid fish species were related to difference of trophic level of their main prey. Sub‐surface deposit‐feeding polychaetes, carnivorous polychaetes, shrimps and brachyurans presented higher δ 15 N values than bivalves, small crustaceans and ophiurids. The lower δ 13 C values observed in M. barbatus compared to M. surmuletus were related to a higher consumption of sub‐surface polychaetes in the former species. Significantly, lower δ 13 C were recorded in fishes collected off the Rhône River, particularly in spring, suggesting an influence of river inputs as a source of particular organic matter for mullids in this zone after the flooding season. Thus, these closely related sympatric fish species displayed diet divergences that were reflected in their stable isotopic signatures.