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CLINTON — Last April, Brad Liermann stuck a radio transmitter in a cutthroat trout’s belly and let it go. On Wednesday, he found it again just a few hundred yards from where he’d released it.

In between, thousands of fish enthusiasts tracked “Rocky” as he beeped his location to the “Race Up Rock Creek” website. Something of a homebody, Rocky only traveled 2.6 miles in six months.

In comparison, “Free Willy” the cutthroat logged more than 60 miles up and down the blue-ribbon fishing stream a half-hour east of Missoula. He went downstream to the Rock Creek Mercantile, turned into the current and swam part-way up Brewster Creek, suddenly reversed course, and then tried an even-smaller tributary in Williams Gulch. There he fertilized some eggs and then returned to the main stem of Rock Creek.

“Lady” made a similar journey with a different ending. She swam eight miles up the Hogback drainage, found a cozy hole, and stayed. For 150 days. She stayed so still that Liermann at one point got a mortality signal from her transmitter. But on further review, an underwater camera caught Lady hanging out under a sunken log, fat and happy.

“There’s been a lot of odd behavior,” said Liermann, a Montana Fish, Wildlife & Parks fisheries biologist. “One thing we’ve learned when we track native fish is they do a lot of different things.”

Liermann wants to know about Rock Creek’s cutthroats because the tributary may spawn a lot of the fish that swim in the Clark Fork River. That waterway has undergone more than $200 million in cleanup and restoration work overseen by the state’s Natural Resource Damage Program to repair a century’s worth of mining pollution. Following those fish day to day — sometimes hour to hour — reveals what they need to thrive.

That place where Free Willy reversed course? That’s where a farm bridge and culvert were blocking passage farther up Brewster Creek. Trout Unlimited partnered with FWP and the landowner to rebuild those structures and open passage to better spawning grounds.

Several TU volunteers spent days catching one-pound cutthroats for science last April. Liermann and his staff took 33 of the best and surgically inserted radio transmitters in their bellies before releasing them back into Rock Creek. Twenty-eight were still swimming in October. A few died after spawning, and at least two wound up in osprey nests, transmitters still beeping.

To encourage public interest, TU set up the Race Up Rock Creek website with daily updates on each fish in the study. Rock Creek Fisherman’s Mercantile owner Carolyn Persico said visitors asked about it all summer, keeping track of their favorite trout.

“Cutthroats will spend between five days and three weeks spawning,” said TU Rock Creek Project Coordinator Tess Scanlon. “But when they hit an obstacle, they may spend only a day or two before they turn around. If you’re not watching closely, you’ll miss it.”

Unlike non-native rainbow and brown trout, westslope cutthroat trout have spent millions of years adapting to mountain stream environments like Rock Creek. The year-to-year channel changes, tiny feeder streams, and forest fire-triggered erosion often encourage cutthroat populations where they suppress non-native survival. That kind of local specialization even occurs within subspecies. Yellowstone cutthroats will spawn in big rivers, while westslope cuts tend to seek out the tiniest streams they can squeeze into.

In the current, predatory brown trout hang in eddys and sloughs resting before pouncing on smaller fish. Insectivore cutthroats surf the faster water, eating every bug that floats by. Rainbows flit between fast and slow flows, dining on other fish, insects and plankton.

Around the point bar where Rocky was holding, Liermann said as many as 15 trout might be lined up by size, with the biggest fish claiming the most ideal combination of current and cover as its territory. If it’s caught or dies, the next biggest fish takes over that spot and every other one moves up a step in the hierarchy.

Human anglers can spot those choice bits of stream geography and turn them into secret or storied fishing holes. Liermann said the biggest benefit of the radio-tracking study was seeing Rock Creek from the fish’s perspective.

“We found a lot of fish went into Williams Gulch,” Liermann said. “We didn’t expect that. It’s such a small, steep drainage, we never thought about it. But that’s where these studies are so unbiased. Fish move where they’re going to move.”

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Natural Resources & Environment Reporter

Natural Resources Reporter for The Missoulian.