Whirling Disease Continues Its Purge

Whirling disease has devastated the rainbow trout population along Montana’s upper Rock Creek and killed browns in the upper Clark Fork. Strangely, it hasn’t hit the Bitterroot, Blackfoot and lower Clark Fork rivers.

The disease is caused by a microscopic parasite that attacks the cartilage of juvenile trout, resulting in deformities, fish spinning uncontrollably and death. Its deadly impact on trout didn’t attract the nation’s biologists and anglers until a 1995 survey along the world-famous Madison River showed a drop from 3,300 rainbows per mile to 300 per mile.

Since then whirling disease has been found in trout in several of Montana’s Blue Ribbon trout streams and in Idaho, Washington, Utah and Colorado streams. In Washington, the disease has been detected in juvenile rainbows along the Grande Ronde River, leading to speculation that the disease may devastate steelhead runs.

For several years, fisheries biologists have known that numbers of juvenile rainbows have been dropping along Rock Creek, an internationally known trout stream just east of Missoula.

“We didn’t know why,” said Dennis Workman, fish manager for the Missoula region. “We considered several possible reasons for the drop. We thought that the drought that lasted several years might have been responsible. We also considered the possibility that ice conditions during severe winters might have caused the drop.

“What we didn’t know was that the whirling disease was killing a high percentage of juvenile rainbows. We finally learned that the trout were dying as the result of the whirling disease infestation. Then, as the result of a survey, we found that the population of rainbows 10 inches and under in the upper river dropped 70 percent from 1993 to 1996.”

What does that mean for the fishing this year? Probably not much of a change from last year’s fishing, Workman said. The catch rates may be down. Because the trout have been infected for several years by the whirling disease parasite, fishermen aren’t likely to notice a change in fishing from the past two or three years.

The parasite has invaded trout in numerous tributaries of the rivers in the Missoula region, but it hasn’t been found in fish along the lower Clark Fork and the Blackfoot and Bitterroot rivers.

“We’re keeping our fingers crossed,” Workman said. “The infestation is fairly severe in the upper Clark Fork, where brown trout have been affected.”

So far, he said, the major problem along the lower Clark Fork was caused by the washing of heavy metals behind Millown Dam just east of Missoula. The metals killed 70 percent of the trout in a relatively short section of the Clark Fork.

Some tubifex worms, which are the only known hosts to the parasite, have been found in the Bitterroot River below the Painted Rocks Reservoir, Workman said. That could mean the parasites are infecting trout in the river.

“There’s some hope that the Bitterroot will remain clean of the parasite,” Workman said.

Although the main Blackfoot seems to be free of the parasite, it has been found in fish in some of its tributaries.

Whirling disease has been identified in trout in the Missouri River below Holter Dam. The Missouri is one of the most productive of Montana’s Blue Ribbon trout streams.

State and federal agencies, as well as the University of Montana and Montana State University, have mounted an impressive campaign to try to beat the disease. Scores of fisheries scientists, biologists and students are studying every possible aspect of the disease.

Anyone who has access to the Internet can learn a lot about the disease and what the group is doing to combat it. More is being done to solve the problem than is being done to solve some diseases of humans. If you’re interested, check the Internet address at http://rivers,oscs.montana.edu/dig/aim/ annelid/whirling.html.

Major waterways that now test WD free include the Yellowstone, Bighorn, Stillwater, Boulder, Gallatin, East Gallatin, Big Hole, upper Missouri, Teton, Smith, Bitterroot, Flathead, Kootenai and Marias rivers; Big Spring Creek and the Madison above Quake Lake.

The researchers have come up with possible good news. Last year they discovered a “significant increase” in population of yearling (5-8-inch) rainbows in the Madison between Quake and Ennis lakes. “Whether this increase marks a beginning of a population recovery,” they said in a report, “or is simply a one-year aberration won’t be known until 1997 or later.”

On the other hand, they reported, “The decline of adult rainbows continued, reaching an all-time population low in the fall of 1996.” On the bright side, they said, “The population of older brown trout remains healthy and yearling browns (6-9 inches) exceed normal historic levels.”

The health of trout means as much, maybe more, to Montana’s economy than many of the historic industries, including mining, agriculture and logging. Trout fishing is a multi-million dollar industry. , DataTimes MEMO: You can contact Fenton Roskelley by voice mail at 459-5577, extension 3814.

The following fields overflowed: CREDIT = Fenton Roskelley The Spokesman-Review