Gunnison River rainbow trout, resistant to deadly whirling disease, are being spawned so that this strain can be stocked in rivers across Colorado. Photo courtesy of Bill Vogrin/Colorado Parks and Wildlife

DENVER, June 18 (UPI) -- Wild rainbow trout are returning to the mountain rivers of Colorado for the first time in more than 20 years after an invasive parasite eradicated them and some other species in the Rocky Mountain West.

In 2015, wild rainbow trout were discovered in the Gunnison River in southwstern Colorado. These trout had a resistance to "whirling disease," a parasite called Myxobolus cerebralis, carried by tube worms, that attacks the cartilage of developing young fish. In salmonid fish, especially rainbow and native cutthroat trout, the disease attacks balance centers in the "fingerling" fish's skull and causes the fish to develop a bizarre tail-chasing whirling behavior.


Fish also develop deformed spines, heads and tails. Fish biologists look out for trout with "dolphin heads" to identify victims of the disease.

The new rainbow trout in the Gunnison River caught the attention of Colorado Parks and Wildlife fish scientists, who quickly captured them with nets and bred 40,000 new super-trout hatchlings.

With help from volunteers from the local Trout Unlimited branch, fishery agents clipped small fins on the fishes' underside for future identification, and in 2016 the young trout were released into another Colorado mountain river, the Arkansas.

Between 1995 and 1998 the number of rainbow trout in Colorado's rivers went from thousands of fish per mile to almost zero, said Josh Nehring, Colorado Parks and Wildlife senior aquatic biologist. The parasite quickly spread, killing trout in river drainages in Montana, Wyoming, New Mexico and Utah. Whirling disease is now present in 25 states, according to the U.S. Fish and Wildlife Service, although it has done more damage in the Rocky Mountain West.


Now Parks and Wildlife fish specialists are waiting to see whether the 3-year-old fish -- which have reached lengths of 14 inches -- are ready to reproduce in the wild.

"This spring would be the first year they spawn in the river," Nehring said. "This fall we may be able to see if we're getting any natural reproduction."

'Biological Chernobyl'

The agency and its state fisheries have been trying to bring back the rainbow trout for two decades since the near-obliteration of the species, which was described by scientists in 1997 as a "biological Chernobyl," according to media accounts at the time.

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The rainbow trout is non-native to Colorado and states west of the Rocky Mountains. It was introduced in the 1800s from the West Coast, along with the brown trout, which came from Europe and doesn't seem susceptible to the parasite.

Whirling disease also was imported from Europe, first showing up in Pennsylvania during the 1950s, and making its way west.

The disease has a complicated life cycle involving two different hosts, one being the tube worm and the other the fish. It begins with microscopic spores on the river bottom, which are eaten by tubeworms. Inside the worms, the spores transform into tiny three-pronged organisms called "triacintomyxons" that escape from the worm and latch themselves onto a young fish's body, working their way through the skin and following the nerves to the fish's skull and cartilage. When the fish dies or is eaten by another predator, the spores return to the river and the cycle starts again.


The parasite does not affect humans.

In 1987, the transfer of live diseased fish to multiple private fisheries in Colorado and Montana is thought to have spread whirling disease to local fish hatching centers, both private and state-owned. Then, millions of adult trout who had been exposed to the parasite were stocked in Colorado's streams, leading to the spread of whirling disease to more than 200 of the local rivers and streams, according to Colorado Parks and Wildlife research published in 2010.

State hatcheries have made improvements to keep hatchlings away from the parasite by raising them through adolescence in well-water tanks, sometimes in fiberglass environments to keep them away from spore-spreading worms. Scientists have even zapped the hatchlings with ultraviolet light to try to kill the parasite. Colorado's Parks and Wildlife alone spent $13 million trying to manage the disease.

State aquatic scientists also tried to crossbreed Colorado trout with a German rainbow trout, which had developed a resistance to whirling disease.

The problem was that the "Hofer" trout were too domesticated and had a hard time living in cold-water rivers, Nehring said. Sports anglers also didn't like fishing for the Hofer-strain fish because they didn't put up a fight on the line.

"It's like reeling in a wet sock," Nehring said.

Return to Madison River

Rainbows have also returned to Montana's Madison River, where the disease knocked out the species in 1995-98. Biologists from the Montana Fish Wildlife Parks aren't sure why. It's possible the new fish developed a genetic resistance to the disease, or fish found spawning grounds in warmer waters, where the parasite is less active. Montana no longer stocks rivers with fish raised in hatcheries, preferring to stock rivers with unhatched eggs, said David Moser, fisheries biologist.


The drop-off of rainbow trout in the 1990s was devastating to the big business of tourist fly fishing, Moser recalls.

"There was quite a bit of blowback [about whirling disease] from clients and outfitters. They didn't want us even saying there was an issue with the disease."

State and federal fishing agencies across the country are trying to keep the whirling disease parasite's spores from spreading further by outlawing the transfer of live fish between bodies of water. Anglers are urged never to leave fish carcasses on the banks of water bodies and to carefully clean their equipment, boots and boats.

If the sexually mature hatchlings in Colorado are able to spawn, creating a wild, naturally reproducing rainbow trout, Colorado Parks and Wildlife would score "a huge wildlife conservation victory," the agency said.

"At this point we just have to manage around it," Nehring said.

In Montana, fly fishing on a 50-mile stretch of the Madison River increases 15 percent every year, Moser said. In 2017, more than 207,000 angler fishing episodes were logged. The average rainbow trout is caught and released back into the river five times a year.

That leads to another factor that can cause disease in a species, Moser said: stress.

For that reason, Montana fish biologists are watching carefully to see if higher angling rates might push rainbow trout back into a weakened state where they could again succumb to whirling disease.

"Once a population gets a disease, they're going to have it forever," Moser said. "The question is whether the pathogen turns into disease, and what causes that."

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