Determining the upper thermal tolerance of Athabasca Rainbow Trout (<i>Oncorhynchus mykiss</i>) across naturally varying stream temperatures

Stream temperature is a key driver of physiological function in ectothermic fish, and fish have clear upper and lower limits to thermal habitat use. Stream temperature increases from climate change are a major threat to coldwater taxa like Athabasca Rainbow Trout ( Oncorhynchus mykiss), listed as Endangered under Canada's Species at Risk Act. To better understand their vulnerability to climate change and prioritize recovery locations, Athabasca Rainbow Trout were collected in August 2021 from three streams in the upper Athabasca River, Alberta, Canada, across a gradient of thermal regimes ranging from cold to warm. Individual thermal tolerance was measured using two physiological metrics: agitation temperature ( T ag ) and critical thermal maxima (CT max ) using a portable streamside laboratory. T ag is a behavioural indicator of a thermal avoidance threshold, whereas CT max is a physiological response that can be interpreted as the upper thermal limit where long term survival is reduced. Results provide strong evidence that CT max is a plastic metric in Athabasca Rainbow Trout; however, T ag is not. This suggests that Athabasca Rainbow Trout demonstrate some thermal plasticity in terms of CT max , yet the lack of plasticity in T ag indicates potential limitations in their adaptability to warming stream temperatures.