On a sparkling July morning on Yellowstone Lake the nine-meter aluminum boat Hammerhead putts over to a yellow-flagged buoy lolling on the glassy surface. The buoy marks a 275-meter-long net that’s been hanging in the lake for the last 24 hours. Two crew members pull the net up with a hydraulic winch—it’s empty.

Most fishermen would be disappointed, but Phil Doepke is encouraged. To Doepke, a fisheries biologist at the National Park Service (NPS), the empty net is a sign that he and his colleagues may be winning their battle against the lake trout, Salvelinus namaycush, an infamous invasive species that has been devastating the lake, one of the world’s iconic ecosystems. “We’ve been targeting the larger lake trout lately,” he explains as he feeds the empty net into a bucket. “But the larger lake trout have been hard to find.”

The lake trout—not a true trout, but a close relative called a char—is a rapacious predator that’s native to the icy waters of Alaska, Canada and the Great Lakes. In the 1980’s an anonymous angler—a “bucket biologist,” as such meddlers are called—dumped a handful into Yellowstone Lake, the 352-square-kilometer jewel at the heart of America’s first national park. The invaders, which regularly exceed 10 kilograms, spelled disaster for the lake’s native inhabitant, Yellowstone cutthroat trout, a golden fish that derives its name from the red slash below its jaw. Cutthroat numbers, once as high as four million, plunged by 90 percent as juvenile cutts became prey.

By the time the NPS began setting gillnets to kill the interlopers in 1996, it was too late. The lake trout population swiftly ballooned to an estimated half million fish.

When cutthroat populations collapsed, they dragged an entire food web down with them. Bears, osprey, eagles and other carnivores that feed on cutthroat in rivers and streams, where the fish go to spawn, went hungry. (Lake trout, by contrast, remain in the lake all their lives, where predators can’t get at them.) One tributary, Clear Creek, once hosted 70,000 migrating cutthroat annually. Today, no more than 500 trout visit the stream.

The loss of cutthroat has rippled throughout the greater Yellowstone ecosystem. Grizzly bears, which historically gorged on spawning cutthroat in the spring, have turned to elk calves instead. The average Yellowstone grizzly bear, which once killed fewer than four elk calves a year, now eats an estimated seven— likely enough to slow the growth of migratory herds. Cutthroat, then, may have even more influence over the Yellowstone ecosystem than wolves, long touted by scientists as the park’s keystone species. Says Yale University biologist Arthur Middleton, who has studied the effects of the invasion, “You have the potential for fish in the waters of Yellowstone Lake to affect how many elk you see 50 miles outside the park.”

First futility, then hope

At first, the NPS’s efforts to control the invasion were futile. Each year they deployed more nets and caught more lake trout: nearly 40,000 in 2005, 60,000 in 2006, almost 80,000 in 2008. But the aliens remained as robust as ever. “We were slowing down the growth rate but the population was still increasing,” says Pat Bigelow, an NPS fisheries biologist.

In 2009 Bigelow and her team turned for help to Hickey Brothers Fisheries, a private fishing crew that was already removing lake trout from Idaho’s Lake Pend Oreille. Their larger boats and gear allowed them to deploy longer nets than the Park Service and to set those nets in different configurations—in particular, a sine wave–shaped curve that reliably catches more fish. And having gathered data on catch rates for nearly two decades, the netters were better able to target Yellowstone lake trout hotspots, like the shallow spawning grounds around Carrington Island.

The result was a dramatic increase in lake trout removal: 224,000 in 2011, 300,000 in 2012, another 300,000 in 2013. Although catch numbers have stayed high, it’s taken more nets and days on the water to snag the same number of fish, suggesting that aliens are finally growing scarcer. Even more important: according to a 2013 report, cutthroat trout numbers recently began ticking upward for the first time in years.

The progress is encouraging but the invaders are likely in Yellowstone to stay. Whereas total eradication may be impossible, the NPS hopes to suppress lake trout—and boost cutthroat—enough to maintain a healthy, functional ecosystem. To that end the agency and its partners are experimenting with other ways of attacking lake trout, including an array of electrodes that would zap their eggs and larvae. “If you could hit them at both ends, where you’re taking the adults out and also killing any eggs and embryos,” Bigelow says, “you could really make a difference.”

By noon, the Hammerhead crew has caught only 11 late trout. Doepke, however, is not celebrating. The shallow waters of the lake’s south arm aren’t prime lake trout habitat, he cautions, and the hot weather has the fish feeling sluggish. Still, as net after net comes up empty, it is easy to imagine that the invader has become truly scarce.

The last net of the day, however, dispels that notion; it contains an eight-kilogram female lake trout—longer than a baseball bat and thick as a human thigh, slate gray and chalked with white spots. Doepke whacks it on the head with a brush-handle and tosses it into a tub. Another lake trout dispatched, the gillnetters’ tally now at around 1.5 million since 1996, and counting.

This story was supported by the Institute for Journalism & Natural Resources.