International audienceStream fish are expected to be significantly influenced by climate change, as they are ectothermic animals whose dispersal is limited within hydrographic networks. Nonetheless, they are also controlled by other physical factors that may prevent them moving to new thermally suitable sites. Using presence–absence records in 655 sites widespread throughout nine French river units, we predicted the potential future distribution of 30 common stream fish species facing temperature warming and change in precipitation regime. We also assessed the potential impacts on fish assemblages' structure and diversity. Only cold-water species, whose diversity is very low in French streams, were predicted to experience a strong reduction in the number of suitable sites. In contrast, most cool-water and warm-water fish species were projected to colonize many newly suitable sites. Considering that cold headwater streams are the most numerous on the Earth's surface, our results suggested that headwater species would undergo a deleterious effect of climate change, whereas downstream species would expand their range by migrating to sites located in intermediate streams or upstream. As a result, local species richness was forecasted to increase greatly and high turnover rates indicated future fundamental changes in assemblages' structure. Changes in assemblage composition were also positively related to the intensity of warming. Overall, these results (1) stressed the importance of accounting for both climatic and topographic factors when assessing the future distribution of riverine fish species and (2) may be viewed as a first estimation of climate change impacts on European freshwater fish assemblages

The real impact on downstream fish populations of a change in management of a hydropower installation, in particular, the impact of an increase in instream minimum flow, can be difficult to assess. Other factors, in addition to the simple habitat limitations in low-flow periods, can influence the evolution of a fish population over time. It has often observed in French streams that the 0þ individuals appear limited by strong discharge in the first months after their emergence (March to May). A dynamic model of a trout populations (MODYPOP) has been applied to the river Roizonne in the French Alps, upstream and downstream of a hydropower station, in order to estimate the effect of different discharge time series on trout population dynamics. Trout populations were monitored by means of annual inventories at two study sites, one with minimum flow and the other with natural flow. At each site, available habitat was described and simulated as it changed with discharge, according to the 'microhabitat' method (EVHA, derived from PHABSIM). Habitat time series were built on the basis of discharge time series. MODYPOP simulations, integrating habitat and discharge time series, were compared with observed fish numbers. Results indicated that fluctuations in discharge during the post-emergence period played a major role in the dynamics of the total population. Predictions were significantly improved when the impact of strong discharge on emerging fry was integrated in the model.