Eleven miles up Specimen Creek, in the pristine backcountry of Yellowstone National Park, people in yellow hazmat suits plod slowly around High Lake dumping poison in the water.

I’m the only person without these fancy yellow duds; most people wear respirators too. So, alternately holding my breath and feeling silly, I climb up a small knoll and, with a long lens, and snipe photos from a safe distance of 50 yards.

I’m here to witness final extermination of High Lake’s Yellowstone cutthroat trout; undoing a mistake made decades ago by well-intentioned Yellowstone fisheries biologists. If all goes well today, not a single fish will swim these waters, and High Lake will become a blank slate for westslope cutthroat, a native fish species on the brink of extinction.

After backpacking through remote forests, setting up my tent on ground peppered with wolf tracks, and sleeping with a large canister of bear spray under my pillow, dawn broke to the surprising drone of outboard motors. The trail sign was little cryptic, but the engines guided me up the final half mile to the lake.

The base camp for this fish-killing operation is infrastructure heavy, with two large tents to house gear, chemicals, and water testing equipment – essentially a fully equipped chemistry lab – all flown in by helicopter earlier this month. Off to one side, a steel bear-proof container, nearly the size of a compact car, contains food and water for roughly a dozen biologists and technicians.

The motors that called me up the trail early this morning power two 12-foot inflatable rubber boats, which will deliver the bulk of a chemical called rotenone, a general-use pescicide made from a member of the legume family.

When I walk into camp, a man in full gear, goggles, and a respirator warns me away. “This is the poison area,” he says, and shows me a trail that will take me to the back side of the lake.

After dodging another yellow suit, this one walking around the perimeter of the pond squirting rotenone from a wand attached to a plastic backpack, I set out around the lake looking for signs of a struggle just beneath the surface.

Today is more a mopping-up operation than a full-on chemical attack. Yellowstone supervisory fisheries biologist Todd Koel and his staff apparently did a thorough job when they first poisoned the lake earlier this month, because the only sign of distress is a huge dragon fly fluttering on the water. For all I know, the dragon fly is laying eggs or bathing.

On the phone last week, Koel explained that rotenone is generally non-toxic, except to fish, of course. The only real danger to the humans on this lake is accidental inhalation, hence the respirators. As if to prove this point, a bird flits by, lands on the lake’s edge, takes a deep drink, and starts to fly away. I wait for it to fall, twitching, out of the sky, but instead it cruises straight and strong into the woods.

Rotenone works by entering the gills as the fish breathe. It then travels through the blood stream until it seeps into each cell, blocking cellular respiration.

During our phone conversation, Koel’s voice betrayed his enthusiasm for the High Lake project. The really neat thing about this type of rotenone, is that it mixes with water instead of a petroleum-based solvent. “We’re the first in Wyoming not to use those solvents,” he explains. “It’s a friendlier fish toxin.”

Not only is this formula better for the environment in general, but, before, fish could smell the solvent in the water and avoid it. Not so at High Lake, where Koel and his biologists collected hundreds of dead Yellowstone cutthroat on the first attempt.

Another cool thing about High Lake is a 12-foot waterfall that keeps non-native fish from swimming up Specimen Creek and contaminating the lake, a perfect refuge for westslope cutthroat, which Koel and his crew will bring in next summer.

Around the backside of the lake, I get a glimpse of a complicated system that will ensure that no fish breathes water at the end of the day. Plastic buckets dripping piscicide keep Yellowstone cutthroat fry from swimming to safety up the two streamlets that feed High Lake.

Back at the camp, Koel and aquatic ecologist Jeff Arnold relax for a moment before they recommence the chemical onslaught. Koel explains that they timed this operation just right to spare any amphibians around the pond, giving them a chance to change from water-breathing tadpoles to air-breathing adults.

Bugs, Arnold says, have fared well too. “As far as the invertebrates go, we saw them in the lake yesterday... caddis flies, dragon flies, damsel flies, water beetles, and still lots of midges, which is one of the better fish foods.”

Further, a nearby wetland should replenish supplies of any water-breathing bugs that should die from the rotenone.

Those bugs could provide a meal for the westslope next summer. Koel plans to bring in streamside incubators, each with 60 to 100 eggs from a recently-discovered population in Yellowstone. “The fry will emerge into the stream,” says Koel. “That’ll be their new home. We’re really excited now to get that population into a spot that we can ensure the long-term existence of the species.”

After Koel and Arnold go back to work, Mike Ruhl, a fisheries biologist at Montana State University, shows me down to the base of the waterfall that pours into Specimen Creek. There, two volunteers tend to a bucket that streams dark red potassium permanganate into the water.

The entire setup mixes the potassium permanganate in the stream and serves as a detox station, deactivating the rotenone to keep it from killing fish throughout the rest of the watershed.

But does it work? Ruhl leads me down a perilous rock path to his “canaries in the cage,” two Yellowstone cutthroat in a fish basket, both still squirming with life.

Ruhl says that the High Lake project is a unique opportunity to save a rare species. “This is the type of thing Yellowstone should be doing,” he says.