Whirling disease-resistant rainbow trout released in North Tongue River

News – Sheridan Media

Earlier this summer, the Wyoming Game and Fish Department released approximately 2,000 rainbow trout with resistance to whirling disease (WD) into the North Tongue River.

Transported from a hatchery in Colorado, the fish are part of efforts to reverse a long-term decline in the river’s rainbow trout population. Sheridan Region Fisheries Biologist Andrew Nikirk said rainbow numbers have dropped steadily over the past 15 years and have not stabilized.

Whirling disease is caused by a parasite that infects salmonid fish, including trout, salmon, grayling and whitefish. No treatment to rid a fish population of WD exists, and once the parasite is introduced, eliminating it from salmonid populations in the wild is not usually possible.

The disease does not pose a health risk to humans, other mammals or fish outside the salmonid family. The parasite cannot infect people, and there is no risk associated with swimming or eating fish caught from infected waters.

According to Game and Fish, managers have attempted to address the decline by extending catch-and-release sections and increased numbers during stockings when available. Whirling disease, first confirmed in the river in 2020, is believed to be the leading factor. In 2023, a Colorado State University study found 41% of sampled trout from the North Tongue River tested positive for the parasite.

“We will first look to see if the WD-resistant rainbows stayed in the release areas,” Nikirk said. “If they did, we might also find some evidence of reproduction, as some of the released fish were in the 10-inch range. These trout are not immune to WD, but show good resistance, so hopefully over the next several years we will begin to see a turnaround in the population numbers.”

According to the department, the N. Tongue is sampled on even years, so in 2026, Nikirk will get a first indication of the possible success of the stocking.

Game and Fish will continue stocking Snake River cutthroat, which have performed well and provided strong fishing this summer.

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Last modified: September 27, 2025