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 years, were studied in 15 non-polluted Salmonid streams in the Pyrenean mountains. The populations of brown trout (Salmo trutta L.) and the physical habitat characteristics were compared between two sections, one upstream of the dams (control section) and one below the dams with constant reduced how (residual section). The average velocity, depth and area of cover decreased significantly below the dams. Similarly, the total abundance of brown trout calculated per linear metre of stream decreased at nine sites for biomass and eight sites for densities. The reductions of biomass and densities per unit area were less at seven sites. The abundances of the main stages of brown trout were modified differently; adults were affected more than fry by constant reduced flow. The modifications of biomass and densities of the age classes were significantly 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 differences in weighted usable area (WUA), average depth, average velocity and area of cover. A multiple linear regression model using differences of WUA and area of cover explained 84% of the difference of biomass per linear metre of stream and 68% of the difference of density. Estimating the relative capacity of a stream to support fish after a reduction in flow by measuring the variations of WUA is a promising approach for predicting the development of brown trout populations. The results are discussed in terms of modifications and the relative capacity of a stream to support a brown trout population.