On July 31, 2025, approximately 2,000 Gunnison River/Hofer-cross rainbow trout were released into the North Tongue River. The fish were transported more than 650 miles the previous day from the Pitkin State Fish Hatchery in Colorado. In addition to being long-distance travelers, the fish are special due to a unique genetic trait - they are resistant to the impacts of whirling disease.
The release is the latest of several management actions aimed at reversing a long-term decline in rainbow trout populations of this popular fishery. Rainbow trout were stocked in the river from the 1970s to the 1990s, and have been self-sustaining since, with one supplemental stocking in 2021. However, over the past several years, the population has declined.
“We have documented a downward trend for rainbows in both population (numbers of fish per mile) and biomass (pounds of fish per mile) over the past fifteen years,” said Sheridan Region Fisheries Biologist Andrew Nikirk, who manages the fishery. “Wild populations ebb and flow, sometimes drastically, but will generally stabilize over time. So far, this hasn’t happened on the North Tongue.”
Fish managers have taken a multipronged approach to addressing the decline, including extending the catch-and-release section of the river in the fishing regulations beginning in 2022 and stocking extra fish when available. But, whirling disease has also been documented in the river and is likely the leading cause of the decline.
Myxobolus cerebralis is the parasite that causes WD. Once established in a water body, the parasite begins a simple, but damaging, life cycle. Aquatic tubifex worms host the parasite where it transforms from a spore into a triactinomyxon (TAM). TAMS are then released by the worms into the water and when they come in contact with a fish, they embed in the body. The TAMs attack soft cartilage of young fish, primarily in the head and spine, and begin making spores. This damage to the cartilage and nervous system is what gives the disease its name - infected fish show neurologic damage, physical deformities, black tails and often swim in circles. These deformities make them more vulnerable to predation and can impact their ability to eat. When the fish dies and decays, the spores are released and taken up again by the worms, morphed into TAMS and the cycle repeats.
Myxobolus cerebralis was first documented in the N. Tongue by the Game and Fish Fish Health Laboratory in Laramie in 2020. Three years later, a research project by Colorado State University collected 112 adult trout on the N. Tongue, of which, forty-one percent of the samples were positive for WD. The CSU research is ongoing, but initial information suggests releasing WD-resistant rainbows in the river may stabilize or provide a boost to the population over time.
“We have not found a ‘smoking-gun’ yet, but whirling disease appears to be having a potentially big impact,” Nikirk said. “The population decline is most likely a combination of factors, and not just WD, including the natural ups and downs of a population and environmental factors such as anchor ice or high stream flows, changes to invertebrate production, connectivity, available spawning habitat and overwintering habitat. High angler use could be contributing as well. Our 2018 creel survey found a 40 percent increase in angling hours and 19 percent increase in the number of anglers since the previous survey in 1999.”
The N. Tongue is sampled on even years, so in 2026, Nikirk will get a first indication of the possible success of the stocking.
“We will first look to see if the WD-resistant rainbows stayed in the release areas,” said Nikirk. “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.”
Snake River cutthroat will continue to be stocked in the river, as they have performed well and anglers have reported excellent fishing this summer.
