Competition and predation as mechanisms for displacement of Greenback Cutthroat Trout by Brook Trout

Abstract.—Cutthroat trout Oncorhynchus clarkii frequently are displaced by nonnative brook trout Salvelinus fontinalis, but the ecological mechanisms of displacement are not understood. Competition for food and predation between greenback cutthroat trout O. c. stomias and brook trout were investigated in montane streams of Colorado. A replicated field study was used to describe the population density, diet, stomach fullness, and body condition of the two species in allopatry and sympatry. Population data confirmed that brook trout displaced greenback cutthroat trout at sites where the species occur together. The diets of the two species were different; cutthroat trout consumed more prey items and a wider variety of prey than brook trout. Sympatry did not influence gut fullness or body condition for either species. Predation occurred at low rates that did not account for declines in populations of greenback cutthroat trout. Instead, population data suggest that the displacement of greenback cutthroat trout by brook trout occurs through a bottleneck on recruitment due to the mortality of eggs or juvenile cutthroat trout. The greenback cutthroat trout Oncorhynchus clarkii stomias, which is endemic to the South Platte and Arkansas rivers, is listed as threatened under the U.S. Endangered Species Act. Like many subspecies o