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

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Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi; hereafter WCT) is an economically and ecologically important trout subspecies in western North America. WCT populations are declining across much of their native range due to a number of threats such as habitat degradation and fragmentation, competition with non-native species, and climate change. Understanding how habitat characteristics (e.g., stream temperature) impact distributions of nonhybridized populations of WCT throughout relatively pristine ecosystems is needed to inform management and conservation efforts for WCT. We analyzed field habitat data, fish presence/absence data, and temperature predictions in a mixed effects logistic regression analysis of WCT presence throughout tributaries of the North Fork Flathead River in Montana, USA and British Columbia, CAN. We analyzed a host of biotic (Bull Trout – Salvelinus confluentus) and abiotic (average stream gradient, pool density, large woody debris density, and August mean stream temperature predictions) factors to see how they affected WCT presence throughout the study system. We compared 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). WCT presence was predicted by gradient, August mean stream temperature, and an interaction of pool density and Bull Trout. To predict future presences, we removed potential habitat for Bull Trout and WCT if it exceeded mean August temperatures of 14°C and 18°C, respectively. Given our negative reach scale associations of Bull Trout and WCT, we assumed that if Bull Trout were extirpated WCT would move into the reach if it did not exceed WCT thermal tolerances. Using this regression model and climate projections under both the moderate and extreme emissions scenarios, WCT presence is predicted to increase by 5.2% and 7.8% respectively in 2035 from current distributions. Thus, the North Fork Flathead River basis is predicted to continue to serve as a WCT stronghold, if other threats (e.g., conflict with non-native species) can be contained.