Seasonal Microhabitat Use of Young-of-the-Year Bull Trout, Salvelinus Confluentus, in a Drought Year

This study examined the microhabitat use of young-of-the-year (YOY) bull trout, Salvelinus confluentus, throughout the summer and fall of 2015 in Gold Creek, a third order Cascade Mountain stream near Snoqualmie Pass, WA. It is crucial to understand YOY habitat requirements, as this life stage is highly vulnerable to fluctuations in streamflow and various ecological risks associated with their dispersal. Previous research has demonstrated that YOY salmonids often occupy stream margins where they are sheltered from these kinds of threats. However, in the face of climate change, Pacific Northwest streams are more susceptible to decreased summer streamflows and elevated, irregular winter streamflows. Thus, preference for stream margin habitats by YOY salmonids may be detrimental in areas characterized by more severe, flashy hydrologic regimes. Juvenile and adult bull trout have been shown to exhibit some of the most stringent requirements for cold-water, complex habitats of any salmonid in the Pacific Northwest, but little is known about the YOY life stage. This study attempted to expand on the body of literature on YOY bull trout and their habitat selection. Microhabitat variables including bottom velocity, water depth, distance to the nearest shoreline, substrate composition and presence or absence of various fish cover types were evaluated to characterize microhabitat preferences of YOY bull trout. YOY were observed inhabiting shallow stream margins with little or no detectable flow. A generalized linear mixed model was developed to predict the probability of presence of YOY bull trout in response to microhabitat variables. Overall, YOY bull trout in Gold Creek were shown to have very specific habitat requirements relative to the stream habitat available. The optimal model suggested that bottom velocity, distance to the nearest shoreline and an interaction between depth and season were some of the most significant predictors of presence of YOY bull trout. The results from this study demonstrate the importance of critical rearing habitat for highly vulnerable YOY bull trout