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ROCK CREEK — Of the thousands of fish stacked up above the confluence with the Clark Fork River, Warren Colyer managed to catch the same one twice.

He knew because the cutthroat trout had a 20-inch radio antenna streaming out of its belly, evidence it had already been enrolled in a study looking at how fish live and breed in Rock Creek, the blue-ribbon angler’s goal east of Missoula. Colyer was among a half-dozen Trout Unlimited volunteers fishing for science on Thursday (really!) with Montana Fish, Wildlife & Parks Department biologists.

The biologists floated the upper reaches of the stream in a drift boat, electro-shocking the water to capture big cutthroats. The volunteers whipped their fly rods along the mouth of Rock Creek, where spring runoff debris made it too risky to maneuver the boat. The target was 15 radio-tagged trout at the upper end of the creek, 10 in the middle and 15 in the lower third.

“We’re trying to figure out where they’re spawning, what tributaries they use, where they find thermal refuge in the summer,” said FWP biologist Brad Liermann. “We also want to see what obstacles they run into, like irrigation ditches or culverts.”

Not all the survey trout were cutthroats. Some were “cutbows,” hybrid offspring of non-native rainbow trout and the native Westslope cutthroats. While the two species usually have separate breeding areas and times, Liermann said the radio tags might reveal where the intermingling happens.

Cutthroats that cross-breed with rainbows lose some of the genetic adaptations that help them thrive in mountainous streams where rainbows struggle.

The fish caught for the project would have made any angler proud. They needed to weigh over 400 grams (nearly a pound) to have enough body space for the 8-gram (third of an ounce) transmitter. FWP biologist Tracy Elam would drop one in a water tank with a clove-oil anesthetic to numb the fish.

Then colleague Rob Clark would slice a slit in its upper abdomen, poke a hollow needle through the lower end, hook the long antenna to the needle, and pull it and a transmitter the size of a ballpoint pen cap into the fish’s body. A couple of stitches and a dab of antibiotic ointment finished the procedure. Elapsed time: less than 2 minutes.

Trout Unlimited project manager Rob Roberts said the organization plans to intensify its work in the Rock Creek area this year. That dovetails with coming investments by the state Natural Resource Damage Program to help remediate decades of mining damage to the Clark Fork River Basin.

“Rock Creek has seen the same warming trends that are a big problem for native trout,” Roberts said. “We’re wondering if there are ways we can buffer that.”

A recent study published by the Transactions of the American Fisheries Society journal found that rising summer temperatures could significantly reduce habitat for trout in the Rocky Mountains.

FWP Fisheries Management Biologist Ladd Knotek said temperature monitors throughout the area have revealed how cold-water fish adapt to the changes. For example, trout in Rattlesnake Creek north of Missoula seek out parts of the stream fed by cool underground springs when the air temperature gets uncomfortable.

But climate change has presented some new problems. The cutthroats staging at the mouth of Rock Creek wait there for a couple different reasons. They depend on the surge of spring meltwater to open up passage to small side channels and refresh the gravel beds where they spawn. But their egg and sperm development gets triggered by the lengthening daylight as the season shifts. If the runoff peaks before the daylight trigger occurs, the whole breeding process can suffer.

“We’re seeing some really alarming and interesting things with brown trout moving into bull trout spawning waters,” Knotek said of the water changes.

“As the temps get unsuitable, the bull trout recede upstream, but that’s limited by things like waterfalls. The hole left behind is filled by brown trout that are more tolerant of warmer water. We’ve seen this over the last two to four decades. The core populations of bull trout have more limited areas to spawn and living space for juveniles. For tributaries that aren’t in good shape, we’re losing those populations.”

Anyone interested in the radio-tagged cutthroats’ activity in Rock Creek can keep up to date through an online map Trout Unlimited has posted on its website. The map tracks individual fish and shows how many daily miles they’ve traveled, how far they’ve migrated and other notes on their activity.

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