SuperTrout: Rainbows Resistant To Whirling Disease Are Thriving

SALIDA, CO – Rainbow trout are thriving in the Arkansas River near Salida, and that's a hopeful sign for wildlife conservation efforts to battle whirling disease, which decimated trout populations in Colorado, starting in the 1980s.

In November, Colorado Parks and Wildlife biologists conducted electroshocking surveys of the river and found thriving, large fish, identified by snipped left pelvic fins, as among 40,000 "Gunnison River Rainbows," stocked as babies at about 3-inches long in 2016.

The fish were special rainbows, the spawn of wild rainbows from the Gunnison River that were found to be resistant to the deadly whirling disease. The disease is caused by a parasite that infects rainbows, leaving them deformed and swimming in circles before it quickly kills the youngest fish, Colorado Parks and Wildlife said.

“This year, the largest [rainbows] that we sampled were 13½ inches long,” Michael Atwood, CPW aquatic biologist said. “It only took two-and-a-half years in the Arkansas River for some of the GRRs to grow 10 inches in length! That is impressive and encouraging.

“The Gunnison River rainbows 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,” Atwood said in a press release.

The disease showed up in Colorado in 1987 at a private hatchery, and quickly ravaged Colorado’s wild rainbow trout population. By 1997, wild rainbows essentially vanished and brown trout, which are resistant to whirling disease, took over most of the state's major rivers.

Scientists and biologists have worked ever since to re-establish wild rainbow trout in Colorado. The state has spent more than $13 million to clean up infected hatcheries and convert them to "spring- and well-water rather than surface water sources," CPW said in a press release.

Scientists are waiting until the rainbows are sexually mature next spring, and hope that they can start are naturally reproducing in the Arkansas.

Bringing back a wild, naturally reproducing rainbow trout, resistant to whirling disease, would be a "huge wildlife conservation victory for CPW," the agency said.

“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 in a statement. “There have been several obstacles in the Arkansas River including competition with brown trout, river flows and whirling disease, which is present in the river."

“These Gunnison River rainbows have shown resistance to the disease. This would be a big breakthrough if this project succeeds.”

Images: 1) 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. Photo courtesy Colorado Parks and Wildlife / Bill Vogrin. 2) 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 3 inches long, to prepare them for stocking in the Arkansas River. A November survey found some of the fish had grown 10 inches. Photo courtesy Colorado Parks and Wildlife / Bill Vogrin 3) In November, a CPW team used electroshock equipment to survey the Arkansas River for rainbow trout missing their left pelvic fins, identifying them as descendants of Gunnison River rainbows and resistant to whirling disease. Photo courtesy the Mountain Mail in Salida / Cody Olivas.