Colorado’s rainbows ready to rise
December 24, 2007
After being decimated by whirling disease, Colorado’s trout rainbow trout fishery could be primed for a big comeback.
State researchers said they’re encouraged by the results of an experimental breeding and stocking program aimed at rebuilding rainbow populations with trout that are resistant to the parasitic spore that deforms and kills young fish.
“There’s a lot of potential for re-establishing wild populations of rainbow trout,” said Colorado Division of Wildlife biologist George Schisler.
Beginning in 2008, the DOW will begin restocking lakes and streams with large numbers of rainbow trout that show strong resistance to the disease that all but wiped out Colorado’s rainbow fishery during the past 15 years.
“Almost every fingerling stocking will be with whirling disease-resistant strains,” said Jon Ewert, DOW fisheries biologist for Summit and Grand counties.
Ewert said he recently sampled a stretch of the Colorado River that was stocked with resistant trout in 2006. He recovered 25 tagged fish, most of which had grown about 1.5 inches since they were released.
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The next big step forward in re-establishing wild rainbow populations is to watch and see if those fish will start spawning next year when they are sexually mature.
Although rainbow trout are not native to Colorado, they quickly became a key piece of the state’s fishery when they were introduced in the late 1800s. As recreational fishing became more important, the speckled trout were highly valued for their longevity, their willingness to rise to dry flies and for their splashy acrobatics once hooked.
But entire rainbow trout populations quickly vanished from some Colorado rivers and lakes beginning in the 1980s, when whirling disease was accidentally introduced to the state in a shipment of hatchery fish from Idaho. Parasitic spores – part of a complex life cycle involving mud-dwelling worms – were infecting the fish and spreading like wildfire. To make matters worse, the DOW continued stocking infected trout for four or five years after they first discovered the problem, thinking that wild populations were resistant.
By the early 1990s, rainbow populations simply collapsed, disappearing entirely from some rivers and lakes, with only a few remnant populations holding on. Some rainbow populations in the high country also managed to avoid the worst of the disease, based partially on the fact that fast-running mountain streams don’t have the layer of mud on the bottom that provide the ideal environment for the parasite-hosting worms.
In 2003, the state wildlife agency started fighting back against the disease by crossing Colorado River rainbows with a resistant strain of trout from Germany. The goal was to blend the resistance of the Hofer trout with the desirable game fish qualities of the Colorado River strain.
The cross-bred fish were exposed to high doses of the parasite, and those that showed the greatest resistance were kept to breed the next generation.
Five years later, the DOW has started to raise large quantities of the resistant trout. About 20,000 resistant rainbows have already been stocked in the Gunnison River, Schisler said.
“Our goal is to reduce the spore burden wherever we stock,” Schisler said. As the resistant fish breed with remnant populations, they will infuse those genetic qualities back into the wild, he explained.
If all goes according to plan, Anglers may once again be able to catch wild rainbows in Colorado’s rivers and streams.