The quiet struggle between brook and cutthroat trout

By Jeff Mitton

Trappers Lake, in the Flat Tops Wilderness area, is arguably the most beautiful lake in Colorado. It is 1.5 miles long, 0.5 miles wide, 170 feet deep and ringed by Flat Top Mountains, the most imposing of which is the Amphitheater, a semicircular mountain that rises 1,500 feet above the lake.

Several cabins were erected on the lake shore in 1889, and in 1903 the Colorado Division of Wildlife (CDOW) occupied one of them as a base for its work with the thriving population of native Colorado River cutthroat trout. The other cabins have been taken down, but the CDOW cabin is still in use today.

In 1919 the Forest Service hired Arthur Carhart as a landscape architect and assigned to him the task of surveying a road around the lake and platting home sites. Carhart, struck by the beauty of the area, recommended that the area not be developed, but preserved as wilderness. The following year the Forest Service prohibited development of the area.

The beauty of Trappers Lake inspired the Wilderness Area system and consequently Trappers Lake is fondly referred to as the "Cradle of Wilderness." The Flat Tops Mountains were declared a wilderness area in 1975.

The CDOW harvested Colorado River cutthroat trout eggs from Trappers Lake from 1903 to 2000, restoring cutthroat to lakes and streams on the Western Slope. But this wellspring of cutthroat was compromised first by the invasion of brook trout in 1984 and the introduction of whirling disease (my next column) in 1999.

Brook trout are native to eastern North America and are introduced in Colorado. They were established in Crescent Lake, but early in 1984 a high-water event flushed them downstream into Trappers Lake.

The number of brook trout swept into Trappers Lake was not known, but that year they probably constituted a minuscule percentage of the trout population. However, by 2003, brook trout constituted 40 percent the fish population. When these two species occupy the same habitat, brookies displace cutthroat.

Brookies have a natural advantage over cutthroat trout because brookies spawn in the fall while cutthroat spawn in spring. Brook trout fry hatch in May in Trappers Lake but cutthroat trout don't emerge from the gravel until August. By the time cutthroat eggs hatch, brook trout are already over 2 inches long, and may be able to eat some of the cutthroat. More importantly, the size disparity gives brookies a huge competitive advantage in their first year of life.

To avoid the extirpation of Colorado River cutthroat from Trappers Lake, CDOW biologists decided to manage the trout by selectively trapping fish and removing brookies. The Fyke nets used resemble a gigantic minnow trap with a net curtain extending 50 feet from the mouth of the trap to shore.

When a fish encounters the curtain it turns toward deeper water to get around the obstruction. Swimming along the wall of netting, the fish passes through a series of funnels culminating in a confined space with netting on all sides. Biologists empty the trap nets and remove brookies while returning cutthroat to the lake. Culled adult brook trout are cleaned, packed in ice, and taken to the food bank in Meeker.

By setting traps and leaving them overnight for a half dozen nights each fall, biologists have cut the percentage of brook trout in the population in half. The cutthroat are safe as long as the management program continues. Regrettably, there is no realistic prospect of eradicating the brookies.

Jeff Mitton (mitton@colorado.edu) is chair of the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

Jan. 8, 2010