Macrobenthic crustaceans in the diet of demersal fish in the Bay of Biscay in relation to abundance in the environment

Abstract

In this study, the importance of the main macrobenthic crustaceans in the Bay of Biscay in the diet of 18 species of demersal predatory fish was investigated by comparing stomach content data from the fish with the results of sampling the bottom with a beam trawl. The Ivlev index was employed to evaluate the degree to which the crustaceans were selected in favour of other prey. Stomach content data were weighted to the abundance of the predators in the environment in accordance with the abundance indices obtained in the area with a bottom trawl survey, when all the data were collected. Selection of the crustaceans as a group varied, depending on whether biomass/volume or number was being studied, with the former value negative and the latter positive, due to the importance of the fish biomass as prey for the predators studied. The highest mean value of the Ivlev index was obtained for the benthic shrimp Processa spp. Other prey species with a high index were Pandalina brevirostris and the Crangonidae, and to a lesser extent Portunidae, Scyllarus arctus, Alpheus glaber, Lophogaster typicus and the Amphipoda. Processa spp. and Galathea spp. occurred in 15 of the 18 fish species analysed. Other more armed or protected species were negatively selected and were less important as prey, e.g. Nephrops norvegicus, Dichelopandalus bonnieri, Plesionika heterocarpus and the Paguridae. The predators studied were also classified according to trophic spectrum and the importance of macrobenthic crustaceans in their diet. Scyliorhinus canicula, Raja clavata and Lepidorhombus boscii showed the highest mean Ivlev index and diet diversity, while the lowest was for Pagellus acarne and Chelidonichthys obscurus.