Exploitation of trophic resources by fish under stressful estuarine conditions

Abstract

Despite their high complexity and variability, estuaries are very productive and play an important role in fish feeding. We investigated how fish optimize their use of the available trophic resources by studying trophic preference variability and feeding strategies of some pelagic and demersal fish in the Gironde estuary (southwest France). Fish and their prey were collected approximately every 2 mo from July 2003 to June 2004 in the upstream area of the saline estuary. Stomach contents were analysed to assess the variability of fish feeding in relation to their size and the time of year. Intra- and interspecific food niche overlap was evaluated using Schoener's index, and a crosscalculation method was used to highlight general fish trends in predation strategy. Stomach content results showed interspecific and intraspecific variability in feeding by fish, which can be explained by their different or ontogenetically changing ecomorphology. Their diets are composed mainly of zooplankton and hyperbenthic crustaceans, with temporal variations in the consumed taxa. Optimization of available trophic resource use, a key element in estuarine resilience, is thus possible due to the temporal adaptation of this structural trophic web. However, in spite of their temporal adaptation capacity, most fish species exhibited a specialist feeding strategy. This result was not expected. Since zooplankton and hyperbenthic crustaceans exhibit a low specific richness in estuaries, especially in the high turbidity of the Gironde estuary, the loss of one of these species could affect the fish trophic web structure and hence the resilience of the system. © Inter-Research 2010.