Temperature preferences of Athabasca Rainbow Trout (Oncorhynchus mykiss) across a gradient of thermal landscapes.

Stream temperature is a key driver of physiological function in ectothermic fish, shaping the limits of thermal habitat. Temperature increases from climate change and habitat degradation are a major threat to federally Endangered Athabasca Rainbow Trout (Oncorhynchus mykiss Walbaum, 1792; ARBT) in Canada. This study investigated thermal preferences of ARBT across three streams spanning a natural temperature gradient (7.1–15.0°C mean July temperature) in the McLeod watershed, Alberta, Canada, at the northern limit of Rainbow Trout. Shuttle box experiments were used to assess preferred (Tpref) and avoidance (Tavoid) temperatures using a portable streamside laboratory, where Tpref is thought to represent the optimal temperature zone for metabolism and growth. The mean Tpref was 18.0 °C, higher than expected for a northern Rainbow Trout ecotype adapted to colder headwater streams. Significant variation in Tpref and lower Tavoid values across streams suggests a degree of thermal plasticity in response to different stream acclimation temperatures. These unexpected findings suggest that northern populations may not necessarily be differentially cold-adapted. However, compounding threats and the cumulative impacts of stream warming still pose significant risks to this endangered ecotype. Understanding ARBT’s thermal behavior is crucial for developing habitat protection and conservation policies as climate change intensifies.