Summer Temperature and Westslope Cutthroat Trout

Each of the three native trout species are thermally sensitive and vulnerable to increased water temperature resulting from land disturbance and climate change (GOA 2023; Alberta Athabasca Rainbow Trout Recovery Team 2014; The Westslope Cutthroat Trout Recovery Team 2013).
Westslope Cutthroat Trout is a thermally sensitive cold-water salmonid vulnerable to increased water temperatures resulting from land disturbance and climate change (ASRD and ACA 2006). The thermal characteristics of Westslope Cutthroat Trout habitat in Alberta were explored by comparing modelled mean summer stream temperatures to all locations where cutthroat trout have been captured between 1946-2016 (FWMIS query, April 2017). In addition, we also compared stream temperatures across Westslope Cutthroat range. The stream temperature model used was created using a step-wise multiple linear regression model. This incorporated mean summer (August and September) air temperature and land use co-variates found in ALCES Online© to create a basin specific (Oldman River or Bow River) model (MacDonald and Jones 2017). The mean water temperature thresholds (8-18°C) were similar to those reported in previous laboratory and field studies investigating the effects of water temperature on Westslope Cutthroat Trout hybridization (e.g. Muhlfeld et al. 2009) and growth and survival (e.g. Bear et al. 2007). The findings of this analysis were used to inform the shape of the dose-response curve below, which characterizes the expected influence of warm temperature on the sustainability of Westslope Cutthroat Trout populations (Figure 1).
The rapid decline in the number of occurrences of the three trout species on the colder ranges, between 10°C to 13°C for Bull Trout, 6°C to 8°C for Athabasca Rainbow Trout and 8°C to 13°C for Westslope Cutthroat Trout is likely due to sampling bias (i.e., there are fewer sampling events in cold, high-elevation areas that are difficult to access). However, in visually inspecting stream temperatures in colonized Jasper National Park streams with excellent Athabasca Rainbow Trout populations they fall within the modelled 5-8°C temperature range. Therefore, lotic habitats with these colder temperatures were still considered to be excellent Athabasca Rainbow Trout habitat and while cold temperatures undoubtedly can limit Bull Trout and Westslope Cutthroat Trout distributions, for the first iteration of this dose-response curve the results were not extended to the cold side of the curve due to lower confidence in this portion of the analysis.