RBC I After being devastated by whirling disease in the 1990s, rainbow trout populations are increasing in most major rivers in the state thanks to a 20-year effort by Colorado Parks and Wildlife aquatic scientists and biologists.
“It’s been a long road, but bringing back populations of fish that were essentially extirpated from Colorado can only be called a huge success,” said George Schisler, CPW’s aquatic research team leader who is based in Fort Collins.
The comeback is positive news for anglers who can once again fish for rainbows and brown trout in Colorado’s big rivers and streams. For the past 15 years, brown trout have dominated most of the state’s rivers. But since last summer, anglers have reported they are catching nice size rainbows in the upper Colorado, Rio Grande, upper Gunnison, Poudre, East, Taylor, Arkansas and Yampa rivers and others.
The whirling disease problem started in 1986, when a private hatchery unknowingly imported infected rainbow trout from Idaho that were stocked in 40 different waters in Colorado. The disease eventually spread throughout the state and even infected CPW hatcheries, which caused more waters to be infected.
Whirling disease is caused by a spore that infects the spine of very young fish. The infection deforms the spine, causing the fish to swim in a whirling pattern. They die shortly after becoming infected. When whirling disease hit Colorado’s rivers, natural reproduction of the species virtually ended. That allowed brown trout, which are not affected by the disease, to become the dominant sport fish.
By the mid 1990s rivers in Colorado and other Western states were thoroughly infected.
At a national conference on whirling disease in Denver in 2002, a German researcher presented information that showed trout at a hatchery in Germany, operated by a family named Hofer, were resistant to the parasite. Colorado’s aquatic staff moved quickly to import eggs from Germany that were hatched at the University of California at Davis. The fingerlings were then brought to CPW’s Bellvue hatchery near Fort Collins.
The fish grew quickly and their disease resistance was proven. By 2006, Schisler stocked some of the Hofers in two reservoirs west of Berthoud. Anglers reported that the fish hit hooks hard and were easy to catch. This made them ideal for stocking in reservoirs where anglers expect to catch fish.
But because the “Hofers” had been domesticated in a hatchery for generations, Schisler and his colleagues knew that the fish did not possess a “flight response” to danger. They would have little chance in creeks and rivers where they need to avoid predators and survive fluctuating water conditions. So CPW researchers started the meticulous process of cross-breeding the Hofers with existing strains of trout that possessed wild characteristics and had been stocked in rivers for years.
Biologists first stocked 5-inch Hofer-crosses, but they didn’t survive. Then in 2010, fingerlings were stocked in the Colorado River near Hot Sulphur Springs. When researchers returned to survey the area 14 months later, they learned the experiment had finally paid off. They found good numbers of 15-inch rainbows and evidence that young fish were hatching in the wild.
CPW biologists have been stocking fingerling Hofer-crosses throughout the state at different sizes and times of year to optimize survival. The young fish are surviving and Schisler is confident that Colorado’s rivers and streams are again home to truly wild rainbows.