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Rainbow trout ready to return?

Bob Berwyn
Vail, CO Colorado
Mark Fox/Aspen Times file photoOne of the larger rainbow trout spawned at the Carbondale Fish Hatchery in October 2005.
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SUMMIT COUNTY ” Local guide Trapper Rudd remembers well the heyday of fishing for rainbow trout in Colorado, before whirling disease spread into the state in 1987 and crippled and deformed the beautiful speckled fish.

In just 10 years, many wild rainbow trout populations across the state were decimated.

“It truly was a magnificent time,” Rudd said. “You could get into the double-digits pretty quick. The rainbow populations here were as strong as anywhere in the country.”



Rainbows are known for their willingness to rise to a dry fly, and for their splashy acrobatic moves once they’re hooked, he said.

“We started seeing it before we know what it was,” Rudd said of whirling disease. “When it was in its bloom, we started catching these small fish that were deformed. We noticed them acting funny around the banks.”



It didn’t take biologists very long to figure what was going on. Parasitic spores ” part of a complex life cycle involving mud-dwelling worms ” were infecting the fish and spreading like wildfire. Before long, state hatcheries were infected, and researchers pinpointed a shipment of infected trout from Idaho as the source.

Worst of all, there seemed to be no way to stop it. Initial reports from biologists suggested that it might not be a problem for wild rainbow trout populations, so the Colorado Division of Wildlife continued stocking infected trout for four or five years after they first discovered the infection.

By the early 1990s, rainbow populations simply collapsed, disappearing entirely from some rivers and lakes. Some rainbow populations in the High Country 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 worms that carry the parasite.



Rudd said other fish quickly filled the niche, with brown trout, for example, thriving in some areas where rainbows previously dominated. In other cases, brook trout populations started to increase, and that’s not always a good thing. Brook trout can crowd out populations of native cutthroat trout, Rudd said.

All the while, researchers looked for answers, with the goal of someday re-establishing rainbow trout in Colorado. In recent years, some of the most promising research has focused on breeding Colorado River rainbows with another strain, called Hofer rainbows, from a hatchery in southern Germany. The Hofer trout, it turns out, are highly resistant to whirling disease.

Working first at Colorado Division of Wildlife research facilities, and recently moving on to field trials, aquatic biologists are mixing the two strains, hoping to blend the resistance of the Hofer trout with the qualities of the Colorado River strain for which the fish are prized.

“We’re closer now than we have been to finding a solution. There’s some light at the end of the tunnel,” said George Schisler, a Division of Wildlife scientist involved in the effort.

The first crosses between the Colorado River rainbows and the Hofer rainbows were made in 2003, with 35 different “families.”

Using genetic markers, the scientists will be able track the offspring from those pairings, even after the fish have been released into the wild. Currently, there are two field trials in progress, in the Gunnison and South Platte rivers, Schisler said.

The researchers are finding a range of resistance in the various families. It’s challenging in part because the resistant Hofer strain has long been bred as a docile food fish. Schisler said the differences are even apparent in a lab setting, where the Colorado River rainbows tend to try and hide in the farthest corner of a tank.

But the Hofer rainbows are nearly tame.

“You can practically pick them up,” Schisler said.

But these efforts to breed disease-resistant rainbow trout are threatened by federal budget cuts. The U.S. Fish and Wildlife Service recently announced that it will eliminate all funding.

The cuts may affect the programs in the Gunnision and South Platte rivers.

“This will have a severe impact … this will likely cause the resistant trout research to go away,” said Dave Kumlien, director of the Montana-based Whirling Disease Foundation.

Kumlien said his organization will go back to the Fish and Wildlife Service and at least seek to restore some minimal “life-support” funding. He said there is some political support from western congressional delegations, including Colorado’s.


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