Idaho Fish and Game plans to kill brook trout in Panhandle stream

A combination of tools old and relatively new will be used to rid a creek in North Idaho of the colorful but invasive brook trout.

In September, the Idaho Department of Fish and Game plans to poison the lower portion of Binarch Creek with rotenone, a piscicide that has long been used to remove unwanted fish.

Next year, they’re looking to bring in a special strain of brook trout that can only produce male offspring to keep any fish that survive the poison from re-establishing a strong population in the creek, which dumps into the Priest River just south of Priest Lake.

Fish and Game biologist Rob Ryan said removing the brook trout is the first step in a long-term plan to make the stream usable for the Priest River drainage’s migratory westslope cutthroat trout. They also have plans to fix a culvert to give those fish a chance to get into the stream, but they need the brook trout to be gone first.

Idaho has stocked the special strain of brookies – known as YY trout, because they have no X chromosome – in other streams over the years. But Ryan said this project may be the first time it’s been paired with rotenone treatments.

Once it’s all over, the hope is that the native cutthroat trout will become the dominant species in the stream, which Ryan said is packed with good habitat.

“There’s kind of this diverse group of habitats,” Ryan said. “There’s a bunch of beaver ponds up there, there’s some marshy areas. There’s some sections that go dry. It’s really just an interesting place.”

Binarch Creek begins high in the Selkirks on the Washington-Idaho border and drains east into the Priest River just below Priest Lake Dam. Ryan said it’s a relatively small drainage, with less than 10 miles of stream. They plan to use rotenone on the lower half.

Brook trout are colorful salmonids native to the eastern portion of North America. They were stocked all over the West, legally and illegally. In Binarch Creek, they were introduced in the 1940s.

They aren’t found in the uppermost portions of the drainage, Ryan said, and cutthroat are doing well up there. But brook trout have taken over in the lower reaches, outcompeting cutthroat for food and space.

Ryan said talk of removing the brook trout picked up after the Forest Service identified the box culvert that allows Binarch Creek to pass under Highway 57 as a barrier to fish migration.

Fixing it would give migrating cutthroat and bull trout a chance to move into Binarch and spawn there or use it as a cold water refuge when water in the Priest River gets too warm, which happens every summer.

But as long as the brookies are there in good numbers, they’ll keep the cutthroat population under their thumb.

Ryan said they will use rotenone on the lower 4.5 miles of the stream. The piscicide kills everything with gills, but there are often a few survivors who are able to evade the poison.

Stocking YY brook trout is meant to deal with that problem. Ryan said the plan is to stock YY brookies for five years. If it works, it would eventually result in an all-male population that eventually blinks out.

In other projects, the YY trout stocking has been paired with electrofishing work to speed up the process for removing trout. Now, Ryan said, researchers are interested in how the stocking would work with rotenone, and Binarch’s variety of habitat make it a good place to test the combination.

“Binarch stands out as a great stream to try this because of the challenges of it being super complex,” Ryan said.

Erin Plue, the Idaho state director for Trout Unlimited, said Binarch is an important stream in the drainage that has a lot of potential for supporting cutthroat.

She added that it’s good to see Fish and Game and the Forest Service working together there, and that combining rotenone and the YY brook trout stocking looks promising.

“It seems like a multipronged approach to managing brook trout,” Plue said. “It’s a good effort.”