Role of temperature and flow regulation on the Salmoniform - Cypriniform transition

Abstract

The effects of both temperature and flow regulation on the longitudinal evolution of the fish community were studied along a section of the River Garonne. The section included an upstream zone dominated by the taxonomic order Salmoniforms, and a downstream zone dominated by Cypriniforms. Point abundance sampling by electrofishing was performed at six sampling sites during summer, autumn and winter. The fish community observed was composed of eleven species of fish and one species of Cyclostome, and dominated by one species of Salmoniform: brown trout (Salmo trutta L.) and two representatives of the Cypriniforms: the European minnow (Phoxinus phoxinus L.), and the stone loach (Barbatula barbatula L.). Multiple regression analysis shows that temperature and flow both influence the Salmoniform-Cypriniform transition. When performed with each of these variables considered separately, simple regression analysis was neither significant for temperature only, nor flow only. From upstream to downstream, temperature is the determining factor inducing the progressive replacement of Salmoniforms by Cypriniforms in the natural flow sites, whereas in regulated flow sites the determining factor is the modification of the morphodynamic conditions. These variations caused by both temperature and flow regulation induce a decreasing oscillating trend of the fish community.