Effets globaux d'une agglomération sur la typologie ichtyenne d'un fleuve : cas de la Garonne à Toulouse (France)

Abstract

A study of the fish community and its relation with habitat in an anthropogenically modified part of the Garonne River was conducted in 1994-1995. Four sectors in a segment of 50 kms were surveyed. These sectors consisted of two types of channel forms: a lotic type situated upstream and downstream (corresponding to the babel zone), and a lentic type in Toulouse city which is divided in two arms, superior arm and inferior arm (corresponding to the bream zone). The downstream and inferior arm sectors were subject to anthropogenic disturbances, while the two others were less disturbed. According to the channel forms, fish were sampled using two different methods; regular abundance sampling using electro-fishing of spot samples (EPA) for the lotic sectors and gill-nets (EPF) for the lentic sectors. Twenty- seven species belonging to 10 families (dominated by Cyprinidae) were caught in winter 1994 and summer 1995. The lotic- sectors were dominated by barbel (Barbus barbus L.) gudgeon (Gobio gobio L.) and chub (Leuciscus cephalus L.) and the lentics sectors by roach (Rutilus rutilus L.), bream (Abramis brama L.) and bleak (Alburnus alburnus L.) The effect of anthropogenic disturbances was evaluated by comparing the secondary and rare species in the different sectors studied this difference was more obvious by comparing the two sectors of the lentic channel forms (the two arms of the Garonne in Toulouse). The inferior arm(especially subjected to industrial pollution) had higher species richness and abundance than the superior arm (unpolluted). For the majority of species, the fish-habitat relationships in the lotic sectors were not obvious. In the lentic sectors, the habitat preference in the superior arm (unpolluted) was oriented to the sites with rocky bank and aquatic vegetation.