Population and Individual Responses of Rainbow Trout to Disturbance: An Integrated Analysis of Observation and Models

Management of aquatic resources in fire-prone areas requires understanding of factors that contribute to fish species’ response to the effects of 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 headwater streams a decade after a major wildfire: three unburned, three burned, and three burned with subsequent, massive channel-reorganizing events.  We then tested proposed drivers for observed differences among streams in an individual-based model designed to provide insight into rainbow trout demographic patterns under the characteristic thermal regimes of each disturbance type. Trout were present throughout disturbed streams at densities comparable to unburned streams, displaying resilience to long-term habitat alterations caused by wildfire, including warmer temperatures. 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.  Rainbow trout in burned streams show patterns of fast growth, low lipid content, and early maturity; these patterns were most dramatic for the warmest streams.  Our modeling experiment suggests that moderate warming associated with wildfire and channel reorganizing leads to faster individual growth, which exacerbates competition for limited food, leading to decreases in overall population densities. The model results provide an intuitive mechanistic explanation for the trends we observed in fish growth and density among streams with contrasting disturbance histories, and the inferred mechanisms suggest the transferability of ecological patterns to a variety of temperature disturbance scenarios.