Summer Temperature and Bull 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).

The thermal characteristics of Bull Trout habitat in Alberta were explored by comparing mean warmest month temperature (MWMT) derived using the program Climate WNA© (Hamann and Wang 2005; Wang et al. 2006) to all locations where Bull Trout have been captured between 1946–2013 (FWMIS query, Nov. 2013). Air temperature was used in this analysis because there is currently no province-wide water temperature dataset or model available. Of note, updated spatial stream network (SSN) temperature models are currently in development, but were not yet complete for use in this version of the Joe model. We expect that refinements to the stream temperature model stressor-response curves and watershed input values will occur once these models are complete. In addition, air and water temperatures are typically correlated over time scales >1 week (Mohseni et al. 1998). The minimum and maximum air temperature thresholds (10°C and 17°C; Figure 1) were similar to those reported in previous laboratory and field studies investigating the effects of water temperature on Bull Trout growth and survival (Selong et al. 2001) and occupancy (Dunham et al. 2003; Wenger et al. 2011). The findings of this analysis were used to inform the shape of the stressor-response curve below, which characterizes the expected influence of warm temperatures on the system capacity of Bull 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.