Projected potential fish distribution under climate change in a heavily regulated, fragmented river system

Climate change and dam construction greatly impact fish distribution. Capturing dam impacts, especially dam-induced changes in flow and thermal regimes, on environmental suitability for freshwater species remains a challenge at regional-scales. So far, regional fish distribution models have mostly relied on air temperature as a proxy for river temperature to estimate environmental suitability. Studies that used river temperature directly largely ignored the impact of dam-induced thermal stratification on downstream temperatures. To address this challenge, we combined a hydrologic modeling approach that explicitly accounts for the effects of river regulation on flow and temperature with a maximum entropy based species distribution model. We also accounted for stream network fragmentation resulting from dam blockage. We used this approach to evaluate potential fish distribution in the highly regulated Tennessee River system in the southeastern United States. In particular, we projected the climate-induced changes in environmental suitability for exotic coldwater rainbow trout [Oncorhynchus mykiss], stocked for sport fisheries, and for endemic coolwater blackfin darter [Etheostoma nigripinne], an indicator species with ecological importance. Less than 10% of historically suitable streams for rainbow trout will remain by the 2080s (2070-2099), mostly located at cold reservoir tailwaters. For blackfin darter, changes in river flow and thermal regimes may increase the environmental suitability of some river segments, which however can be inaccessible due to dam blockages. In addition, we used the Shapley decomposition from cooperative game theory to determine the main driver of changes in environmental suitability. Changes in river temperature rather than flow regime form the primary driver of changes in environmental suitability for both species. In conclusion, this study combined a state-of-art hydrologic modeling approach and a species distribution model, aiming to better inform the decision-making in freshwater conservation in a heavily regulated river system.