Brook Trout (Salvelinus fontinalis) and Brown Trout (Salmo trutta) summer thermal habitat use in streams with sympatric populations.

This study addresses the need to examine the thermally mediated interactions of fish with their natural environment by investigating the shift in thermal habitat occupation for sympatric Brook and Brown Trout populations. We observed upstream Brook Trout and Brown Trout population shifts during the summer, with thermal habitats showing an increased number of Brown Trout, while some sites also displayed a decrease in the number of Brook Trout. Overall, there was an increased incidence of overlapping habitat occupation at the end of the summer. Brown Trout occupied optimal resting and feeding thermal habitat locations, which can potentially affect growth rates and Brook Trout's survival. Population shifts did not occur at elevated water temperatures as expected but seem driven by temperatures that are optimal for growth. Observed population shifts can lead to increased interactions between the two species throughout the summer. The results provide a better understanding of how future, longer-term, thermal habitat modifications may modify species interactions, which are critical for salmonid conservation efforts.