Bull trout - Mid-Columbia Recovery Unit (Salvelinus confluentus pop. 3)

Sensitivity of bull trout is primarily driven by water temperature. Bull trout are the southern-most species of Western North American char and have lower thermal tolerance than other salmonids they co-occur with. The upper incipient lethal temperature for bull trout was found to be 21˚C, whereas the optimal temperatures for growth were in the range of 10-15˚C. Thus bull trout have a similar thermal optima to the salmonids they co-occur with, yet a lower thermal tolerance, indicating they have a narrower thermal niche and higher sensitivity to temperature. Indeed the geographic distribution of bull trout, and the persistence of populations during contemporary warming has been most strongly related to maximum water temperature. The ability of bull trout to persist in sub-optimally warm temperatures likely depends on food abundance. As temperature increases metabolic costs, the extent to which bull trout can maintain positive energy balance depends on its ability to find food. Bull trout historically relied heavily on salmon as a food resource and may be less resilient to temperatures in areas where foraging opportunities of salmon eggs and juveniles have declined. Invasive chars (brook and lake trout) now reside in many headwater streams and lakes, and may exclude bull trout from these potential coldwater refuges, increasing their sensitivity to warming. Bull trout sensitivity to flows is likely to occur during two critical periods: (1) direct effects of altered runoff timing and magnitude on emerging fry in late winter/spring, and (2) indirect effects of low summer flows on all life phases of bull trout by mediating the duration and magnitude of thermal stress events.