Fire is a key natural disturbance that affects the distribution and abundance of native fishes in the Rocky Mountain West. In the absence of migratory individuals from undisturbed portions of a watershed, persistence of native fish populations depends on the conditions of the post-fire stream environment. Stream temperatures typically warm after fire, and remain elevated until riparian vegetation recovers. An additional threat to native species is that nonnative fishes have invaded many waters, and these species tolerate or prefer warmer water temperatures. Thus, forecasting the long-term effects of fire on native fish populations requires an understanding of fire dynamics (size, distribution, frequency, and severity), the extent and location of changes in riparian forest structure and time to recovery, changes in stream temperatures associated with these forest changes, and how native and nonnative fish respond to changes in water temperature. To perform spatially explicit simulation modeling that examined the relations among fire disturbance, stream temperature, and fish communities, we upgraded and then linked the fire-forest succession model FireBGCv2 to a stream temperature model to project changes in water temperature in the East Fork Bitterroot River basin in Montana under an array of climate and fire management scenarios. Model projections indicated that although climate led to increases in fire severity, frequency, or size, water temperature increases at the basin scale were primarily a consequence of climate-driven atmospheric warming rather than changes in fire regime. Consequently, variation in fire management—fuel treatment or fire suppression—had little effect at this scale, but assumed greater importance at the scale of riparian stands. By revisiting a large number of previously sampled sites in the East Fork Bitterroot River basin in Montana, we evaluated whether bull trout persistence and other native and nonnative fish distributions were related to temperature changes associated with fire and recent climatic trends. Although fires were related to marked increases in summer water temperatures, these changes had a positive effect (westslope cutthroat trout) or a negligible effect (bull trout) on the abundance and distribution of native fish species, whereas the abundance of nonnative brook trout markedly declined in some instances. Fire-related changes in factors other than the thermal regime may have contributed to these patterns. In contrast, at the scale of the entire basin we observed an upward-directed contraction in the distribution of bull trout that was unrelated to fire. We concluded that this may be a response to temperature increases related to climate change.
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