Encroachment of Exotic Rainbow Trout into Stream Populations of Native Brook Trout in the Southern Appalachian Mountains

Abstract

Since 1900, the distribution of native brook trout Salvelinus fontinalis has shrunk in the southern Appalachian Mountains area. Initial reductions were caused by logging and heavy fishing pressure but, in later years, encroachment by exotic rainbow trout Salma gairdneri may have contributed. Historical and current field evidence supports the hypothesis that the modern contraction of the brook troutˈs range is due to encroachment by rainbow trout. In Great Smoky Mountains National Park, Tennessee-North Carolina, brook trout formerly occurred from headwaters to low-gradient streams near the park boundary. They retain this longitudinal distribution in a few streams today, but only in those lacking rainbow trout. The typical pattern in other streams is for allopatric brook trout to be in upper stream reaches, allopatric rainbow trout in lower reaches, and a zone of sympatry (up to about 3.2 km long) between the two pure populations. Few age-0 rainbow trout occur in zones of sympatry unless adult brook trout are rare, and vice versa. In stream segments with similar physical characteristics, allopatric brook trout and allopatric rainbow trout have similar numerical densities, but the average size of rainbow trout is larger, and rainbow trout have biomasses about 1.8 times greater than that of brook trout. Adult allopatric brook trout occupy main channel habitats, but they occur predominately in peripheral stream areas when sympatric with rainbow trout. We believe the encroachment of rainbow trout is dynamic and continuing in the park and elsewhere in the southern Appalachian Mountains, and that brook trout ultimately will be reduced to a few small inbreeding populations in headwater refugia, and exterminated from many streams, if no management measures are taken. Received September 13, 1983 Accepted November 13, 1984