GREAT FALLS — WOLF CREEK – As dusk falls over the Missouri River, a specially outfitted jet boat operated by Montana Fish, Wildlife and Parks (FWP) begins its nightly mission. While it may look like a typical fisheries vessel by day, after sunset it becomes a high-tech tool for one of FWP’s most important long-term studies: tracking the population of brown trout.
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"We're doing our annual brown trout population estimates," said Adam Geik, a fisheries biologist with FWP. "Weeks ago we did two nights of marking—putting a little clip on the tail. Now, we’re back for the recapture phase."
This mark-recapture method allows biologists to estimate trout populations by comparing the ratio of previously marked fish to unmarked ones caught on subsequent nights.
It’s a technique that has been used on this stretch of river for over 40 years, providing invaluable long-term data about fish trends in relation to water conditions and habitat changes.
Back to Average – But Still Big
According to Geik, trout numbers are stabilizing after recent years of abundance.
“Last year, our brown trout and rainbow trout numbers were right about average,” Geik said. “A few years before that, we saw near-record numbers."
He noted, "But this year, while numbers have returned to average, we're seeing some really impressive sizes, especially among the brown trout—most of them between 18 and 21 inches.”
Higher water years often correlate with bigger, healthier fish, and this year’s data reflects that trend.
Science After Sunset
As the sun sets, FWP crews load boats and prep equipment. Each boat is a self-contained electrofishing unit with metal booms that extend into the water, emitting a mild electrical current that temporarily stuns the fish.
Once stunned, the fish are quickly netted and transferred into a holding tank with a sedative in the water to keep them calm while they are weighed, measured, and recorded.
Then they’re returned to the river—swimming away unharmed.
“It’s a smooth, practiced process,” said Geik. “We’re usually out there until dawn.”
The crews are made up of biologists, technicians, and volunteers, each playing a role in ensuring the data is accurately and humanely collected.
Why It Matters
Decades of trout population data help scientists monitor the health of the river, understand the impacts of climate and habitat changes, and adjust management strategies for future generations of anglers and conservationists.
“These surveys let us see how things like high water years influence trout populations over time,” said Geik. “It’s critical to keep this long-term trend going.”
So the next time you’re fishing one of Montana’s famous rivers, remember—there’s a good chance the trout you catch was counted, measured, and safely released by a crew working through the night, decades in the making.
