Wildfire size and frequency are increasing and may be exceeding historic norms. Populations of native fishes may not be capable of recovering from unprecedented wildfires, especially where human activity has reduced habitat quality and connectivity. We used a population viability analysis to predict the probability of persistence of Bonneville cutthroat trout Oncorhynchus clarkii utah in steep watersheds of south-central Utah, USA. Treatment watersheds were burned in 2010 in the Twitchell wildfire, which was followed one year later by intense rainfall, large floods, and debris flows. Abrupt changes in channel morphology and water quality severely reduced or extirpated trout populations from entire watersheds. We represented a spatially-structured population of Bonneville cutthroat trout based on known habitat preferences, including the presence of natural and artificial waterfalls (fish barriers). Together with burn severity data, we used measurements of valley confinement, channel morphology, and distribution of debris flows (from high-resolution aerial lidar) to predict stream reaches where trout were extirpated after the Twitchell Fire. We combined these physical and biological habitat characteristics in a stochastic, stage-based matrix model to predict the probability that this spatially-structured fish population would persist after a suite of future wildfire scenarios.
Results/Conclusions
We demonstrate that fire occurring in synchrony across multiple watersheds forced Bonneville cutthroat trout populations past a quasi-extinction threshold in 45% of iterations; asynchronous fire caused extinctions in 0% of trials. Local extirpations occurred most frequently in confined valleys downstream of debris flows; fish barriers permanently reduced carrying capacity by preventing immigration to previously occupied reaches. Our results indicate spatial heterogeneity in fire severity and vital rates increase population viability; barriers and valley confinement decrease viability. This information expands our understanding of the relationship between fish and fire to include inland watersheds containing numerous imperiled fish species.