Predicting the Potential Impacts of Climate Change on Stream Fish Assemblages

—Stream fish are expected to be influenced by climate change as they are ectothermic animals living in lotic systems. Using fish presence–absence records in 1,110 stream sites across France, our study aimed at (1) modeling current and future distributions of 35 stream fish species, (2) using an ensemble forecasting approach (i.e., several general circulation models [GCM] × greenhouse gas emission scenarios [GES] × statistical species distribution models [SDM] combinations) to quantify the variability in the future fish species distribution due to each component, and (3) assessing the potential impacts of climate change on fish species distribution and assemblage structure by using a consensus method that accounted for the variability in future projections. We found that future projections of fish species distribution were relatively consistent among GCM × GES × SDM combinations, with 57% of the total variability between projections being consensual. The statistical method used was the main driver of the variability between future projections, accounting for 70% of the total variation. The projections were next influenced by the GCMs, whereas the importance of GES was weak. Nonetheless, both the amount of consistency among projections and the relative contribution of each uncertainty component to the variability in projections were different depending on the species considered. Concerning the impacts of climate change, only the scarce coldwater species (e.g., brown trout Salmo trutta fario) were predicted to experience a strong reduction in their distributional area, whereas most coolwater and warmwater fish species (e.g., barbel Barbus barbus, European chub Leuciscus cephalus) were predicted to colonize many newly suitable sites located in intermediate streams or upstream. As a result, local species richness was forecasted to increase greatly, and high turnover rates indicated fundamental changes in the structure of assemblages in the future. Moreover, we found that climate change could result in remarkably different impacts on the structure of fish assemblages depending on their position along the upstream– downstream gradient. These findings may be viewed as a first estimation of climate-change impacts on European freshwater fish biodiversity. They also illustrate the need to account for different sources of uncertainty when estimating the potential impacts of climate change on species distribution modifications. * Corresponding author: [email protected]