The great utility of running waters has resulted in the extensive exploitation of streams and rivers throughout the world, a process greatly facilitated by the construction of hundreds of thousands of dams globally. It is well established that dams have a fragmenting effect on the landscape by blocking the movement of individuals between essential habitats required to complete their life cycle (e.g., salmon species in the Pacific Northwest). Moreover, many dams throughout the world have dramatically modified downstream environments by selectively releasing cold water from reservoirs in order to establish productive trout fisheries in the tailwater reaches of the dammed river. Our recent research in southeastern United States shows that the conversion from warmwater to coldwater habitats can result in significant losses to fish populations in the tailwaters, whereas the adjoining, warmwater tributaries provide thermal refugia and contain extremely dense and diverse fish assemblages. These results led to the hypothesis that coldwater-release dams may have an additional fragmenting effect by restricting fish movement through the cold tailwaters, which over time could lead to the genetic differentiation of the tributary fish populations. We address this hypothesis by examining the genetic structure of 3 fish species that exhibit different thermal preferences and mobility's in a series of tributaries that are separated by different tailwater temperatures and geographic distances. This is the first examination of how human-induced thermal barriers to movement may shape the genetic structure of fish populations in fragmented tributaries, and has important implications for focusing future management and conservation efforts aimed at improving the thermal connectivity of warmwater tributaries in tailwater environments in order to ensure the long-term persistence of warmwater fish assemblages.
