Experts puzzle the influence and impact of whirling disease

| July 03, 1997

Armed with the potential to devastate trout fisheries across the state, whirling disease has scared the hell out of Montanans who care about trout fishing since it was discovered on the Madison River in 1994.

During the past three years, the microscopic parasite has invited Big Sky outfitters to envision disaster, influenced anglers to wet their flies elsewhere and spurred Gov. Marc Racicot to assemble a pair of volunteer task forces to study the disease’s impact.

As you might expect of an infestation that has attracted the attention of 20 experts in this state alone, there have been reams of reports generated on the economic impacts, history and biology of whirling disease.

Discovered in Germany in 1893, whirling disease was first reported in the United States in 1958, when it was found in Pennsylvania. It has since been identified in 21 states, 14 of which, including Idaho and Wyoming, are still contending with the parasite.

Originally thought to be exclusive to hatcheries, whirling disease was discovered in Colorado in a section of the Colorado River in 1993. Since then, 10 other states have reported whirling disease in the wild, with Connecticut, Maryland, and Virginia claiming continued infection only in their hatcheries.

Countless reports and symposia later, few generalizations can be made about the future of whirling disease in Montana. While the potential for disaster remains real, fishing on most Montana streams is still surprisingly good--largely because native trout and exotic browns haven’t been hit as hard as rainbow trout.

Biologists suggest that the disease provides a reason for preserving and restoring Montana’s waterways. The parasite which causes whirling disease does not seem to do as well in clean waterways, and native fish which have been crowded out by exotic species seem to be staging a slight comeback.

While there is no consensus on how to deal with problem yet, everybody agrees that Montana’s fishermen, resource managers and other citizens would do well to learn a whole lot more about the parasite.


Swimming in circles

If you were to catch an infected trout, the fish in your creel might suffer from a mishapen skull or twisted posture. Fish afflicted by whirling disease swim in circles--hence the illness’ name. They’re vulnerable to predators, and have trouble feeding because their nervous systems are messed up. Sick fish also tend to have black tails.

Whirling disease is caused by a microscopic parasite, Myxobolus cerebralis, which attacks the cartilage of trout, causing skeletal deformities, particularly of the head and spine.

As young trout develop, their cartilage becomes bone, reducing their susceptibility to the disease. But in infected fish, the resultant deformities put pressure on their nervous systems, causing them to develop a frenetic tail-chasing behavior as they mature.

One of the hardest hit rivers in the state has been the Madison, where whirling disease is thought to be responsible for a 90 percent decline in the once-legendary rainbow trout population. This reflects the potential havoc wreaked by the illness, which has been found more recently in many other streams around the state, notably Rock Creek and the Big Hole River.

Soon after the disease was discovered, the governor responded to concerns about Montana’s world-class trout fisheries and the state’s $250 million annual income from coldwater fisheries by assembling the Montana Whirling Disease Task Force.

Since its inception, the panel has been headed by Marshall Bloom, a disease researcher for the National Institute of Health’s lab in Hamilton and an active member of Trout Unlimited. The task force’s first incarnation met for two years, researching the spread of the disease in Montana as well as the parasite’s life cycle.

In June 1996, with some handle on the basic science of the disease, the task force released its recommendations for action, suggesting a shift toward studying the socio-economic impacts of whirling disease and educating the public about the parasite and how to avoid spreading it.

These measures have largely been carried out by researchers at the University of Montana and the Montana Department of Fish, Wildlife and Parks.

As for concrete recommendations, the first task force issued only one. The team produced guidelines for anglers suggesting that they rinse their boats and waders to prevent transporting the disease from one river to another.

Earlier this year, realizing the state still didn’t know enough, Bloom assembled the second task force. This time he brought together a broader range of expertise and perspectives.

“For the second go-around,” Bloom explains, “my priority was to really expand the education and communication efforts, to focus on management issues, and also to focus on analyzing the economic impacts of the disease.”

Economically, the fish mortality caused by the disease has caused out-of-state anglers to choose to spend their recreation dollars elsewhere. Fishing guides estimate that anywhere from 10 to 40 percent of their business has been lost.

For the most part, though, Montana’s fishing guides, fly shops, lodges, and travel agents have played down the impact of the disease and played up the fact that there are still plenty of big fish to be caught.

Those suffering most, however, are the trout themselves, who couldn’t give a damn about dollars. For the fish affected by the parasite, the question is one of basic survival.

For the last 30 years, rainbow trout have been the most prolific and highly reputed species of trout in Montana streams. Thanks to a policy to stop stocking the state’s waterways with hatchery fish implemented in the early 1970s, populations of rainbow trout soared as the game fish began outcompeting other species. In rivers like the Madison, Montana’s rainbows earned a reputation for being wild, large, wily and high-leaping, and for offering a hearty challenge to any angler.

These days, the biggest challenge on the Madison is to find a rainbow trout. After soaring to an almost obscene 3,300 fish per mile in the 56-mile stretch between Quake Lake and Ennis Lake in 1991, the population plummeted to 300 rainbows per mile by 1994.

When talking about fish, though, it’s important to distinguish between the native trout--Westslope cutthroat, bull trout, Yellowstone cutthroat, Arctic grayling and whitefish--and species such as rainbow, brook, and brown trout that were first introduced to Montana’s waterways just prior to the 20th century.

Still, in a pamphlet entitled “Whirling Disease & Montana’s Wild Trout,” the spin doctors at the department of fish and wildlife point out that “the Madison’s total number of catcheable wild trout still favorably compares to other Montana blue ribbon trout streams, like the Big Hole, the Yellowstone, and the Gallatin rivers.”

Rock Creek not at rock bottom

“It’s becoming clear now that different drainages are responding differently to whirling disease,” says Bill Grannath, a UM researcher and a member of the second task force. “Unfortunately, people immediately assume that it’s going to do the same thing on Rock Creek that it’s done on the Madison.

“Well, it hasn’t done that. Why different drainages respond differently, we don’t know for sure.”

Between 1989 and 1996, Rock Creek’s population of rainbow trout declined about 50 percent, which seems devastating if not on the same scale as the 90 percent decline in the Madison over the same period. Yet, while the Madison River’s brown trout population has remained steady, Rock Creek boasts more than five times as many brown trout as it had in 1989.

Many say the difference stems from the fact that on the Madison, the rainbows tend to spawn in the main stem of the river, which is thick with whirling disease. Thus, the young rainbows are exposed to the disease at an age when they’re thought to be ex- tremely susceptible.

In the Rock Creek system, the rainbows tend to spawn in tributaries, which are relatively free of whirling disease. The young trout then develop in the tributaries, where they can build up a resistance to whirling disease before heading into the infected sections of Rock Creek.

Why the brown trout numbers are increasing in Rock Creek is another mystery yet to be answered. “I’m not sure how the brown trout are benefiting from the decline in the rainbows,” says Don Peters, a state fisheries biologist. “It sort of defies explanation right now.”

More easily explained are the increases in the native Westslope cutthroat that have been observed in the Madison River. “On the Madison River, what’s happened is that as the numbers of rainbows have gone way down, the numbers of cutthroat appear to be starting to go up,” says Bloom.

“The [cutthroat] numbers were extremely low. So we’re talking about a difference from about three fish per three miles to about 50 to 60 fish per three miles. Before whirling disease, nobody ever really caught cutthroats there, and now people are starting to catch them.”

Initially, the success of cutthroats was a surprise, considering that in the laboratory they proved to be as susceptible as rainbow trout. Their survival, like that of the rainbows in Rock Creek, may lie in their dependence on the Madison’s tributary streams for their spawning grounds. Bull trout, which Bloom describes as having a susceptibility to whirling disease somewhere in between that rainbows and the more resistant browns, also tend to spawn in feeder streams.

Thus, in reducing the number of rainbow trout, whirling disease may actually be doing Montana’s fisheries a favor over time. “What it suggests is that the native trout, the cutthroat, may turn out to be the basis of the fishery in the Madison River in the future,” says Bloom. “To me, that’s a really, really encouraging sign.”

Indeed, to anyone who cares about Montana’s native trout and the restoration of original species, whirling disease could turn out to be the opposite of the disaster it seemed initially. Bloom suggests, when imagining the parasite’s potential benefit to Montana’s native species, “We might end up with a fishery very close to what we had in back in the days of Lewis and Clark.”

As the worm turns

The key to saving Montana’s fisheries may lie in scientists’ capacity to understand and stem the spread of the disease from fish to fish. But researches have struggled from the get-go to establish how exactly the parasite attaches itself to trout.

The life cycle of whirling disease is complex, and researchers spent many years of frustration before establishing the mechanism by which the disease is transferred from one fish to another.

The disease is similar to malaria in that it requires two hosts to support its life cycle. Just as malaria cannot be passed directly from person to person, whirling disease can’t be transferred from fish to fish. In the case of malaria, the disease reproduces sexually in the primary host, humans, then requires an intermediate host--the mosquito--within which it reproduces asexually. This intermediate host then aids in transmitting the disease back to the primary host.

The difficulty in pinpointing the life cycle of whirling disease lies in the fact that the parasite’s life cycle is unlike that of most. Rather than reproducing sexually in the primary host and asexually in the intermediate host, whirling disease reproduces sexually in both the trout and the tubifex worms, an arrangement that Grannath, a disease researcher, had never seen before.

When a fish infected with whirling disease dies, its decomposing corpse releases spores which are eaten by the worm Tubifex tubifex, a small cousin of the common earthworm. The spores are extremely hearty, and may survive freezing temperatures as well as drying out completely. Once inside the digestive tract of the tubifex worms, the spores shoot out what are called polar filaments into the intest-inal walls of the worms.

After about three months, the worm, which is unharmed, excretes the infectious form of the parasite. Whirling disease is then taken up by fish through their gills, their digestive tracts and their skin. The spores attach wherever they can, shooting filaments into whatever flesh they can find.

“To me, the worm is really going to prove to be the key to this problem in the state of Montana,” says Bloom, who points out that the spores can be transported from one stream to another on fishing boats or waders, or through the transport of infected fish. “We know that all of our rivers are susceptible to whirling disease.”

First identified as a necessary component of the parasite’s life cycle in the mid-80s, tubifex worms, once known as “sewer worms,” may be the key to disrupting the spread of the disease. While little is known about how exactly the parasites kill trout--aside from making them vulnerable to predators by disrupting their natural behavior--clean water, which is good for trout, is bad for whirling disease.

“Tubifex worms are very poor competitors with other invertebrates,” says Grannath. “So in a pristine environment, where you have caddis flies, mayflies and stone flies, they feed on the worms.”

While tubifex can survive in just about any aquatic habitat--including springs with almost no nutrients--stone flies, mayflies, and caddis flies rely on relatively pristine habitats with low levels of siltation and organic compounds. Where populations of these insects are healthy, such as in the Bitterroot River, the tubifex populations are low and the incidence of whirling disease is minimal.

The only way to ensure healthy populations of stone flies, mayflies, and caddis flies is to provide them with healthy, unpolluted streams to live in. In other words, the more Montana’s streams and streamside habitats can be preserved and rehabilitated, the better populations of tubifex worms--and whirling disease--can be held in check.

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