Influence of Fire on Native and Nonnative Salmonid Populations and Habitat in a Western Montana Basin

Anticipated increases in the frequency and severity of wildfire may threaten the persistence of native salmonid populations in headwater streams in western North America. This study used extensive pre‐ and postfire data to assess whether wildfire leads to hypothesized declines in native westslope cutthroat trout Oncorhynchus clarkii lewisi and bull trout Salvelinus confluentus populations along with increases in the prevalence and abundance of nonnative brook trout S. fontinalis . Postfire cutthroat trout density was negatively correlated with the proportion of basin area that burned at moderate to high severity, but the declines in density after fires were less pronounced for bull trout and brook trout. Recovery of cutthroat trout was generally rapid in severely affected reaches. Contrary to expectation, there was no evidence of a marked increase in abundance or invasion by brook trout after wildfire. Brook trout exhibited the most severe declines in debris flow–affected reaches among all species and exhibited less recovery in severely burned reaches than did cutthroat trout. Increased stream temperature was the most significant habitat change that followed wildfire, the mean maximum water temperature during summer months increasing by 2–6°C in severely burned reaches. In contrast, burned area percentage was unrelated to large woody debris density, the percentage of surface fines, substrate diversity, or the percentage of pool habitat. The characteristically high variability in fish and habitat responses to wildfire will continue to pose a challenge for the understanding and management of fire in aquatic ecosystems.