By Logan Smith

SALIDA, Colo. (CBS4) — Thirty-one years and millions of dollars into the fight against whirling disease, a strain of the iconic Colorado rainbow trout is at last showing signs of natural resistance.

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Biologist from the Colorado Parks and Wildlife recently made a fish count in the Arkansas River near Salida. Their conclusion?

The fish are “thriving.”

(credit – Colorado Parks & Wildlife file photo)

Whirling disease was accidentally introduced to Colorado’s rivers by a private hatchery in 1987, according to the agency. Ten years later, wild rainbows were essentially gone.

All of Colorado’s trout — mainly rainbows, brook, lake, and native cutthroats — are susceptible, with one exception. Brown trout, which came from Europe in the late 1800s, are naturally resistant.

Europe happens to be where the disease originated.

Three rainbow trout, offspring of Gunnison River rainbows, are shown last February in the Mount Ouray Hatchery in Salida. With help from the Collegiate Peaks Chapter of Trout Unlimited volunteers, CPW snipped the left pelvic fins from 40,000 Gunnison River rainbows, each around three inches long, to prepare them for stocking in the Arkansas River. A November survey found some of the fish had grown 10 inches. (credit – Bill Vogrin/Colorado Parks & Wildlife)

“CPW has been trying to create a wild reproducing population of rainbow trout in the Arkansas and other major Colorado rivers for decades,” said Josh Nehring, senior aquatic biologist in CPW’s Southeast Region. “There have been several obstacles in the Arkansas River.”

The state has spent $13 million cleaning up and upgrading infected hatcheries.


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CPW worked hard to combat the disease and re-establish the wild rainbows in Colorado’s waterways. At the time, that effort proved to be a standoff between biologists and the disease.

Then a strangely resistant strain of the fish was found in the Gunnison River. The focus turned to this strain. Momentum has now swung in the state’s favor.

In March of this year, thousands of Gunnison River Rainbows (GRR) were marked and released into the Arkansas near Salida.

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)

CPW aquatic biologist Michael Atwood conducted electroshocking surveys last month and expressed hope.

“This year, the largest GRRs that we sampled were 13½ inches long,” Atwood said. “It only took 2½ years in the Arkansas River for some of the GRRs to grow 10 inches in length! That is impressive and encouraging.”

A Gunnison River rainbow trout after it was caught last May 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)

“The Gunnison River rainbows,” he added, “actually thrive and grow faster in the river than they do in the hatchery under controlled conditions, a hopeful indicator of how well-suited they are to the Arkansas River.”

This batch of rainbows will reach sexual maturity next spring. Only with proof those fish are reproducing will CPW declare victory.

“We suspect it will take a couple more years before we can say anything definitive about the success of GRRs in the Arkansas,” Atwood said. “But it sure is encouraging.”

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Logan Smith is an assignment desk editor at CBS4 with more than 30 years of journalism experience in print and television media.