Whirling Disease Puzzles Idaho Experts Perplexed Why Trout Populations Not Tumbling Like In Montana Streams

Whirling disease is present in Idaho rivers from the Panhandle to the Wyoming border, but biologists are perplexed why trout populations are not tumbling like in other states’ blue-ribbon streams.

“That’s the $64,000 question. If we knew that one, maybe we could move forward,” said Steve Elle, Idaho Department of Fish and Game fisheries research biologist. “But we haven’t seen the total collapse in Idaho’s population (that is) evident in Colorado and Montana.”

The agency is trying to glean information as quickly as possible to avert such a catastrophe. It also is worried the infection could strike chinook salmon in the hatcheries on the upper Salmon River.

Whirling disease strikes when a tiny parasite - Myxobolus cerebralis - attacks fish cartilage, resulting in pressure on the nervous system. In the most extreme cases, infected fish have a blackened tail and deformed head. They whirl in a death spiral, wearing them out and making them vulnerable to predators.

Young trout may die from the disease, but adult fish may only get low-grade infections and become carriers.

Often, whirling disease is barely detectable. In perhaps the worst infestation, rainbow trout numbers in Idaho’s Big Lost tributary fell 70 percent from 1988 to 1996. At the same time, the population of brook trout increased, compounding Fish and Game’s puzzle.

Now, the department is trying to patch holes in its understanding of whirling disease.

“Every time we come up with a theory, we shoot a hole in it,” Elle said.

Rainbows are vulnerable to whirling disease. Brook trout are, too, but they are thriving in the Big Lost. Brown trout evolved with the disease over eons in Europe and should be out of danger, yet they are dying in some Colorado streams.

Cutthroat and whitefish can become infected. Young steelhead, actually ocean-run rainbows, are catching the disease in the Upper Salmon, Elle said.

It is difficult to gauge how many fish die from whirling disease, because most merely disappear into the current.

But in Montana, where streams are fly-fishing shrines, the parasite is blamed for a 90 percent decline in the Madison River’s rainbow trout numbers. That translates to the loss of millions of dollars spent there by anglers every year.

A list of Idaho waters where Myxobolus cerebralis has been detected includes the North Fork of the Coeur d’Alene, St. Joe, Big Wood River, Silver Creek, East Fork of the Big Lost, Little Lost, South Fork of the Boise, Lemhi, Pahsimeroi, Salmon River in the Stanley Basin, South Fork of the Snake downstream from Palisades Reservoir, the Teton River and Henry’s Lake.

While trout have not completely vanished in those Idaho waters, they are ailing in several reaches. For example, in the Big Lost and Teton rivers, the agency in 1990 established fishing regulations that should have boosted the fisheries there. But that hasn’t happened.

The Stanley and Pahsimeroi hatcheries on the Salmon River try to use only pure well water to avoid infection. But they may have to draw some river water to rear chinook in 1998, which increases the chances of an outbreak at those locations.

“We’re not kidding ourselves,” Elle said. “We’re pretty sure if we have to use surface water there will be some infection in carrier fish.”

Making whirling disease more confounding is the debate over which factors in trout habitat foster the infection.

Some experts contend the disease abounds directly below reservoirs. Yet, Fish and Game has not detected infected trout below Anderson Ranch Dam on the Boise River, while it has farther downstream. Others believe spring-fed streams where sediment is not continually flushed are hazardous. But world-famous Silver Creek near Hailey matches that description and has not been the scene of die-offs.

Another debate is how whirling disease is spread. “My own personal opinion, from where we’ve found the infection, the primary source of spreading the parasite was moving live or dead fish between drainages,” Elle said.

Elle said biologists theorize that whirling disease may have been carried in frozen trout from Europe in the 1950s, but the refrigeration broke down and fish spoiled during the trip. The spoiled trout were ground up and fed to fish in hatcheries.

“The way we used to feed fish was to grind up protein,” he said. “They could be horses, dead cows. We fed it to the fish and that basically spread the parasite.”

He suspects the way it has spread in the West is through private pond owners who took infected trout from one river drainage, ultimately transplanting them into another basin.

“I’m a big believer that we must inhibit the movement of live and dead fish,” Elle said.

The province of Alberta, Canada, has not had a reported case of whirling disease. Fish managers there block the transfer of live fish.

MEMO: This sidebar appeared with the story: THE SYMPTOMS Whirling disease includes several stages and requires two hosts: a Tubifex, or “sludge worm,” and a trout or salmon. The fish ingest the microscopic worm infected with the protozoan parasite, or they ingest the parasite after the worm releases it into the water. The parasite chews through cartilage, putting pressure on the nervous system. Resulting deformities cause the fish to swim in circles, making it tired and vulnerable to predators. Extreme symptoms include a black tail and deformed head. When the infected fish dies and decomposes, the parasite’s spores are released into the water and taken in by the worms, completing the cycle. Steve Elle, an Idaho Fish and Game biologist, believes transporting live or dead fish between drainages has spread the disease around the West.