HETEROGENEITY AND TYPOLOGY OF FISH HABITAT IN THE MAIN STREAM OF A BRETON COASTAL RIVER (ELORN-FINISTERE, FRANCE)

To study the salmonids potential production of a coastal river main st ream from simple characterization of the fish habitat, it was necessar y to rate the hydrosystem's spatial heterogeneity. The presence of a n umber of dams on the river Elorn did not mask the two main levels of h eterogeneity. Longitudinal zonation was the first level of heterogenei ty assessed. Two zones were characterized and delimited based on inter -dam descriptors: slope, width, macrohabitat linear density, stream or der, water quality,... The second heterogeneity level was the macrohab itat. These were quantitatively described for one inter-dam unit withi n each zone. Fish macrohabitat typology could be determined within eac h zone, but was much more difficult for the whole main stream. Beside water velocity, depth and substrate size, the relevant criteria were t he surface aspect and light-related vegetation. These results, obtaine d by Principal Componant Analysis and Cluster Analysis, permit conside ring a graduated sampling protocol.