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LAST OCTOBER, when biologists counted fish in the Missouri River above Cascade, Montana, they found record numbers of big, heavy rainbow trout. "Off the chart" was the notation for one section of the river that outdoor writers were calling the best trout stream in America. And the fish were in tremendous condition; huge.

"They always are heavy," says George Liknes, a fisheries biologist for Montana Fish, Wildlife & Parks. "These were like pond fish. You had to use two hands to handle them and get their lengths. It actually slowed us down."

But there was something missing on the Upper Missouri. Scientists didn't find many little fish--the fish that grow up to be big fish, the fish needed to sustain the historic population of rainbow trout anglers found there last year.

Whirling disease, confirmed three years ago in the Missouri River below Holter Dam, is taking its toll. Populations of two-year-old and older rainbow trout were up 200 percent over the historic long-term on one stretch of the river and 300 percent on another. Biologists estimate that 38 percent of the long-term historic number of yearling rainbow trout survive in the Craig section of the Missouri and 73 percent survive in the Cascade section.

"The infection rate and intensity of whirling disease have risen on the Missouri," Liknes says. It was already quite high in Little Prickly Pear Creek, a crucial rainbow spawning ground. And while the tributaries of Little Prickly Pear have low infection rates, as do the Dearborn River and Sheep Creek, whirling disease has escalated in the river.

Yearling numbers have declined, especially in the Craig section, but they still all are within historic levels. "It is the sharpness of the decline. I am not aware of any other factor besides whirling disease that would cause that," Liknes says.

Whirling disease, a European invader first detected in North America in 1956, is caused by a microscopic parasite that completes part of its life cycle in an aquatic worm, then preys on young trout or salmon by destroying cartilage in their heads and spines before it hardens into bone. Infected young fish become crippled and end up swimming in circles, making them easy prey.

Montana's first known outbreak of whirling disease struck the Madison River in 1994 and quickly decimated young rainbow trout, which along with cutthroat trout are among the species most vulnerable to the disease. Eventually, rainbow numbers in the Madison declined 90 percent or more. On Little Prickly Pear, rainbow trout numbers also have crashed 90 percent.

Will rainbow-trout populations on the Missouri crash? Will we be writing and reading next year about how it used to be? Probably not. Still, Liknes offers a sobering assessment. "We have a real potential for going from the best of times to the worst of times, but we still are in transition," he says. "It is a top-heavy population that you cannot sustain.

"We have the best fishing we have ever had, but it's probably time to prepare people to see real potential impacts. The growth we have seen may actually be the realization of whirling disease, because there is less competition."

It's Scary

GREAT FALLS FISHING GUIDE and author Neale Streeks says things can not stay great forever. "The obvious scene we are looking at is, you have a good sendoff," Streeks says. "The population is as high as I have ever seen it, and I have been guiding 21 years. You have to expect a pretty good drop-off."

"It is scary," Streeks says. "But at this point the Dearborn is not very infected, so it is not like all of the fish are going to disappear.

"There are lots of little brown trout and that is the other scenario--that the browns would fill the gap," Streeks says. "It's not like of all of sudden there aren't any little rainbows.

"The Missouri is such a rich river that with anything drastic, it would rebound in a reasonable amount of time. The habitat is good enough that browns would fill the void to some degree. And other spawning tributaries are still healthy enough at this point, so it is not some kind of a light switch.

"The dark cloud is that it would be a real drastic decline, and then take years and years to build back up. That would be the worst-case scenario," Streeks says.

The Numbers

EACH FALL, BIOLOGISTS CONDUCT fish counts on three sections of the Missouri. They stun the fish with electricity and mark 2,000 to 2,500 rainbows. About ten days later, they shock again. By comparing the number of marked to unmarked fish, they can determine population levels.

Biologists have studied two sections of the Missouri since the early 1980s: the Craig section, which covers five to six miles between the Wolf Creek and Craig bridges; and the Cascade section, which is four miles of river straddling Pelican Point.

A third section of river (the Hardy section) from below the mouth of the Dearborn River to mid-Canon had been studied sporadically until 1992, when annual examinations began.

Trends based on population estimates taken last fall show that on the Craig section in 1999 there were 4,800 rainbow trout per mile that were two years old or older or at least 11 to 12 inches long. The long-term mean average is 2,332 fish of that age per mile.

"We are at 200 percent of the long-term mean. That is much above anything we have ever seen before. It looks like an historic high," Liknes says.

From 1982 until 1999, biologists estimated 1,500 yearling fish per mile, but in 1999, that number tumbled to 582 fish per mile, only 38 percent of the long-term.

"You've got to be a fool if you don't expect whirling disease to be a factor," Liknes says.

On the Cascade section in 1999, biologists found 3,346 catchable rainbow trout per mile, or more than 300 percent of the 989 long-term historical mean. But there were only 772 year-old fish per mile, or 73 percent of the long-term mean of 1,059.

On the Hardy section, biologists found 2,800 two-year-old rainbow trout per mile in 1999. Historically, there have been 1,000 to 1,100 per mile. But biologists found about 830 yearling fish per mile in a section that Liknes says would be similar to the Cascade section. That is the section that has not been studied over the long-term.

Liknes attributed the large numbers of fish and thus the incredible fishing of late summer and fall to favorable flows in the Missouri and its tributaries, mild winters, and above-average recruitment of new fish. Still, he says, he was surprised by the decline in young fish.

"You would expect it to be dynamic, you would expect some fluctuation," Liknes says. "But looking at the data, with no other factors, you wouldn't expect this."

Liknes says the decline of young fish in the Hardy and Cascade sections of the Missouri are-within the historical averages--that the fluctuations are well within what biologists might expect to see in a trout population.

"The bottom isn't dropping out, but the decline is going to be steady," Liknes says. "You would expect that, rather than a precipitous crash, you will see a long decline over a number of years.

"Whirling disease is not an instantaneous impact to a population. It takes some time to assess where it will affect a wild-trout population. We still haven't had impacts to catchable populations, but we are seeing future catchable fish declining, especially in areas where the infection and its intensity have escalated," he says.

John Kowalski, a partner in Paul Roos Outfitters, was saddened but philosophical about these latest numbers.

"On any river, the quality of the fishing dictates the amount of use," Kowaiski says. "The Missouri has been a prime example of that. The last few years we have seen a dramatic increase in use on the river and the quality of fishing has been superb.

"Absolutely, it will affect our business--maybe not this year, but in following years. If the fishing declines, we won't promote the Missouri as much.

"We will be honest with our clients. Some it will affect, and some it won't. It will be a decrease in business, but they will be off to the next hot spot, which isn't all that bad.

"The pressure on the river was quite substantial, so maybe it is nature's way of telling us to back off a little bit," Kowalski says. "We kind of expected it. You never know how this is going to affect a fishery, but obviously it wasn't going to do it any good."

[This article has been reprinted courtesy of the Great Falls Tribune, Copyright 2000. THE EDITOR.]

MICHAEL BABCOCK is the outdoor editor for the Great Falls Tribune in Great Falls, Montana.


FOR UP-TO-DATEWHIRLING DISEASE news and information contact the following organizations:

Montana Whirling Disease Task Force

(406) 444-2449


Whirling Disease Foundation

(406) 585-0860

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Title Annotation:rainbow trout numbers
Publication:Fly Fisherman
Geographic Code:1U8MT
Date:Sep 1, 2000
Previous Article:TIGHT LINES.

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