Disease-resistant rainbow trout have been stocked in the West Fork River for the first time, according to the Colorado Parks and Wildlife.
Last week, Colorado Parks and Wildlife stocked more than 22,000 Gunnison River rainbow trout into the West Fork River, a tributary of the Dolores River. The fish were raised in the Durango Hatchery, and were 2.5 inches long when released.
Gunnison River rainbow trout are a self-sustaining population of trout found in the Gunnison River.
They have a natural resistance to whirling disease and fare very well in the wild, said CPW aquatic biologist Jim White. This is the first time they have been stocked in the West Fork.
Genetics of the Gunnison River rainbow are traced to an imported rainbow trout from Germany called the Hofer strain, which have evolved to resist whirling disease.
“That gene was passed on to the Gunnison River rainbow over time,” White said. “They are a wilder fish than the original Hofer rainbow strain, which were vulnerable to predation by brown trout and did not survive overwintering well.”
Whirling disease is native to Europe, and fish from Europe have developed resistance to it, whereas North American rainbow trout have not evolved to resist it.
The CPW Durango Hatchery obtains eggs for Gunnison River Rainbow Trout and raise them for later release into area rivers.
White said the program is an effective way to reduce whirling disease in Colorado rivers and streams. It is presumed all major tributaries in the San Juan Mountains are positive for the disease.
The disease is caused by a parasite that infects rainbows, leaving them deformed and swimming in circles before it kills the youngest fish.
The disease was introduced in Colorado by a private hatchery in 1987, and it quickly ravaged Colorado’s wild rainbow trout population, according to CPW.
By 1997, wild rainbows essentially vanished, and brown trout, which are resistant to whirling disease, took over most of the state's major rivers. CPW aquatic research scientists and biologists have worked ever since to combat whirling disease and re-establish wild rainbow trout in Colorado.
The effort included spending more than $13 million to clean up infected hatcheries and convert them to spring and well water rather than surface water sources.
A breakthrough came in recent years when CPW biologists verified that a population of wild rainbows in the Gunnison River with resistance to whirling disease were reproducing on their own. CPW staff has begun spawning and raising Gunnison River rainbows and stocking them around the state. In March 2017, CPW began stocking them in the Arkansas, east of Salida.
A 2018 survey of the GRR trout in the Arkansas showed it took 2.5 years for 3.5 inch fish to grow 10 inches, an impressive growth rate, according to CPW aquatic biologists.
The CPW hatchery staff in Glenwood Springs has developed a broodstock, and the first captive spawn of those fish is planned this spring, which will provide higher numbers of GRR to stock statewide.
“They are a very colorful fish, lots of freckling spots, with a very dark red stripe,” White said.