Responses of fish communities to longitudinal thermal gradients in coldwater, lake outflow streams

Longitudinal water temperature gradients in streams are widespread and are associated with changes in fish community structure. A major cause of these changes is variation in the thermal tolerances and preferences of coldwater, coolwater, and warmwater fish species. Much less is known about water temperature gradients in lake outflow coldwater streams and associated variation in fish communities. Our main objective was to assess longitudinal temperature gradients in lake outflow streams and to measure the corresponding variation in fish community structure. We quantified temperature gradients and the fish communities in two lake outflow streams in central Wisconsin from June through October 2019. We measured longitudinal water temperature gradients spanning up to 6 °C or more during the summer but the gradients weakened in late summer and were not present in autumn. Warmwater and coolwater species such as rock bass ( Ambloplites rupestris ), bluegill ( Lepomis macrochirus ), and creek chub ( Semotilus atromaculatus ) were more common in reaches nearest to lake outflows in both streams. Coldwater species such as brown trout ( Salmo trutta ), brook trout ( Salvelinus fontinalis ), and mottled sculpin ( Cottus bairdii ) were more common in downstream reaches. Increasing water temperatures in lakes due to global climate change will likely increase the magnitude of longitudinal temperature gradients in coldwater streams that are fed by warmwater lakes, which will probably impact the distributions of coldwater fish species in these systems.