Two decades ago, rainbow trout in the upper Madison River were struggling. Whirling disease had been found in the stream, caused by a microorganism that latches onto fish. Brown trout aren’t affected by it, but rainbow trout are, and numbers of adult rainbows dwindled. In one section of the river, estimates of adult rainbows — those larger than 14 inches — fell to fewer than 150 a mile.
That was about the time Tim Weiss, a fisheries technician with Montana Fish, Wildlife and Parks, started working on the Madison. A little more than two decades later, the density of adult rainbow trout in that river has changed dramatically.
“Back then, it was hard to find rainbows over 14 inches,” Weiss said. “And now, they’re all over the river.”
The number of rainbow trout in the river has once again reached eye-level with the number of browns in the river. The encouraging numbers allowed the department to loosen some fishing regulations on the stream, allowing more year-round opportunity for anglers.
FWP fisheries biologist Dave Moser said the rebound is something to celebrate.
“Populations in the Madison are doing well,” Moser said. “And people are catching fish.”
In 1994, near the peak of the disease outbreak, FWP estimated there were about 142 adult rainbow trout per mile in the Pine Butte section of the upper Madison, about three miles of river upstream of Lyons Bridge.
In 2014, the estimate had climbed to 807 adults per mile, even higher than the 1988 estimate of 748. Last year’s was 662. Per mile estimates for rainbows of all ages came in at 1,943 in 2015.
The numbers for observed deformities related to whirling disease has decrease as well, from 10 percent a half decade ago to about 2 percent now.
Biologists and anglers alike are thrilled with the data, but it doesn’t mean they are completely out of the woods.
Whirling disease is known to be in about 150 rivers and streams across the state, and the tiny organisms that infect fish don’t just go away. They can remain in the stream for decades, and perhaps even longer.
“They are going to persist at a low level forever,” Moser said.
So there is a chance that the rainbows in the Madison River could be hit by the disease again, but Moser and Weiss are fairly confident the fish have found some way to beat it. They just can’t pinpoint how.
Moser said it could be that rainbows are spawning in areas where the organisms causing the disease are less prevalent. The trout also could have developed some sort of adaptation that makes them more resistant to the disease.
Bruce Farling, the executive director of Montana Trout Unlimited, said a trout’s susceptibility to infection has a lot to do with when and where it spawns each year.
“It’s often a matter of timing or life history,” Farling said.
He added that fish surviving today are likely descendants of those that survived the outbreaks in the 1990s, which would mean how those fish live is already resistant to the disease.
Moser said that another possibility is the stress level of the trout. Fish that are tired or living in extremely warm water may be more susceptible to infection.
“In the fish disease world, when an animal is stressed, that’s generally when the disease has the most impact,” Moser said.
They have some help with that. Hebgen Dam is a regulating force for the upper Madison, often meaning the temperatures stay low there while other rivers heat up.
In late summer, FWP places “hoot owl” restrictions — nighttime and afternoon fishing closures — on some streams because water temperatures near intolerable levels. The upper Madison is rarely one of the streams that see those restrictions.
Moser said they have concerns that high temperatures may become a problem in the future, but for now, things are looking good for those fish.
“I don’t think they are stressed,” he said. “I think they are doing well.”
Anglers, too, are enjoying themselves. Weiss said they’ve heard plenty of positive reports from folks fishing the stream. Fish larger than 14 inches are likely between 3 and 4 years old. They are the ones anglers like to catch, and Weiss said those fish are getting caught.
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