Dynamics of harvested demersal fish communities: analysis of the species diversity in the Bay of Biscay (Atlantic Ocean) and in the Gulf of Lions (Mediterranean Sea).

Species diversity variations between the demersal fish assemblages of the Bay of Biscay (Atlantic Ocean) and of the Gulf of Lions (Mediterranean Sea) are analysed on a decade scale. The aim of this study is to interpret the dynamics of these two communities characterised by different level of fishing intensity. Data come from trawl surveys carried out for the direct evaluation of the demersal stocks abundance from 1983 to 1997. Diversity indices used are the Hill's indices N1 and N2, the species richness S, the evenness J', the K-dominance curves and the life strategy of the dominant species. Three demersal fish assemblages are defined in the community of the Bay of Biscay and in that of the Gulf of Lions: coastal, continental shelf, and continental slope. There are neither significant differences between the assemblages of the Bay of Biscay nor between those of the Gulf of Lions. There are no significant differences between years. The assemblages of the Gulf of Lions are characterised by higher values of N2 and S than the assemblages of the Bay of Biscay. The K-dominance curves show that there is a lower number of dominant species in the Bay of Biscay than in the Gulf of Lions. Finally, there are more long-lived species among the dominant one in the Gulf of Lions than in the Bay of Biscay. Then the community of the Bay of Biscay corresponds to a system adjusted to perturbations (unpredictable mortalities) while the Gulf of Lions corresponds to a system adjusted to a less perturbed environment. There is no interpretation of the relationship between the level of fishing intensity and the chosen indices at the scale of this study. It is concluded that dynamics patterns of the communities can be interpreted from the species diversity indices when associated with K-dominance curves and life strategy characteristics. (C) 2001 Ifremer/CNRS/Inra/IRD/Cemagref/Editions scientifiques et medicales Elsevier SAS.