EFFECTS OF REDUCED FLOW ON BROWN TROUT (SALMO-TRUTTA L) POPULATIONS DOWNSTREAM DAMS IN FRENCH PYRENEES

The effects of 16 hydroelectric power plants, in operation for 75 year s, were studied in 15 non-polluted Salmonid streams in the Pyrenean mo untains. The populations of brown trout (Salmo trutta L.) and the phys ical habitat characteristics were compared between two sections, one u pstream of the dams (control section) and one below the dams with cons tant reduced how (residual section). The average velocity, depth and a rea of cover decreased significantly below the dams. Similarly, the to tal abundance of brown trout calculated per linear metre of stream dec reased at nine sites for biomass and eight sites for densities. The re ductions of biomass and densities per unit area were less at seven sit es. The abundances of the main stages of brown trout were modified dif ferently; adults were affected more than fry by constant reduced flow. The modifications of biomass and densities of the age classes were si gnificantly related to the instream flow below the dam, expressed as a proportion of the mean annual flow at the control section. Similarly, the differences of total, adult, juvenile and fry abundances between the control and residual section were significantly related to the dif ferences in weighted usable area (WUA), average depth, average velocit y and area of cover. A multiple linear regression model using differen ces of WUA and area of cover explained 84% of the difference of biomas s per linear metre of stream and 68% of the difference of density. Est imating the relative capacity of a stream to support fish after a redu ction in flow by measuring the variations of WUA is a promising approa ch for predicting the development of brown trout populations. The resu lts are discussed in terms of modifications and the relative capacity of a stream to support a brown trout population.