Greenback Cutthroat Trout

Oncorhynchus clarki stomias

Description

The greenback cutthroat trout, Oncorhynchus clarki stomias, typically ranges from 10-15 in (25-38 cm) in length. The largest individuals may weigh up to 4 lbs (1.5 kg). The body and head are elon-gated. Of all cutthroat trout subspecies, the green-back generally has the largest spots and the most numerous scales. It displays a dark "cutthroat" mark under each side of the jaw. The belly of the mature male is often a vivid, blood-red color.

Behavior

The greenback cutthroat trout feeds on aquatic insects in streams and zooplankton, benthic crustaceans, and insects in lakes. Larger individuals may feed on larval tiger salamanders and darters. If food supplies are adequate and suitable spawning gravel is available near the female's home range, there is no migration. Because trout are an opportunistic feeder, the quantity and quality of food are not a serious limiting factor to the species' survival.

The greenback cutthroat trout reaches sexual maturity at 2-3 years of age. Spawning occurs annually in the spring and early summer until death, at 4-5 years in streams and 8-10 years in lakes. Females construct nests in gravel substrates while the males observe her progress. The dominant male constantly drives away subordinate males, and he fertilizes the eggs during spawning. Other males may fertilize the eggs as well. Females construct nests in suitable gravel and spawn several times over a 2-3 day period. After the eggs are spawned and fertilized, the female covers the eggs with gravel and abandons the nest. Spawning peaks when daily water temperature exceeds 45°F (7°C). The average fecundity of females is 700-1,000 eggs per pound of body weight. The incubating eggs require adequate water flow to maintain sufficient oxygen levels.

In head-to-head competition with the brook trout, the greenback cutthroat is invariably the loser. Although adults rarely interact, brook trout juveniles are more aggressive and drive greenback juveniles out of shallow, protective streams into larger creeks and rivers, where they are devoured by predators.

Habitat

This trout inhabits undisturbed headwaters at elevations of 7,000-11,000 ft (2,100-3,200 m) in the Rocky Mountain National Park and in one spring-fed pond at Fort Carson. It prefers clear, swiftflowing mountain streams but the greenback cutthroat can survive in any habitat and tolerate any water conditions that support other species of trout. Young and juvenile fish occupy shallow, more open habitat, while older fish prefer deeper water with more cover, particularly overhanging banks and vegetation. Riffle areas are used for spawning. Juveniles tend to shelter in shallow backwaters until large enough to fend for themselves in the mainstream.

Distribution

The greenback cutthroat trout is the only trout endemic to the Rocky Mountain sources of the South Platte and Arkansas River systems of north-central and central Colorado. Its range extended from the headwaters of both rivers to the foothills along the Front Range.

The greenback cutthroat trout is found in the headwater streams above barriers that prevent non-native trout from invading the habitat. The species occurs in the South Platte River, including the east slope drainage of the Rocky Mountain National Park (Cow, Hidden Valley, Pear Reservoir, West and Fern Creeks, Fern, Bear, Caddis and Odessa Lakes, and the Big Thompson River). It is known from Como Creek in the North Boulder Creek watershed (Boulder County), South Boxelder Creek (Douglas County), and the South Fork of Cache la Poudre River in Roosevelt National Forest and Black Hollow Creek (Larimer County). The head-waters of the Arkansas River, including South Huerfano and Cascade Creeks in San Isabel National Forest, and Hourglass Creek, also support populations of the greenback cutthroat trout.

There are no recent estimates, but the greenback population has increased because of successful rein-troduction efforts.

Threats

In the late nineteenth century, the greenback cutthroat's numbers were dramatically reduced by toxic mine pollution and water diversion for agricultural irrigation, so that by 1937 the species was thought to be extinct. The agricultural practices resulted in water diversion, reduced water levels, altered water temperature, siltation, and erosion. Logging activities altered the vegetation and hydroelectric power diverted stream flows. Physical damage was inflicted on the watershed by highway construction, ski resort development, and housing projects. Most threatening to the greenback cutthroat was the widespread introduction of non-native trout—brook, rainbow, brown, and other subspecies of cutthroat—throughout the range of the greenback cutthroat. Brook trout quickly replaced the greenback in small tributary streams and brown trout replaced it in rivers. Rainbow trout hybridized with the greenback and the purity of the species was lost in many populations.

Conservation and Recovery

Efforts to conserve the greenback began as early as 1959, when fingerlings from the Forest Canyon headwaters of the Big Thompson River were stocked in Fay Lake after non-native trout were removed. Unable to survive in Fay Lake, the trout managed to establish a self-sustaining population in Caddis Lake, immediately downstream. In 1967, a brook trout population was eliminated from Black Hollow Creek, and the greenback cutthroat was reintroduced, using stock from Como Creek. A barrier was constructed to prevent the return of brook trout, and the stream was designated a sanctuary for greenback cutthroat trout.

After brook trout were eliminated from Hidden Valley Creek in 1973 and barriers were constructed to prevent the return of competitive species, the greenback population in the stream recovered. In 1975, state and federal biologists successfully transplanted greenbacks to Bear Lake after removing brook trout. Because of these successes, the green-back cutthroat trout was "downlisted" from Endangered to Threatened in 1978. Recovery efforts continue under the direction of the U.S. Fish and Wildlife Service and the Park Service with cooperation of researchers at Colorado State University.

Contact

U.S. Fish and Wildlife Service
Division of Endangered Species
Denver Federal Center
P.O. Box 25486
Denver, Colorado 80225
http://www.r6.fws.gov/

References

Gagnon, J. G. 1973. "The Greenback." Trout: Quarterly Publication of Trout Unlimited 144:12, 13-28, 30.

U.S. Fish and Wildlife Service. 1977, 1983 Revision. "Greenback Cutthroat Trout Recovery Plan." U.S. Fish and Wildlife Service, Denver.