Dan Isaak

US Forest Service
Rocky Mountain Research Station

Co-authors: Bruce Rieman, Charlie Luce

Abstract: Past declines in bull trout populations related to harvest, habitat degradation, and invasive species resulted in ESA listing in 1998. The potential effects of a warming climate were not considered in the original listing, but recognition that environmental changes associated with this phenomenon may exacerbate or exceed previous threats is growing. Bull trout could be especially vulnerable to a warming climate, given a strong dependence on cold water temperatures, fragmented population structures, and confinement to headwater streams that are vulnerable to periodic disturbance. We are engaged in several projects to understand the effects of climate on bull trout within the Interior Columbia Basin. In recently published work, a strong correlation between spatial trends in mean annual air temperature and contemporary bull trout distributions was documented. Model projections based on this relationship suggest substantial and spatially heterogeneous habitat losses (40 – 60%) by mid-century. Other work describing trends in stream temperatures indicate losses of thermal habitat may already be occurring. From 1993 - 2006, we estimate that average summer stream temperatures increased by 1.4°C within a 6th-order central Idaho watershed and the stream length of suitable summer habitat decreased by 45% in response to recent climate trends. If trends continue, bull trout populations should begin to contract and work has begun to monitor distributions. Results from initial resurveys of streams originally sampled in 1997 revealed no detectible shift, but biological responses could lag environmental trends due to interannual climate variability and intergenerational constrains on extinction/colonization dynamics. Bull trout within the face an uncertain future and more research is needed to understand how distributions may change, which environmental features are most important, and how conservation resources are best allocated. Until more is known, enhancing the adaptability and flexibility of existing populations by maintaining diverse, large, and well-connected habitats should remain a priority.

Video Length: 20 Minutes, 26 Seconds

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The eastside experience: bull trout and climate change within the interior Columbia Basin