Competition and Predation as Mechanisms for Displacement of Greenback Cutthroat Trout by Brook Trout

—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 of cutthroat trout O. clarkii, greenback cutthroat trout declined in abundance during the late 1800s and early 1900s because of harvest, habitat alteration, and introduction of nonnative salmonids (Behnke 2002). Only 18 populations are known to have survived. Recovery efforts initiated in the 1970s have focused on identification of suitable habitat, removal of nonnative salmonids from restoration areas by means of antimycin, and stocking of hatchery-reared greenback cutthroat trout to establish self-sustaining populations. Restoration sites typically are headwater streams and lakes that are isolated from nonnative fishes by barriers, such as waterfalls or dams. As of 2005, about 60 such sites contained greenback cutthroat trout, but many of these populations are not considered to be stable because of small population size, lack of reproduction, or the presence of nonnative salmonids (USFWS 1998; Young and Harig 2001; Young et al. 2002). Efforts to restore greenback cutthroat trout have been hindered by the presence of nonnative salmonids, including rainbow trout O. mykiss, Yellowstone cutthroat trout O. c. bouvieri, brown trout Salmo trutta and, most frequently, brook trout Salvelinus fontinalis. Brook trout occur at about 25% of greenback cutthroat trout sites as a result of incomplete eradication, migration upstream past ineffective barriers, or reintroduction by anglers (Trotter 1987; Young 1995; USFWS 1998; Young et al. 2002). Greenback cutthroat trout and brook trout have similar habitat requirements, and both species are sensitive to habitat disturbance (Behnke 2002). Both species feed on a variety of aquatic and terrestrial invertebrates and may sometimes eat small fish or amphibians (Bulkley 1959; Cummings 1987; Young and Harig 2001). Greenback cutthroat trout spawn in late spring or early summer and fry emerge in August, while brook trout spawn in the fall and fry emerge in early summer. Typically, greenback cutthroat trout spawn at age 3 and older and brook trout at age 2 and older (USFWS 1998; Behnke 2002). The mechanisms of displacement of native fishes by introduced species include hybridization, disease transmission, competition, and predation. Fall-spawning brook trout do not hybridize with greenback cutthroat trout, although rainbow trout and other cutthroat trout subspecies readily hybridize with greenback cutthroat trout (Allendorf et al. 2001). Disease transmission from brook trout does not account for declines in greenback cutthroat trout (USFWS, unpublished data) or the closely related Colorado River cutthroat trout (O. c. pleuriticus; Peterson and Fausch 2002). Competition and predation both are plausible mechanisms for displacement of cutthroat trout by brook trout. * Corresponding author: [email protected] 1 Present address: U.S. Forest Service, Rocky Mountain Research Station, 322 East Front Street, Suite 401, Boise, Idaho 83702, USA. Received January 18, 2007; accepted May 11, 2007 Published online August 30, 2007 1381 Transactions of the American Fisheries Society 136:1381–1392, 2007 Copyright by the American Fisheries Society 2007 DOI: 10.1577/T07-017.1 [Article]