Biotic and Abiotic Associations with Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi) in the North Fork Flathead River Basin in northwestern Montana, USA and southeastern British Columbia, CAN under current and future climate scenarios.

Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi; WCT) populations are declining across much of their native range due to threats such as habitat degradation, competition with non-native species, and climate change. Understanding how habitat characteristics impact distributions of nonhybridized WCT populations throughout a relatively pristine core conservation area is needed to inform management and conservation efforts. We investigated whether abiotic (e.g., gradient) and biotic (i.e., Bull Trout – Salvelinus confluentus) variables predicted WCT presence and predicted how future stream temperature projections for the area might be expected to alter distributions. We compared logistic regression models of WCT presence throughout tributaries of the North Fork Flathead River in Montana, USA and British Columbia, CAN models using a variety of metrics (e.g., Akaike Information Criterion). WCT were widespread throughout the 293 reaches analyzed (present in 69.3% of reaches). Their presence was predicted by gradient, summer temperature, and an interaction of pool density and Bull Trout. Using this regression model and climate projections under both moderate and extreme emissions scenarios, WCT presence is predicted to increase by 13.0% and 14.1% respectively in 2075 from current distributions based on changes in temperature alone. When changes in Bull Trout distributions and temperatures are considered, WCT distributions are predicted to increase by 13.4% and 17.5% under the moderate and high emissions scenario, respectively. This conservation area is predicted to continue to serve as a WCT stronghold, if other threats can be contained. Introduction Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi; hereafter WCT) is a native salmonid species of conservation concern in the Rocky Mountain west as its populations are declining across much of their native range due to a number of threats such as invasive hybridization with introduced Rainbow Trout Oncorhynchus mykiss; RBT), habitat degradation and fragmentation, competition with non-native species, and climate change (Muhlfeld et al. 2016). Many of these threats are interrelated and their impacts on WCT can be additive. For example, in addition to decreasing the amount of suitable habitat available to WCT based on physiological tolerances (Wenger et al. 2011), warming stream temperatures can also accelerate invasive hybridization with RBT (Muhlfeld et al. 2014). Nonhybridized populations of WCT make up a small portion of their native range, less than 10% in the United States and less than 20% in Canada (Muhlfeld et al. 2009). Habitat loss from activities such as dam construction, irrigation diversions, and improperly sized culverts, has occurred throughout WCT’s historical range (Young 1995). This fragmentation is especially detrimental to the migrant members of WCT populations by restricting their ability to move through river systems. Additionally, Young (1995) suggests that the loss of these migratory life forms within a WCT population (via a lack of gene flow between migratory and resident trout) may decrease the persistence of these populations across their range. As we work to conserve WCT, it is critical to understand how habitat characteristics (e.g., stream temperature) impact distributions of nonhybridized populations of WCT throughout relatively pristine ecosystems (e.g., North Fork Flathead River system) to inform management and conservation efforts for