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The war is still being waged against lake trout in Yellowstone Lake, but fisheries biologists are seeing new trends that offer optimism, and anglers hunting cutthroat trout are seeing more and bigger fish.

During the 1990s or perhaps earlier, lake trout were introduced into Yellowstone Lake illegally, and the invasive species had catastrophic effects on the fishery. Yellowstone National Park’s website on the subject illustrates the point with what happened in one spawning tributary to the lake: “The number of Yellowstone cutthroat trout spawning at Clear Creek peaked at more than 70,000 in 1978 and fell to 538 by 2007.” That scenario was being repeated in all of the tributaries to the lake — a population crash with extinction knocking at the door. The voracious lake trout were eating cutthroat right out of the lake.

Todd Koel, Yellowstone National Park fisheries supervisor, said 24 years of gillnetting operations are starting to turn the tide with a reduction of lake trout numbers.

“We caught 400,000 (lake trout) in 2017 and a lot of those fish were small,” Koel said. “So then in 2018 was the first year that in the netting boats we caught 100,000 less fish than we did the year before. A 25 percent decline in catch over one year’s time from 2017 to 2018. Not because of lack of effort. There was more effort, more net out there, more crews out there last year than the year before.”

Fewer lake trout is causing a rebound in cutthroat trout numbers in the system. Diana Miller, a Jackson region Wyoming Game and Fish Department fisheries biologist, and her father Dave Sweet, a Trout Unlimited volunteer, had heard rumors that the cutthroat were returning. In the early summer of 2018, they ventured into the remote Thorofare River region below the southeast corner of the park during spawning season. The area is a major tributary of Yellowstone Lake and once was a thriving spawning region before the cutthroat population crash. They reported catching more than 100 fish.

“There were some beautiful Yellowstone cutthroat back up in that country,” Miller said. “It was really just phenomenal. They were all really healthy big fish. I think our average fish was probably 22-23 inches, something like that. Just beautiful, big Yellowstone cutthroat.”

Sweet said in the mid-2000s, outfitters quit taking anglers into the area because the fish were gone.

“So our goal was really twofold. One was to have a good father-daughter trip back into the Thorofare,” he said. “We hadn’t been together for 15 years. And the second, of course, was to find out if the rumors were true, if those cutthroats were returning into the Thorofare in significant numbers. Indeed we found them. That was pretty exciting.”

Because gillnetting operations are expensive, costing more than $1 million a year, the park is developing other weapons to crash the invaders’ population.

With the help of “Judas fish,” park biologists have tracked fish tagged with an acoustic tag that sends out sound in the water. They have learned where the fish spawn and where the schools congregate and how best to kill them. They have learned that spawning sites for lake trout are relatively small and defined areas.

“They have to go to those spots, and they have to spawn and reproduce young on those small areas,” Koel said. “We see that as being a key toward being able to suppress the population and keep the population low over the long term at a reduced cost.”

Koel said biologists are developing ways to kill eggs before they hatch by reducing the oxygen in the water. One successful method is to drop decomposing dead fish or small pellets of organic matter over the spawning sites. As the things decompose, oxygen is depleted and the eggs die. After decomposition, oxygen returns.

“I want to get to the point where we’re treating these spawning sites every fall to cause the mortality of the embryos in these specific areas and then reduce the amount of the young produced by those,” Koel said. “So then we can really cut down our netting program during the summer to a fraction of what it is moving forward.”

One challenge to attacking spawning sites is Yellowstone’s early winters. The lake can freeze over in late October.

“The lake trout’s peak spawning is at the end of September, and we have about a two- to three-week window and then into October to treat these spawning sites, and then we have to leave the area and get off the lake because it gets too dangerous,” Koel said.

With the depletion of cutthroat in the lake, the ecosystem surrounding the trout changed dramatically. Bears, eagles and otters, who once dined on spawning cutthroat, changed their habits or disappeared. Osprey nests declined.

“By 2007-2009, grizzly bears had shifted to alternative prey, and the proportion of cutthroat trout in their diet had declined to zero,” researchers wrote in the scientific journal “Science Advances” on the Yellowstone Lake situation. “Elk then accounted for 84 percent of all ungulates consumed by bears in the Yellowstone Lake area, suggesting lake trout had some level of indirect, negative impact on migratory elk using this area when spawning cutthroat trout were rare.”

With the trend upward in cutthroat numbers, Koel said the ecosystem is slowly righting itself.

“The bear activity in the spawning streams is trending upward,” he said. “It’s nowhere near where it was 30 years ago, but at least the trends are in the right direction.”

Koel says he doesn’t expect lake trout to be completely eradicated but must be held in check.

“I’m not going to say we’re out of the woods yet,” he said. “There’s a lot of work to do. All the indicators are up, and we’ve got some good technology. We’re trying to reduce long-term costs and make it sustainable for the future. Lake trout are always going to be there. We can’t get rid of those. Any let up in the program and they would just rebound. That is obviously not acceptable.”