One step forward, two steps back. In what has to be a disappointing development for fisheries managers in Yellowstone National Park, non-native brook trout are once again swimming the waters of Soda Butte Creek.

The National Park Service declared the stream, a tributary of the Lamar River, free of the invasive char in 2016, after years of exhaustive removal efforts. Brook trout pose a competitive threat to native Yellowstone cutthroat trout, and their presence in Soda Butte Creek is particularly alarming because of the stream's immediate connection to the Lamar River, which is home to one of the last intact big-water populations of native cutthroats.

Brookies are common in the West, but they are native to Appalachia and eastern Canada. All over the Rockies, brook trout have displaced native cutthroat trout, and they are proving to be stubborn invaders that are difficult to eradicate.

In Soda Butte Creek, which starts east of the park's boundary in Montana, brookies have been targeted for removal for the better part of two decades, as part of the NPS plan to restore native trout to its waters whenever and wherever it's practical. For instance, the Park Service is working to restore native fish in smaller, headwater streams, like the extreme upper reaches of the Gibbon River. It is not, however, looking to eradicate non-native brown and rainbow trout from the Firehole or Madison rivers, where the naturalized fish are cherished by park anglers.

As a result of the discovery in Soda Butte Creek, the Park Service, in cooperation with the Custer National Forest and Montana Fish, Wildlife & Parks, will close the stream from the park's northwestern boundary to Ice Box Canyon to public access and fishing from Aug. 14-18. During that time, fisheries professionals will apply the EPA-approved piscicide rotenone to the creek to kill all the fish in the stream.

Without this urgent action, the National Park Service says “brook trout will quickly displace native Yellowstone cutthroat trout and eventually invade the entire Lamar River watershed, threatening the largest remaining riverine population of Yellowstone cutthroat trout in existence.”

Prior to the treatment, fish techs will remove as many native cutthroat trout from the streams they can and relocate them to upstream tributaries of the Soda Butte that are not targeted for treatment. Techs will use electrofishing equipment to stun and net the fish before they are relocated.

"Cutthroat trout are the only trout species native to the park. They are the most ecologically important fish of the Greater Yellowstone Ecosystem and are highly regarded by anglers," a National Park Service statement on the removal effort reads. "Genetically pure Yellowstone cutthroat trout populations have declined throughout their natural range in the Intermountain West, succumbing to competition with and predation by nonnative fish species, a loss of genetic integrity through hybridization, habitat degradation and predation."

In addition to Yellowstone cutthroat trout, the park is also home to a native population of westslope cutthroat trout and a restored population of Arctic grayling. Both of these fish are found in the northwest corner of the park. The southwest corner of Yellowstone is home to native Snake River fine-spotted cutthroat trout.

The NPS has worked for 20 years to restore native Yellowstone cutthroat trout in the Yellowstone Lake drainage, where invasive lake trout were discovered in 1994. That effort is proving to be successful, as spawning cutthroats are returning to the watershed. That said, the effort, which involves gillnetting and targeting lake trout on their fall spawning beds, will be a persistent project that will likely always require attention.