Fish ecology and terrestrial carbon use in coastal areas: implications for marine fish production

1. To investigate the role of fish ecology in controlling terrestrial particulate organic matter (POM) uptake by benthic fish species and population responses to changes in continental inputs to coastal areas, depth distribution, diet and stable isotope (delta(13)C and delta(15)N) data from the juveniles and adults of five important flatfish species (Arnoglossus laterna, Buglossidium luteum, Citharus linguatula, Solea lascaris and S. solea) were compared off the Rhone River delta (NW Mediterranean). 2. In moderate river discharge, isotopic signatures differed with fish species and benthic life stage, suggesting intra- and interspecific differences in trophic levels and food sources. Food web analysis indicated terrestrial POM uptake in all life stages of S. solea and A. laterna (delta(13)C = -20.1 to -20.0% and -18.9 to -18.2%, respectively), in the adults of B. luteum (delta(13)C = -19.0%) and in the juveniles of S. lascaris (delta(13)C = -18.7%), whereas all life stages of C. linguatula (delta(13)C = -17.6%), S. lascaris adults (delta(13)C = -16.9%) and B. luteum juveniles (delta(13)C = -17.5%) exploited marine POM almost exclusively. 3. Fish diet and depth distribution accounted fully for the intra- and interspecific differences in terrestrial POM uptake observed. Greater terrestrial signatures were observed in fish eating larger quantities of deposit-feeding polychaetes (the main prey exploiting terrestrial POM for growth) and occurring mainly in the 30-50 m depth zone (where river POM sedimentation and its uptake by the benthos were the highest). Due to intra- and interspecific differences in fish depth distribution and diet, Rhone River floods were predicted to have little effect on C. linguatula populations but to increase the other four species' local stocks for several years, with a maximum impact on S. solea. 4. This study shows that the ability of benthic fish to exploit terrestrial POM inputs to the coastal zone depends strongly on their ecology. As this parameter will determine fish populations' reactions to river floods, it must be assessed carefully before making hypotheses about the consequences of variations in river discharge on coastal fisheries' production.