VARIABILITY OF MESOHABITAT USED BY BROWN TROUT POPULATIONS IN THE FRENCH CENTRAL PYRENEES

The variability of mesohabitat used by populations of brown trout Salm o trutta was studied in seven rivers of the central Pyrenees, France. Physical habitat preferences of individuals, intraspecific competition , season, and diversity of adjacent channel unit segments explained th e trout density variation among different types of mesohabitat. Total trout density, size-class densities, and redd densities differed signi ficantly among mesohabitats and 18 subgroups of a hierarchical classif ication. The total density followed a gradient from deep channel morph odynamic units to shallow rapids including step channel, glides, riffl es, and cascades. The different size-classes of trout expressed a stro ng spatial segregation between fry and adult stages. Fry were more oft en found in riffles and glides, whereas adults concentrated in deep ha bitats. During the reproduction period, the highest redd densities wer e observed in glides and riffles. The density of trout in deep mesohab itat increased in relation to the diversity of mesohabitat type and th e percentage of riffles in the adjacent segment. These results clearly showed the importance of the diversity of hydrodynamic conditions for the life cycle of brown trout in mountain streams.