0-FISH ASSEMBLAGES IN A SECTOR OF THE RHONE RIVER INFLUENCED BY THE BREGNIER-CORDON HYDROELECTRIC SCHEME()

A survey was undertaken to assess the effects of the construction of t he Bregnier-Cordon hydroelectric power station in 1984 on the spawning and nursery habitats of the young of the year fish (YOY) in the River Rhone floodplain area. No obvious effect could be identified in the m ost important aquatic habitats studied, possibly due to the downstream drift and dispersion of juvenile fish. Some new habitats were created with a high production capacity, which proved suitable as nursery are as for YOY rheophilous fishes (e.g. riparian zones of the headrace can al and reservoir). The conditions for young fish were also improved in the section of the main channel, now by-passed, in which the discharg e is stabilized and sufficiently high. However, a rapid decline in the abundance and species diversity of YOY fish assemblages was noted in parapotamic side-arms, caused by the side-arms drying up and becoming invaded by terrestrial plants. High YOY abundance, indicating good rep roductive success, was nevertheless noted in some fishes considered as sensitive to such hydroengineering activities.