Changes in native and nonnative fish assemblages and habitat following wildfire in the Bitterroot River basin, Montana

Abstract

Wildfire frequency and severity have increased over the past decade but few studies have assessed the effects of large, intense fires on native salmonids in the Intermountain West. I utilized a unique data set with 1-11 years of pre-fire population data in 24 small streams in the Bitterroot River basin in western Montana to determine if habitat changes caused by a large (1,108 km 2) wildfire and associated debris flows favored nonnative brook trout Salvelinus fontinalis over native westslope cutthroat trout Oncorhynchus clarkii lewisii and bull trout S. confluentus. I used before-after control-impact (BACI) and extensive post-treatment study designs to determine whether changes in species abundance and habitat increased with increasing burn severity and debris flows. Species abundance was estimated pre- and post-fire with mark-recapture electrofishing and habitat conditions post-fire were assessed by measuring substrate, temperature, large woody debris, and habitat type. Stream temperature and sedimentation generally increased with burn severity whereas habitat complexity decreased with increasing burn severity and presence of debris flows. However, recovery of native trout populations was rapid with populations approaching or surpassing predisturbance levels within three years. In contrast, brook trout recovery was less apparent especially in debris flow reaches as the proportion of brook trout to the total salmonid assemblage decreased each year post-fire. However, one notable exception occurred in a high burn severity reach on Rye Creek, where brook trout increased by 499% and apparently replaced bull trout. Model results indicated that brook trout abundance was negatively related to stream gradient, elevation, and the proportion of a basin that was burned and positively related to watershed area, water temperature, and pool frequency. Spread of nonnative species to reaches where undetected pre-fire (n = 7) occurred irrespective of wildfire disturbance with brown trout Salmo trutta being the primary invading species (n = 5) and only one occurrence of brook trout and rainbow trout Oncorhynchus mykiss invasion. Although changes in aquatic habitat following wildfire have the potential to favor nonnative fishes, connected cutthroat trout and bull trout populations in the Bitterroot River basin were resilient to disturbance and generally recovered more rapidly than nonnative brook trout.