Mechanisms for Resilience of Rainbow Trout in the Boise National Forest to Wildfire

Management of aquatic resources in fire-prone areas requires understanding of factors that contribute to fish species’ response to wildfire and associated disturbances. We sought insight into these factors using an approach that pairs empirical observation with a mechanistic simulation.  We examined population and individual characteristics of rainbow trout in nine streams a decade after a wildfire: three unburned, three burned, and three burned with channel-reorganizing events.  We tested drivers for observed differences among streams in an individual-based model of rainbow trout demographic patterns under the thermal regimes of each disturbance type. Trout displayed resilience to long-term habitat alterations caused by wildfire. Older age classes were less abundant in burned and reorganized streams than in burned and unburned streams, whilst age 1+ individuals were most abundant in unburned streams.  Trout in burned streams show patterns of fast growth, low lipid content, and early maturity.  Our modeling experiment suggests that warming associated with wildfire and channel reorganizing leads to faster individual growth, which exacerbates competition for food, leading to decreases in population densities. The results provide a mechanistic explanation for the trends we observed in fish growth and density and suggest the transferability of ecological patterns to other temperature disturbance scenarios.