Yellowstone National Park and state of Montana fisheries crews devoted an entire week to poisoning unwanted, nonnative brook trout out of Soda Butte Creek near Cooke City back in 2015.

This story also appeared in WyoFile

When rotenone, a chemical used to poison fish, coursed through the 38 stream-miles of the Lamar River tributary that summer, some 450 brook trout went belly up. 

The next summer, in 2016, two surviving brook trout were detected. The northwestern Wyoming and southern Montana watershed got another dose of poison to ensure that the creek would remain a stronghold for the native species: Yellowstone cutthroat trout. 

For five straight years afterward, no brook trout, a cutthroat competitor, were detected via routine monitoring and “eDNA” technology, which detects the whole suite of species in a waterway with great precision.

Yellowstone had won the fight against brook trout, at least in the embattled Soda Butte Creek watershed, or so it seemed. 

Brook trout, pictured, are a fall-spawning fish that can outcompete native cutthroat trout, especially in smaller headwaters streams. Credit: National Park Service

Then, last fall, Yellowstone Fisheries Supervisor Todd Koel got discouraging news. 

In a one-mile stretch of Soda Butte Creek, electrofishing crews discovered 15 or 16 brook trout. And they were a few different sizes, suggesting different age classes. The pod of brookies materialized from out of nowhere — or they got a helping hand from a human illegally tinkering with the fishery. 

“It’s a good chance of that, unfortunately,” Koel said. “It’s a good chance. That’s how lake trout got to Yellowstone Lake, right?” 

Talking with WyoFile, Koel and Montana Fish, Wildlife and Parks Regional Fisheries Manager Shannon Blackburn were clear: There’s no hard evidence that newly discovered brook trout in the split Montana-Wyoming stream were the work of a bucket biologist. 

At least not yet. 

“I really wish we knew where they came from,” Blackburn said. “It’s so hard to say, but hopefully between potential genetic analysis and some other microchemistry, we can get a better idea of the source.” 

Potentially, they can pinpoint the source with precision. Fisheries scientists can analyze bones called otoliths in brook trout ears to tell them exactly where the unwanted salmonids came from. And there are some likely sources. 

“There’s a gravel road that goes right from the campground right at Cooke City,” Koel said. “You can go right up the road to access lakes up high.” 

Those lakes are in another watershed, he said, and they contain brook trout. 

With few exceptions, Koel said, the 2015 operation to remove brook trout from Soda Butte was broadly supported

“There’s just one person that I know of who, for some reason, liked brook trout to be in upper Soda Butte Creek,” he said, “and he’s not from the area.”

Notably, the reinvaded or reintroduced brook trout in Soda Butte Creek haven’t yet gained much ground. Using eDNA, Koel and Blackburn’s crews determined they’re confined to a short and isolated stretch of the stream in Yellowstone National Park. Once again, they’re bringing out the rotenone. 

“It’s kind of a smaller spot treatment to remove these fish,” Koel said. “We’re not treating all of upper Soda Butte, we’re only treating the main stem from the park boundary down to Ice Box Falls.” 

Nearly 10 miles of Soda Butte Creek will be closed the week of Aug. 14-18, spanning from the Yellowstone National Park boundary downstream to Ice Box Canyon. Credit: National Park Service

For the first time in seven years, fisheries crews will be dispensing the piscicide in those 9.6 stream miles this week. The operation is expected to span the week of Aug. 14 to 18, during which time Soda Butte Creek will be closed to angling and swimming. 

Mostly, the cutthroat will be spared. While Koel and Blackburn were talking to reporters last week, their crews were wading Soda Butte Creek’s waters to electroshock the stream and salvage the native fish. 

Other crews are out seeing if they could find any more brook trout. 

“So far, nothing,” Blackburn said. “Just cutthroat.” 

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