Response of 53 Brown Trout Populations to Various Degrees of Hydroelectric Influence Under Different Habitat Conditions

Freshwater ecosystems have a high degree of heterogeneity resulting from many natural contexts (regimes of flow, sediment and temperature). These regimes control the biological responses at different spatial and temporal scales. In these hierarchically organized and nested systems, large scale processes govern the physical and biological dynamics on a smaller scale. This whole ecological context must be accounted for when studying the relative impact of hydroelectric facilities on freshwater ecosytems. The dams presence and operating interfere to different degrees with the hydrophysical environment. Up to now, impacts of dams have been widely studied on a local scale (habitat models, dynamic population models). Control mechanisms for Brown trout populations have been identified through local extensive site analysis. In order to learn more general lessons on trout populations functioning on a larger scale, the aim here was to identify different "filters" that would explain observed biological responses, and any distortion created by the presence and operating of hydroelectric facilities. An analysis of relationships between trout populations and their physical habitat characteristics was conducted using 26 monitoring stations located in by-passed sections. A similar analysis was conducted on control stations (n=27). Results were compared to assess the relative impact of hydroelectric powerplants. These analyses showed several structuring factors. In agreement with the habitat templet theory (Southwood, 1977) which argues that habitat characteristics shape the biological traits of species occupying this habitat, the influence of different parameters was tested. It showed that stream size and position in the network summarised by the width is the first filter level explaining the variability of observed densities. The second filter is relative to the hydrological dynamics: We have highlighted the role of floods (absolute value) on recruitment. The populations in bypassed sections appear to have comparable resilience as in natural systems, but their resistance to a given perturbation appears to be less strong, since we have been able to show a gradation of the effect of floods between sites in bypassed sections, as opposed to control sites. Furthermore, the severity of breakdown in river continuity estimated by the size of dam affects variability of juveniles abundance. Finally, downstream flow released in the bypassed section partly determines trout abundance. These results have enabled determining the relative importance of deviation in population density due to different types of habitat conditions and degree of hydroelectric influence.