ICHTHYOGEOGRAPHIC REGIONS AND WATERSHED SIZE IN THE FRENCH RIVER RHONE NETWORK

Fish community structure can be considered at several scales. The watershed is the largest geographic unit connecting freshwater systems. It describes the upstream-downstream gradient resulting from changes in flow rates. The watershed can also extend over distinct ecological regions. The number of native fish species recorded in each sub-watershed directly connected to the Rhone is strongly correlated with the area of the sub-watershed. Three major groups of sub-watersheds can be retained as ichthyogeographic regions: the Saone region, dominated by a large alluvial plain and rich in species, the Isere region representing the north and central Alpine rivers and the Durance region, strongly influenced by the Mediterranean climate. The River Rhone basin reveals a high diversity of native faunal regions which have to he considered in monitoring and managing the network.