Response of Wild Trout to Stream Restoration over Two Decades in the Blackfoot River Basin, Montana

Anthropogenic degradation of aquatic habitats has prompted worldwide efforts to improve or restore stream habitats for fisheries. However, little information exists on the long-term responses of salmonids to restoration in North American streams. To recover wild trout populations in the Blackfoot River in western Montana, a collaborative approach to stream restoration began in 1990 to improve degraded stream habitats, primarily on private land. To assess the efficacy of various restoration techniques (channel reconstruction and placement of instream habitat structures, restoration of natural instream flows, installation of fish ladders and screens at irrigation diversions, and modification of grazing practices) in the recovery of wild trout, we examined long-term (>5 years) trends in trout abundance on 18 tributaries treated between 1990 and 2005 and subsequently monitored between 1989 and 2010. At pretreatment conditions, average trout abundance was significantly lower in treatment versus reference sites (0.19 versus 0.62 trout/m; P = 0.0001). By 3 years posttreatment, trout abundance had increased significantly to an average of 0.47 trout/m across treatment sites (P = 0.01) and was no longer significantly different from the reference average (P = 0.12). These initial rapid increases were sustained over the long term (5–21 years) in 15 streams. However, trout abundance declined below pretreatment levels on three streams presumably due to the return of human impacts from heavy riparian grazing and detrimental irrigation practices. Although long-term (12 year) average response trends were positive, trends varied spatially and native trout responded more strongly in the upper portion of the basin. Study results indicate that restoration should focus in the mid to upper basin and emulate features of natural channels to promote life history diversity and the recovery of native trout. Finally, long-term monitoring led to adaptive management on most (10 of 18) projects, and thus proved vital to the overall sustainability of wild trout fisheries throughout the basin.