Rainbow rebound - In Madison River, new trout species makes whirling disease stand
Rainbow rebound - In Madison River, new trout species makes whirling disease stand

ENNIS - An evolutionary accident - an unexpected biological treasure at the end of the rainbows - has saved a famed Madison River fishery from spiraling into memory.

"It's truly remarkable," said Dick Vincent. "A decade ago, whirling disease had wiped out

90 percent of the Madison's rainbow trout. Today, we have a population that's highly resistant and bouncing back quite nicely."

Vincent is recently retired from his longtime post of whirling disease coordinator at the Montana Department of Fish, Wildlife and Parks. Never in his wildest dreams did he imagine that before he stepped down he'd see Madison River rainbow populations at 70 percent of their historic numbers.

He calls the rebound "remarkable," and "amazing," and "just wonderful," and also "a bit of a mystery."

"Who knows?" Vincent said. "Maybe the Madison just got lucky."

And maybe, just maybe, some of that rainbow luck can color other waters.

Dick Vincent was an FWP biologist back in 1991 when he first noticed that the Madison's young rainbow trout seemed to be dying off. He scratched his head a bit, nursed some quiet suspicions, kept careful track of the numbers.

But years passed, and it wasn't until 1994 when he finally put a name to the problem - whirling disease. That's what they called it down in Colorado, where rainbows and other salmonid fish were circling the evolutionary drain.

The disease, first described in Germany in 1903, begins with a microscopic parasite, Myxobolus cerebralis. After being introduced into North America in the 1950s, the parasite quickly grabbed hold of trout, especially rainbow trout.

And here's what happened, time and again and again:

The parasite wriggles its way into fish flesh, injecting spores that travel along neural pathways to attack and eat cartilage.

The attack is especially tough on young fish, which are still more cartilage than bone. Their tails blacken, their bodies deform, and they die.

As nerve damage increases, the fish often whirl in circles, unable to eat or avoid predators. Thus whirling disease.

When the trout finally dies, the spores release into the water, where they're gobbled up by tiny Tubifex tubifex worms. Those worms play host while the next generation parasite grows, ready to attack more fish.

The infection won't hurt humans but it can decimate fisheries and there is, Vincent said, no cure - and no way of scouring an infected waterway. An estimated 150 Montana streams have been affected.

"In some places," he said, "it's caused almost no problems. In other places, it's been very severe."

Severe, certainly, in the Madison. That's where Vincent found it, the first confirmed Montana infection, a full 15 years ago. "It certainly was in other places by then," he said, "but we just didn't recognize it."

Rainbow trout are not native to Montana. They came from California, a century ago and more, sloshing along the rails in water-filled milk jugs. At the time, a rainbow was a rainbow was a rainbow, despite the fact that distinct sub-species came from distinct watersheds.

One of those coastal watersheds - no one now knows which one - provided the rainbow trout that arrived at Wyoming's DeSmet Reservoir, out near Sheridan, back in the early 1890s.

Eventually Wyoming's fishery biologists killed off the DeSmet strain, in favor of a rainbow easier to catch, but not before Dick Vincent got his hands on a few.

In 1977, Vincent trucked a load of Wyoming's DeSmet rainbows into Montana's Willow Creek Reservoir, near the tiny town of Harrison. That was long before the disease, and Vincent just wanted some good sport fish for the lake.

But the whirling disease parasite eventually arrived at the reservoir, too, "and it did wreak a bit of havoc, but not nearly as much as we expected."

That's because 30 percent of those wild DeSmet rainbows tested naturally resistant to the parasite. Everywhere else - including the Madison - only 1 percent of fish showed any resistance; which is why, in some Colorado rivers, some

98 percent of rainbows have been wiped out by whirling disease.

Today - what with the California roots lost to history and the Wyoming fish killed off - Montana remains the last known home of DeSmet rainbows. They persist only in Willow Creek Reservoir, and in a single high-mountain wilderness lake - and in both places they have proved highly resistant to whirling disease, killing the parasite before it burrows through the skin.

And although he has no definite proof as yet, Vincent is convinced their genetic heritage survives in one other river - the Madison. Perhaps, he said, they arrived by way of Hebgen or Ennis lakes, where a few Harrison fish were later stocked.

"Personally," he said, "I'm pretty sure some of those DeSmet fish escaped from Harrison and ended up in the Madison. Then evolution went to work, and selected out the ones that can handle the disease."

At Willow Creek Reservoir, where a solid DeSmet population initially proved 30 percent resistant, many rainbows died. But those that survived passed their resistance on to their offspring, and now some 98 percent have resistance.

Down in the Madison, where only a few escaped DeSmet fish are thought to have lived among other rainbows, just 1 percent of the fishery showed initial resistance. The fishery collapsed, with whirling disease claiming all but

10 percent of the river's rainbows.

But again, with the presumed help of a few DeSmet genes, the survivors passed on their good fortune and now

95 percent test resistant.

"They're very well recovered from the darkest depths of the whirling disease," Vincent said. "The Madison is a surprising success story."

It is, he said, the only Western river known to have recovered on its own.

Vincent has tried to track the genetic history of those DeSmet rainbows, hoping to unlock the clues of disease resistance, but historic records are incomplete at best. He's not sure where in California they came from, for instance, or even if they survive there today.

"It's a real puzzle, actually."

He's also not sure how the DeSmet rainbows will fare in the long run, as they obviously are not evolved for the particulars of the Madison. How will they deal with seasonal water level changes, for instance, or warm water flows?

"That's one area we'll need to look at," he said. "Just who are these new Madison River rainbows? Because genetically, they sure aren't the same fish that were there 20 years ago. There's been a genetic bottleneck. Will evolution iron things out? I guess only time will tell."

That's why he's still advising caution, before biologists rush out to stock Montana streams with Harrison's DeSmet fish. Down in Colorado, and in Utah, where whirling disease has hit so hard, they've already started introducing Harrison's fish in hopes of also introducing parasite resistance.

In Montana, however, "it's not that bad, yet," Vincent said. "We know we have the stock, and we know it's not going anywhere, so let's not be hasty. Let's do some basic research before we go moving fish around willy-nilly."

After all, he said, it was moving species around that got us into this mess in the first place.

"The Madison is coming back," Vincent said. "Let's see how that turns out over time, before we take any drastic steps."

Reporter Michael Jamison can be reached at 1-800-366-7186 or at mjamison@missoulian.com.

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