Are Estuarine Fish Opportunistic Feeders? The Case of a Low Anthropized Nursery Ground (the Canche Estuary, France)

Stomach contents of the main fish species (i.e. sprat Sprattus sprattus, sea-bass Dicentrarchus labrax, common goby Pomatoschistus microps and flounder Platichthys flesus) in the Canche nursery ground (France) were examined from May 2006 to March 2007 according to food availability (i.e. macrozoobenthos and zooplankton) to analyse diet composition, trophic competition and prey selection. Overall, 44 prey fish species were identified from the 684 stomach contents examined. Fish diet was characterized by a wide diversity of prey, a relatively high feeding activity (vacuity index ranged between 16.9 and 29.0 % according to species) and spatiotemporal changes, except for D. labrax which did not vary between periods or estuarine zones. Copepods Temora longicornis and Euterpina acutifrons were the most important items in the diet of S. sprattus; for D. labrax, the dominant prey was the amphipod Gammarus sp., while Platichthys flesus and Pomatoschistus microps showed a more similar trophic niche (similarity percentage = 39 %) with diets consisting of nematodes and amphipods. Spatiotemporal sharing of trophic resources by fish and high food resources in the Canche estuary was examined, and very limited diet overlap was observed, indicating that competition for food resources was limited. Differences between the available preys and fish diets demonstrated that fish do not select the most abundant food resources. S. sprattus showed a selection for Tachidius sp. and cladocerans; D. labrax for Gammarus sp., Hediste diversicolor and to a lesser extent nematodes and Corophium sp.; Pomatoschistus microps for nematodes and Gammarus sp.; and Platichthys flesus for nematodes, Gammarus sp., H. diversicolor and Corophium sp. It was concluded that the studied fish exhibit a specialist rather than opportunistic feeding strategy.