FISH COMMUNITY STRUCTURE AT THE SCALE OF A LARGE EUROPEAN BASIN - LONGITUDINAL ORGANIZATION, SLOPE INFLUENCE AND REGIONAL TENDENCIES

On a broad scale, the European fish community is under three major inf luences. First, the upstream-downstream gradient in habitat generated by water flow organizes the community in its longitudinal dimension. T hen, discontinuities appear in this organization under the influence o f 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 contain s, over a small area, the regional heterogeneities which dominate in E urope. In the longitudinal dimension, slope and distance from the sour ce interact on the fish communities of the upper and median courses. A variation in slope has different effects according to the distances f rom the source. An great part of the variation in communities is also explained by differences in geographic zones, thus showing the influen ce of local conditions which are not taken into account by the longitu dinal dimension and the slope. These regional variations fall within t he three ichthyogeographic regions previously identified within the ba sin: the Alpine, southern and oceanic climatic subdivisions, with a co ntinental modulation.