TEMPORAL VARIABILITY OF FISH COMMUNITIES IN THE MEDIAN PART OF THE LOWER RHONE RIVER

A study in the regulated part of the Lower Rhone (hydropower darns of Montelimar and Donzere-Mondragon) allowed us to identify the between y ear variability of fish assemblages from 1983 to 1994. Observations co ncerned three stations representative of the artificially modified mai n channel : dyked lotic channel, storage reservoir and diversion canal .Twenty-nine species among the thirty-three known from this part of th e river were captured in the three stations during spring. Five euryoe cious species (Rutilus rutilus, Alburnus alburnus, Leuciscus cephalus, Anguilla anguilla and Lepomis gibbosus) with high occurrences in samp les (more than 60%) represented 92% of catches. During the last decade , three new species appeared in the Lower Rhone : Silurus glanis (1987 ), Pseudorasbora parva (1989) and Carassius auratus (1990). Apart from anthropogenic disturbances, interannual variability in species richne ss and fish abundances were mainly determined by : - successive and si gnificant hydroclimatic events (hydrological deficit from 1989 to 1993 ) and high floods (October 1993 and January 1994), - ecological strate gies of species facing abiotic and biotic constraints. The results pro ve the interest of long-term studies to appreciate variability in larg e river fish assemblages and the impact of river equipment.