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Fishy Victory: Colorado Parks & Wildlife Salvages State's Rainbow Trout Population

February 14, 2020 / 2:51 PM MST / CBS Colorado

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(CBS4) -- State researchers and biologists declared victory this week over whirling disease, a malady that decades ago sent Colorado's rainbow trout population into a tailspin.

"The wild rainbows basically 'blinked out'" Joe Lewandowski of Colorado Parks and Wildlife told CBS4 Friday. "In all of Colorado's major rivers, wild rainbow trout were essentially gone."

Not anymore.

Rivers and streams throughout Colorado will receive 1.3 million healthy rainbow fingerlings this summer.

"I feel like we've done some good work and these fish are ready to be stocked statewide," researcher Eric Fetherman said.

CPW Aquatic Biologist Eric Gardunio holds a whirling-disease resistant rainbow trout caught in the Gunnison River Gorge. (credit: Colorado Parks and Wildlife)

It is a victory decades in the making.

Whirling disease was inadvertently introduced into the state's system in the late 1980s by imported fish from a private hatchery. It is parasitic disease; tiny worms bear spores which attack the nervous systems of young rainbows, deforming their skulls and spinal columns, and causing them to swim erratically in circles. The disease itself is not deadly, but the accumulated symptoms often lead to death.

(credit: Colorado Parks & Wildlife)

By the mid-90s, the impact was obvious. While CPW maintained a stock of reproductive fish in its hatcheries, the state's waterways emptied of the colorful rainbows.

"We were having no natural reproduction of rainbow trout in the state, that we could determine," Lewandowski said.

Ironically, even though an import was to blame for the near-demise, an import also led to its comeback.

At a national conference in Denver in 2002, a researcher from Europe who studied whirling disease gave a presentation about a strain of disease- resistant rainbow trout he'd found at a hatchery in Germany. George Schisler, CPW's aquatic research chief, imported eggs and tested the hatchlings -- named the Hofer strain after their German hatchery.

A CPW team used electroshock equipment to survey the Arkansas River in November for rainbow trout missing their left pelvic fins, identifying them as descendants of Gunnison River rainbows and resistant to whirling disease. (credit -The Mountain Mail/Cody Olivas)

Those Hofer fish, however, did little once distributed into the wild. Lewandowski said the Hofer hatchlings lacked the flight instincts to survive. They were raised in hatcheries without predators and made for easy meals for other breeds, particularly the brown trout that dominated the rivers in the rainbow's absence.

But once the Hofers were cross-bred with the naturally instinctive locals, researchers had obtained a strain of rainbow with all the attributes to recover in the wild.

They are named Gunnison River Rainbows after the section of river where they showed success.

"We know these fish are maintaining their resistance to whirling disease," said Schisler.

CPW's Glenwood Springs hatchery is now propagating the Gunnison River and other related disease-resistant strains for stocking this summer.

The ultimate goal, according to Lewandowski, is to restore natural reproduction in the rivers and eliminate the need to stock the fish in the future. But that point, he said, is itself decades away.

A Gunnison River rainbow trout after it was caught in May 2018 during spawning operations by Colorado Parks and Wildlife biologists. Because they are resistant to deadly whirling disease, Gunnison River rainbow trout are being spawned so that strain of rainbows can be stocked in rivers across the state. (credit - Bill Vogrin/Colorado Parks & Wildlife)