In the '90s, whirling disease hit trout populations hard in Montana, at one point leading to a 95% decline in rainbow trout in the Madison River. It sparked concern among biologists, anger in tourist towns and even played a role in a murder mystery novel. It also inspired this week's question: What's happening with whirling disease and other threats to trout?
Austin Amestoy: Welcome to The Big Why, a series from Montana Public Radio — and introducing our friends at Yellowstone Public Radio — where we find out what we can discover together. I'm your host, Austin Amestoy. This is a show about listener-powered reporting. We'll answer questions, big or small, about anything under the Big Sky. By Montanans, for Montana, this is The Big Why.
Joining me this week is Jackie Coffin, News Director at Yellowstone Public Radio. Welcome Jackie.
Jackie Coffin: Thanks for having me, Austin! I’m excited to be here and so is Yellowstone Public Radio! We are stepping into the Big Why to answer listener questions from central and eastern Montana.
Austin Amestoy: And Jackie, your question today comes to us from Great Falls, more specifically, the banks of the Big Muddy – the Missouri River.
Jackie Coffin: Yes it does, Austin. My question is from Mark Ozog, who is an avid fisherman of the Missouri.
Mark Ozog: I get all over the state, to be honest with you, but the Missouri is my home river and that's why I like to fish. But I really have a hankering for small little streams if I can find one that's good.
Jackie Coffin: He has a question about Montana’s trout health.
Mark Ozog: I want to find out what's going on now with whirling disease and any other invasive problems that are facing trout. And if anybody knows the answer, it's gonna be The Big Why.
Jackie Coffin: And for Mark, this question had a pretty intriguing inspiration. His chapter of Trout Unlimited has a book club and they read Keith McCafferty’s The Royal Wulff Murders. That’s W-U-L-F-F like the popular dry fly. The story is set in a fictional Montana town along the Madison River and whirling disease plays the part of an ecological clue in a series of mysterious deaths.
Austin Amestoy: Ooh, that is intriguing. So this fly fishing murder mystery novel sparked the question for Mark to learn more about trout health in real life.
Jackie Coffin: That’s right Austin. And so, just like the Royal Wulff Murders, answering Ozog’s question about whirling disease puts us on the course of tracking a killer.
Pat Byorth: The Madison was kind of the ground zero for whirling disease, and it had a profound effect on the trout population from the mid to late '90s, where the population initially declined about 95% — the population of rainbow trout.
Austin Amestoy: It sounds like you found someone who can help us do this detective work.
Jackie Coffin: Yep, that’s Pat Byorth. He’s the Montana Water Director for Trout Unlimited, and he was a fisheries biologist for Montana Fish Wildlife and Parks on the Madison River right in the midst of the whirling disease outbreak, which started as the invasive parasite was found on the river in December 1994.
Pat Byorth: So, whirling disease, the name comes from when very young trout are exposed to whirling disease, their brain gets squished as the skull gets deformed as they grow. And they whirl as they swim, they can't orient themselves.
Jackie Coffin: Whirling disease is caused by an invasive parasitic spore, brought over to North America from Europe. And by the time it hit Montana, it had already done a lot of destruction to other Western trout populations The impact on the Madison river was enormous.
Pat Byorth: It's a big, very popular — one of the most popular trout streams in America, if not across the world.
Jackie Coffin: Byorth describes a massive research effort, huge amounts of funding; scientists, anglers, stakeholders from all over the world collaborating to try to figure what this disease could do, and how to stop it.
Pat Byorth: We knew so little about how these spores could be transmitted across the landscape.
Austin Amestoy: Wow, Jackie, it sounds like the stakes were really high here. What was going on off the water, though? How did the public respond to all this?
Jackie Coffin: Things got very tense. In the town of Ennis, a lot of businesses and livelihoods are tied to fishing the Madison and its tributaries. As biologists were working to figure out what was happening to trout populations and how to stop it, they closed sections of the river and nearby streams to fishing, triggering a lot of social and political upheaval.
Pat Byorth: People needed somebody to blame. So who did they blame? You blame the field biologist, blame the fish and game.
Jackie Coffin: Byorth remembers a particular moment when emotions boiled over.
Pat Byorth: I remember this brutal public meeting down in Ennis … and this gentleman stood up and he was spitting mad, just angry, 'shut the hell up. You don't need to talk about this. This is not important.' And I just told him, I said, 'we have a job to do to keep these trout here, not just for now, but forever.' And kind of stormed out.
Austin Amestoy: Wow, what a saga! What ended up happening with whirling disease? Is it still wreaking havoc on our trout populations? What’s going on?
Jackie Coffin: That’s what our listener Mark wanted to know. As Byorth said, one of the top priorities during the initial outbreak was to stop the spread of whirling disease to other waterways. In those early days, scientists learned that it can be transmitted through things like dirty boots, waders, and boats — and there’s no cure. There was a big public push to stop transmission, much like the battle against invasive mussels today. But ultimately, over the course of the last 10-12 years, it was the fish who started getting the upper hand against this killer.
Mike Duncan: The populations rebounded after about a decade, and it seems like there's some naturally built in resistance with the populations that remain.
Jackie Coffin: That’s Mike Duncan.
Mike Duncan: I'm the region three Fisheries Manager for Montana Fish Wildlife and Parks.
Jacki Coffin: Duncan says where it stands in 2025, whirling disease is widespread through Montana’s waterways, but it’s not as threatening to trout populations or concerning to wildlife managers as it once was.
Mike Duncan: It's very, very few fish of the thousands of fish that we handle that show signs that might be indicative of whirling disease. There's some build-in resistance at this point at the population level.
Austin Amestoy: So the fish managed to build up resistance to whirling disease. How did that happen, and is it still as deadly as it was in the '90s?
Jackie Coffin: Well, trout adapted. Vulnerable baby trout were far more resilient if their parents spawned in colder-water tributaries of the river.
Pat Byorth: Fish that spawned in these tributaries throughout the Madison that are much colder and purer, have fewer worms. And so, even though whirling disease might have worked its way into those tributaries, they weren't really set up in the same way.
Jackie Coffin: Byorth mentioned to me that you can catch rainbow trout on the Madison today that have these distinct, warped heads that is an indicator of present or past whirling disease that the fish has survived.
Austin Amestoy: Well, I have to say, that's a pretty cheery end to a murder mystery story.
Jackie Coffin: It is! It is a big obstacle Montana’s trout population overcame. But the hard part of river life is there are other obstacles too.
Austin Amestoy: Ok, this sounds like the setup for a sequel to our trout health mystery saga.
Jackie Coffin: Yeah, Austin, it does. Our listener also wanted to know what other invasive species issues trout are facing .
Austin Amestoy: What did your trout biologists have to say about that?
Jackie Coffin: Mike Duncan with FWP said the biggest threats facing our cold-water fisheries right now aren’t necessarily invasive-related at all. High temperatures and low flows can put a lot of stress on trout.
The Big Hole River, and other rivers in the area, are seeing big declines in trout populations as fish show up with head lesions and fungal infections. Some people call them "zombie trout." So Duncan said the task ahead is spending a lot of time on the water, "testing both infected fish and seemingly healthy fish to see if we can get a better understanding of exactly what the pathogens are that's causing those infections and then scaling it up.
Austin Amestoy: Another trout mystery in the making.
Jackie Coffin: That’s right Austin, we’ll follow up on that one as time goes on. But for now, we can close the case on what happened to whirling disease in Montana.
Austin Amestoy: Well, Jackie, we appreciate the in-depth investigation for Yellowstone Public Radio's first time on The Big Why. Glad to have you here.
Jackie Coffin: So glad to be here Austin.
Austin Amestoy: Now we want to know what makes you curious about Montana. Submit your questions below, or leave a message at 406-640-8933. Let's see what we can discover together!
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