A preliminary investigation of the fish food web in the Gironde estuary, France, using dietary and stable isotope analyses

Abstract

Carbon and Nitrogen stable isotopes and stomach contents analyses were used to investigate an estuarine fish food web and identify the contribution of these two methods to the knowledge and understanding of the food web's structure and its functioning. The nine most abundant fish species during the warm period in the Gironde estuary (southwest France, Europe) are examined. Observation of the stomach contents reflects a variety of feeding modes between fish species that consume a diverse assortment of prey, with limited dietary overlap. Nevertheless, when regarding the whole fish community, few prey species dominate the stomach contents. Nitrogen isotope ratios indicate a high intraspecific variability inducing an interspecific covering of the signatures. However, a tendency to δ 15N enrichment according to the trophic position of the species studied was observed. Fish assemblages show a trend towards enrichment of their carbon isotopic signatures from the upper estuary (−20.8 ± 1.8‰) towards the lower estuary (−18.3 ± 1.6‰). But whatever the capture zone considered, most of the individual δ 13C values for each fish analysed are comprised between −22 and −16‰. Only few specimens, belonging to migratory amphihaline species, have significantly lighter values. The stomach contents method of analysis has the advantage of giving an initial view of the ichthyological trophic structure of the system by describing the food relations between a fish species and its prey. From these results, hypotheses can be drawn about the network's functioning, suggesting a sharing of resources between species and a “wasp-waist” control of this estuarine food web. The stable isotope analysis method enables us to improve our structural knowledge by positioning the different species in a food web, with their position being determined by the number of energy transfers (analysis of δ 15N). Conversely, in environments as complex and changing as estuaries, it appears difficult to precisely identify and quantify the sources of the organic matter at the base of the fish estuarine food webs using analysis of δ 13C isotopic signatures. Nevertheless, the results obtained by using these two methods in parallel suggest that more detailed functional ecological studies could be carried out in future.