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JACKSON, Wyo. (AP) — Diana Miller stood on the spongy banks of oval-shaped Dime Lake in the Teton Wilderness. It boiled intermittently with writhing fish.

The smell of a hard-to-place chemical was in the air, and conifers along western shores of the shallow, seldom-visited lake were blackened and dead from the 2016 Berry Fire. The fish Miller pointed out were dead or dying, too, the result of poison that had been discharged to eliminate a population of brook trout her forebears had introduced a half-century ago.

“Look for white bellies,” Miller said. “They’re not big. Not big.”

While small, the thousands of non-native trout that the Wyoming Game and Fish Department fisheries biologist and two dozen colleagues spent all day Aug. 30 eliminating were considered a long-term threat to the prized cutthroat trout population that inhabits the Upper Snake River watershed.

“It’s one of the biggest sources of brook trout in the Upper Snake,” Miller said.

Not anymore. Dime Lake harbored an easy and obvious population of brook trout to wipe out, she said, because it was virtually never fished, didn’t require killing cutthroat trout and had a natural waterfall on its outlet creek that will prevent the exotic trout from repopulating.

Elsewhere in the Upper Snake — the watershed above Palisades Reservoir — the solution to curbing invasive brook trout isn’t nearly as clear. Although the species has occupied the Snake and its tributaries for a century without taking over, there’s no saying if or when it could become a more commonplace, pervasive species in Jackson Hole.

There’s a real chance that brook trout, native to the Atlantic seaboard and Great Lakes, could eventually bump cutthroat out of large portions of the watershed. Jackson fisheries managers need look no farther than adjacent river basins, like the Green and the Wind, to see what can happen when environmental conditions are right: Brook trout became dominant, and cutthroat are harder to find — or worse.

Miller knows she’s fortunate to oversee a fishery where that’s not yet the case.

“We’re lucky,” she said, “because there’s so few places that have cutthroat populations that are not filled with brook trout or rainbow trout.”

Typically streams that lend themselves to brook trout invasions are slower flowing, weaving through “unconfined” meadows and valleys. Such characteristics also tend to attract beavers and the ponds they create, another brook trout boon.

There are portions of the Upper Snake that have become overrun by brook trout, and some indications that a slow-speed invasion is afoot. There are brook-trout-dominated streams in all corners of the watershed, from the North Fork of Spread Creek to the Teton Wilderness’ Cub Creek to Game Creek just south of Jackson.

Miller even finds them in aquatic habitats that she wouldn’t expect, such as the Salt River tributaries Swift and Strawberry creeks.

“We’re also seeing them in high-mountain streams with really fast, cold water,” she said.

Anglers reported this year that the upper Gros Ventre River above the Darwin Ranch was fishing well not just for cutthroat, but brook trout. And Miller’s electrofishing crew in mid-August sampled an unusually high number of brook trout on the Hoback River, including fish up to 17 inches.

As fall spawners, brook trout can have a leg up on cutthroat by giving birth to fry that outsize their spring-spawning native competitor. They’re also prolific and have the potential to increase rapidly in ranks and establish high densities in short periods of time. And they know how to spread out, said Montana State University professor and U.S. Geological Survey research fisheries biologist Robert Al-Chokhachy.


“They’re extremely good at dispersing,” Al-Chokhachy said. “They’re looking for that window of opportunity, and when conditions are good for them they’re going to expand. They can just hang around and hang around, but if they have a year or two where conditions are good for them they will just take off.”

Brook trout invasion, hybridization from rainbow trout, habitat degradation and physical degradation are among the reasons why cutthroat today occupy only a fraction of the western rivers and streams they did historically. Cutthroat subspecies like greenback, Colorado River and Lahontan have faced the steepest declines, and some have flirted with or are protected by the Endangered Species Act.

The Yellowstone cutthroat (which the Snake River fine-spotted is genetically identical to) have held on slightly better, occupying an estimated 43 percent of their historical range. The Upper Snake is the single largest stronghold, according to a map published in a native fish edition of Yellowstone Science.

Al-Chokhachy said it’s tough to judge whether the warming of the planet and other environmental changes will inevitably make the Upper Snake more hospitable to brook trout, displacing cutthroat along the way.

“I could easily say yes,” Al-Chokhachy said, “but some of the rivers and conditions are going to change so much.

“The systems are changing,” he said. “It’s unclear (climate change) would actually be beneficial for brook trout.”

Using Pacific Creek as an example, he said peak runoff is happening 2 1/2 weeks earlier than it was “not that long ago.”


Changes to hydrological patterns and their effects on fisheries are unpredictable. If a warmer future in Jackson Hole means more rain falling on ice and larger pulses of water in the wintertime, Al-Chokhachy said, that could be a bad thing for brook trout because their eggs are already deposited in the cobble and dependent on stable flows.

Fish poisoning projects like the one that scrubbed Dime Lake and Dime Creek of their brook trout, aren’t an option everywhere. Some stream systems are too complex, too filled with cutthroat trout and too interconnected to the larger Snake watershed, lacking barriers to prevent brook trout from reinvading.

That was determined to be the case in Cub Creek, a Buffalo Fork tributary where brook trout reign.

“Tracy (Stephens) and I spent a lot of time up there trying to figure out if that was feasible,” Miller said, “and the end result was there was way too much water with way too many brook trout.”

There aren’t many options for controlling invasions in similar streams and rivers. Stunning fish and selectively killing brook trout is only temporarily effective because it’s impossible to get them all. Allowing anglers to harvest 16 brook trout a day has proven equally ineffective, especially for remote populations, because they tend to run small and therefore aren’t desirable to keep.

“Just because we are concerned about them doesn’t mean there’s something feasible that we can do to actually remove them,” Miller said. “Our options are pretty limited.”

One technique Wyoming managers are paying attention to is being debuted by their Idaho counterparts. Genetically modified brook trout that can give birth only to male fish have been stocked in four trial streams since 2014.

“If you continually stock these fish, over time in theory you can only have males, and the population will die out,” Miller said. “They think within 10 years they can remove a population.”

Back along the shore of Dime Lake, a frog jumped from the shoreline, splashing into the rotenone-treated waters below at the approach of researchers.

“Oh, he’ll survive,” Miller said. “As long as they don’t have gills, they’re not impacted.”

In the aftermath of the poisoning Dime Lake won’t receive cutthroat trout and will become a fishless body of water, its natural condition. That’s partially because it wasn’t much of a fishery to begin with. It is shallow, holds only small fish and requires a bushwhack to get to. But the plan to leave it barren is also intended to help another class of native wildlife.

“As we started to do more work up here,” Miller said, “we realized what a stronghold it was for amphibians, particularly Columbia spotted frogs.”

Brook trout — and any trout — can do a number on amphibian populations by preying on tadpoles and young, metamorphosing adults. Wendy Estes-Zumpf, Game and Fish’s herpetological coordinator, will keep close tabs on frogs in Dime Lake to see if they thrive, or don’t, in the absence of fish.

“I would expect we’ll see some sort of an increase,” Estes-Zumpf said at Dime Lake. “How much, we’ll have to wait and see.”


Information from: Jackson Hole (Wyo.) News And Guide,