Assessing the Effect of Flow Regimes on Fish Density

Human activities such as hydro-electric power production are known to modify the temporal periodicity of river discharge (i.e. their flow regime). Flow regime may have direct influences on fishes as well as trigger complex cascades of interactions involving different features of the river ecosystem such as sediments, water, vegetation etc. To assess the effect of different flow regimes on river ecosystems, we performed analyses of (total) fish count density (fish m^-2) and biomass density (g m^-2). Data were collected in 989 sites located along 28 Canadian rivers. Each site (300 m²) was observed (snorkeling) and fished (electro-fishing) and variables that were expected to influence local fish density (e.g. water velocity, depth, substrate composition) were taken. We also accounted for spatial pseudo-replication, which is an inherent feature of observational studies spanning a broad geographic range (3 497 km) and involving repeated local measurements (25-50 sites / river). In models, we represented the influence of the flow regimes using either 1) a qualitative variable (levels: natural, run-of-the-river, or storage or peaking) or 2) a quantitative variable based on hydrogram's frequency spectra. Preliminary results using qualitative flow regimes suggest that peaking rivers have lower biomass densities than for the other flow types.