Colorado researchers spent decades trying to save disappearing rainbow trout. Finally, they’re making progress — The #Colorado Sun

In the Gunnison River gorge, CPW Aquatic Biologist Eric Gardunio, holds a whirling-disease resistant rainbow trout. CPW is stocking fish resistant to the disease throughout the state. Photo credit: Colorado Parks and Wildlife

Here’s an in-depth look at CPW’s efforts to recover Rainbow trout from Kevin Simpson that’s running in The Colorado Sun. Click through and read the whole article for the photos and details about the disease. Here’s an excerpt:

Genetics from Germany and a hardy cross with Gunnison River trout seem to be overcoming a nightmarish parasite that causes deadly whirling disease

Through dogged research that called on experts throughout the U.S. and even Europe, the rainbow has staged a remarkable recovery that required years of genetic testing, cross breeding and painstaking reintroduction into Colorado’s waters. Only recently have those efforts shown signs of enduring success against the parasite that nearly destroyed it.

And in large part, it has been developments in the rugged Gunnison River waters, where researchers cultivated a strain of rainbow — dubbed the HXG — that’s both disease-resistant and hardy enough to survive in the wild, that have pushed the effort toward sustainability.

More than 1.3 million of the new fish will be introduced into Colorado’s waterways this summer.

“It’s been an ongoing sort of thing, an evolution of little successes over time,” says George Schisler, chief of aquatic research for CPW and one of the key players in the long-running drama. “Now that we’ve got a lot of these HXGs in production, that’s the tipping point. We’re starting to see more and more little rainbows surviving in the wild.”

To get here, the fish beloved by anglers for its colorful appearance, relative ease to hook and admirable fight, had to overcome a nasty parasite, hungry browns and a whole lot of trial and error…

The department’s electrofishing along a 2-mile stretch of the Colorado near Kremmling in 1993 shocked well over 1,000 fish to the surface. Nehring recalls counting a huge population of wild rainbows from 16 to 24 inches long — but only five under 12 inches.

It was a mystery what happened to the little ones. The results of the count were nothing like they’d seen on that same stretch of river in the early ‘80s, when there were plenty of big rainbows, but most fell in the 9-inch range — evidence that the young ones were thriving.

At the same time, the brown trout population was virtually unchanged over that period. Nehring looked everywhere for possible culprits: water temperature, flow fluctuations during rainbow spawning and egg incubation, pollution, floods. But he could find no factors that seemed to make sense. That stretch of the Colorado seemed to be missing two years’ worth of wild rainbow trout fry — recently hatched fish — with no similar impact on the browns.

When Nehring called in to report the conundrum, his boss wondered if the answer might be something called whirling disease that plagued tiny rainbows but not browns.

“At that time I didn’t even know what whirling disease was,” Nehring says.

Although the life cycle of the parasite — Myxobolus cerebralis — has been understood only since 1984, whirling disease dates its discovery to the late 19th century at a trout farm in Germany. Scientific literature was sparse. In Colorado, a state fish pathologist determined after testing some infected rainbows that there was less than a 5% chance that whirling disease was responsible for the disappearance of the young rainbow population…

Although Nehring sounded the alarm, most fish and wildlife experts didn’t pick up on the damage that whirling disease was doing to the state’s rainbow trout population until years after they’d studied bottom-feeding tubifex worms. The worms, which live in the mud and sediment of river and lake beds, had proved unwitting distributors of the spores that infected young fish and fed on the cartilage that later would mature into bone.

The result is a misshapen skeletal structure, with deformities that include a telltale lateral curvature of the spine. Eventually, inflammation causes nerve malfunction. The result is a rainbow that whirls in endless circles, and either dies of the infection or becomes prey — often to the large population of brown trout. Cutthroat and some other species are vulnerable to the disease, too, but rainbows are particularly susceptible.

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