First-Winter Survival of Caged Wild and Hatchery Cutthroat Trout in Allopatry and in Sympatry with Brook Trout

Abstract

Winter survival of wild and hatchery-reared cutthroat trout (Oncorhynchus darki) both alone and in sympatry with wild brook trout (Salvelinus fontinalis) was compared in cages in a second-order spring-influenced tributary to Henry's Lake Outlet, Idaho. Wild cutthroat trout were about half the length of the hatchery cutthroat and wild brook trout at the beginning of the experiment, and all groups of fish exhibited growth through winter. Survival was not sizedependent. Survival rates of groups containing hatchery cutthroat trout in sympatry with brook trout and hatchery cutthroat or wild cutthroat in allopatry were 84 to 94 percent. However, survival of wild cutthroat held in sympatry with brook trout was 6 percent. We attributed low survival of sympatric wild cutthroat trout to competitive effects of brook trout. These effects may have been exacerbated by confinement in the cages, cage stocking densities, or cage mesh-size that may have reduced food availability.