Monday, January 28, 2008

Call it a Comeback: The Madison River Fights Back against Whirling Disease

If you’re like me, the first time you heard the words 'whirling disease' it was probably followed immediately by the words 'Madison River'. In 1994, whirling disease was discovered in the Madison, and all hell broke loose. I was 18 years old at the time and had experienced my first trip to the Madison a few years prior.

My father, on the other hand, had been fishing the Madison for years. I remember his tales of fishing the '50-mile riffle' complete with characters that seemed to have just stepped out of a Norman Maclean story. For a kid hooked on trout fishing, this was like heroin. But the discovery of whirling disease threatened to end any chances of creating my own fishing tales on one of the West’s most fabled rivers. In the years directly following the discovery, rainbow trout populations in the Madison plummeted, and the fishery was largely written off by destination anglers.

A Madison River Brown

So what happened? Well no one is certain, but there seem to be a couple of factors associated with the Madison’s Lazarus-style return. One new discovery: the flows matter. The Whirling Disease Association stated:

One important study found that higher than normal flow levels at the time of rainbow fry emergence appeared to dilute the whirling disease infection thereby increasing survival rates. These phenomena likely produced the sizeable rainbow trout year classes in 1998 and 1999.

While this was good news for the trout of ’98 and ’99, it is potentially bad news for the trout of just last year. A guide I talked to last summer said that Madison never even turned brown with run-off last year. And when I fished it in August, the caddis hatch was running a month ahead. Between the high water temps and the tendency of the WD parasite to thrive in low water, its possible that last year’s class of Rainbow trout may be a light one (as if you needed another reason to pray for a wet Spring).

Amazingly, the resurgence may also be tied to the adaptability of the fish. This article, by the Montana Wildlife Federation from 2007, discusses a mystery surrounding the increased survival rates.

Whirling disease infection rates are measured on a scale of 0 to 5, with 5 being the highest rate of infection. Rates of 2.8 and higher usually show severe consequences; rates under that are usually benign. In the Madison, the rate dropped into the low 2s during the high water years, Vincent said, which explains why there were good years for young-of-the-year survival. But, predictably, once the water level returned to average in 2005, the infection rate rose to the mid 4s. But something mysterious happened.

"There was good recruitment," Vincent said, meaning the fingerling that would usually be highly susceptible to the disease survived.

Was this a sign of whirling disease resistance? To find out, biologists took some of the Madison rainbows' eggs to a lab near Pony. They hatched the eggs, raised the fish to 60 days of age and then artificially exposed them to high levels of whirling disease. The fishes' infection rate was in the 1s. In a similar study conducted in 1998, the exposed fish had infection rates in the 4s, Vincent said.

"We don't know why that happened," Vincent said. "We have a lot more young fish 6 to 10 inches in length than we did during the peak of whirling disease. We've never witnessed anything like it in any other place in the U.S. We're looking into why."

Does this mean that the Madison is in the clear? All the experts say no. We don’t know enough about whirling disease to predict how a fishery might respond. While the Madison was initially decimated by whirling disease, the Missouri has been infected with similar rates of the disease yet has not had nearly the population declines that the Madison suffered. Some researchers think the answer lies in a WD-resistant strain of trout, but no one is sure how or why some trout remain unscathed. The possibility of another decline in trout populations is a specter that hangs over all rivers infected the disease.

What we do know is that Madison is back to being a premiere fishery, although the fish are not quite as prolific or as large as year’s past. And for me, I have the chance to tell some of my own stories about a river my father fell in love with more than 30 years ago.

For more info on this topic, go here, here, and here.

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