Fish community structure at the scale of a large European basin: Longitudinal organisation, slope influence and regional tendencies.

On a broad scale, the European fish community is under three major influences. First, the upstream-downstream gradient in habitat generated by water flow organizes the community in its longitudinal dimension. Then, discontinuities appear in this organization under the influence of slope variations. Finally, the different ecological regions have an effect on the distribution of the species. The Rhone Basin can be used as a model for the study of these three influences because it contains, over a small area, the regional heterogeneities which dominate in Europe. In the longitudinal dimension, slope and distance from the source interact on the fish communities of the upper and median courses. A variation in slope has different effects according to the distances from the source. An great part of the variation in communities is also explained by differences in geographic zones, thus showing the influence of local conditions which are not taken into account by the longitudinal dimension and the slope. These regional variations fall within the three ichthyogeographic regions previously identified within the basin: the Alpine, southern and oceanic climatic subdivisions, with a continental modulation.