STUDY OF THE HABITAT OF BROWN TROUT (SALM O-TRUTTA FARIO) - ROLE OF ARTIFICIAL AND NATURAL COVERS ON TROUT POPULATIONS

The aim of this study was to quantify the role of various cover types and examine their selection by brown trout. The study was developed on two river course sections in the Pyrenees : the river PIQUE (altitude 920 m NGF) and the river GER (altitude 390 m NGF), situated in the de partment of the Haute-Garonne (31). For the natural cover, we have exp erimented on three sets which have been transitionnally blocked by wir e netting ; rocky river bank, undercut bank and woody debris. This has been performed on the river GER. Initially, the sector, for all these types of covers, had an average density of 2469 ind/ha for a biomass of 278 kg/ha. When the access to the covers is protected, the density stabilises to 1000 ind/ha, being a decrease in the order of 55% in den sity and biomass. According to the type of cover, the recolonisation, after 21 days of reopening of the covers, is estimated between 28 and 72% for the biomass and between 42 and 62% lor the density. For the ar tificial covers, six types of building materials (brick, tile ... ) ha ve been experimented on the river PIQUE section (all natural covers ha ving been removed). By type of covers and their location, the trout de nsity varied from 3 to 23 ind/M2 of cover, and from 0.09 to 0.2 ind/M2 outside cover. Among the material used, we observed a clear preferenc e for brick and, depending on the dispositions tested, the best result s have been obtained with the covers placed near the banks, or being i n the totality of the bed. The experiment has shown the major role the natural and artificial covers have on trout populations.