Bull trout require the coldest water of all species native to the Rocky Mountains (summer temps less than 58 degrees F and spawning temps less than 48 degrees) and are therefore a leading indicator species for aquatic ecosystem health within the Rocky Mountain region. They spawn in the fall and often migrate long distances to lay their eggs in loose, silt-free gravels. Both juveniles and adults are often found in or near deep pools and overhead cover such as logjams, boulders or undercut banks, where they find protection from predators in cool water temperatures.
Because of their dependency on cold, clean water and pristine habitat, bull trout have been largely extirpated from much of their historic range. The best bull trout habitat occurs within large watersheds that have cold, clean water. Especially important are unrestricted well-vegetated floodplains, meaning that stream channels have room to move and create new pools, side channels and other habitat features. Therefore, current bull trout strongholds are often within or downstream from protected wilderness and roadless areas, where dams, road building, grazing, logging and other landscape impacts are limited or nonexistent.
As late summer flows are affected by global warming, fewer rivers will be able to provide ample cold water for bull trout. Bull trout distribution is also related to air temperature, so the heightened ambient air temperatures of the bull trout’s habitat caused by global warming are reducing their survivable habitat. The warming climate also affects precipitation and timing in the Rockies, which is predominately driven by snowfall and snowmelt. The timing and duration of spring runoff could dramatically affect stream temperatures, habitat creation, and therefore the spawning activities of the bull trout.