Short-term impact of reservoir cleaning on the microhabitat use of three non-salmonid fishes in a piedmont river in south west France

As with many rivers of the Pyrenees Mountains, reservoirs on the River Arie ge (SW France) are used for hydroelectricity generation, irrigation and to maintain summer flows in the River Garonne. Two river reservoirs on the Riv er Ariege were simultaneously emptied in April 1991 to remove accumulated s ediment. Fish and ten environmental variables were sampled using Point Abun dance Sampling by electrofishing before (February 1991) and one year later (February 1992) at three study sites downstream of the dams. We examined th e short-term impact of reservoir cleaning on the relative abundance, habita t availability and habitat use of three species of small fish: minnow Phoxi nus phoxinus (L.), gudgeon Gobio gobio (L.) and stone leach Barbatula barba tula (L.). After the reservoir cleaning, areas covered by gravel, sand and mud increas ed significantly as the result of the sediment release. The three species d id not react in the same manner to these environmental changes. Stone leach appeared to be the most sensitive species, with its relative abundance dec reasing at all sites. Conversely, minnow seemed to be the most resistant sp ecies and as such is a leading pioneer species for colonizing new or impact ed stretches of rivers channels. Finally, the number of gudgeon decreased p articularly at the most downstream site where large deposits of mud were re corded.