POTENTIAL BROWN TROUT HABITAT (SALMO-TRUT TA-FARIO, L 1758) IN 3 MASSIF-CENTRAL STREAMS - METHODOLOGICAL APPROACH AND FIRST RESULTS ON INFLUENCE OF RIPARIAN CORRIDORS

An approach using a French version of PHABSIM has enabled us to estima te the potential habitat relative to the mineral and hydraulic structu re of channel in three salmonid streams located in Correze (Massif Cen tral, France). PHABSIM estimates a potential carrying capacity or weig hted usable area (WUA) by coupling measurements of depth, velocity and substrate with brown trout habitat preference curves. The WUA calcula tions enable to extricate from the morphohydrological differences betw een sites; so it becomes possible to compare and assess the influence of other environmental factors with a saving on the number of sites an d operations carried out. The ratio of observed biomass of adult brown trouts to weighted usable area (WUA) has been compared for the differ ent streams. The results showed differences between studied sites, due to the morphohydrological habitat and different vegetative surroundin gs. The estimation of different shelter structures showed the importan ce of riparian corridor in the structuration of the brown trout popula tion. The lack of riparian corridor seemed to be a limiting factor in one studied site with a low trout biomass of 13 g/m(2) WUA. Conversely , in another site, the riparian corridor with dense canopy and with we ll developed root system compensated a poor physical carrying capacity of the channel dominated by sand (65 g adult trout/m(2) WUA). Potenti al WUA of this type of river should therefore be interpreted by the mo rphohydrological habitat estimated by PHABSIM like methodology and the absence or presence of riparian trees estimated by the shelter struct ures inside the stream channel.