This study prospects the research on fish populations in the ancient arms of the Garonne river between Toulouse and the confluence of the Gimone - six sites were surveyed. Investigating fish populations by electro-fishing of spot samples and the use of gillnets, made its possible to establish the ecological indices and species associations. Species distribution profiles had been associated with the descriptive environmental variables. The results reflect the diversity of fish populations and differences according to the considered side-arms of the Garonne river. Ancient arms which have an important exchange in their downstream part with the river are the richest and most varied. Spatial distribution of the main species presents two different groups. In the ancient arms of the Garonne river, the deep zones on a gravel substrate, are favourable for the roach, bream, bleak, rudd and black-bass. The shorelines are gently sloping shallow and colonised by aquatic vegetation which is an attractive habitat for bottom-feeders and other carnivores (perch and sunfish).
