After spending more than two decades and $20 million battling nonnative lake trout, Yellowstone National Park officials say they’re making substantial progress.

The annual removal of nonnative lake trout from Yellowstone Lake recently concluded, with data pointing to a declining population.

Yellowstone National Park and contract crews removed 282,960 fish between May and October, compared to 297,110 in 2018, and 396,950 in 2017. That’s a 29 percent decline over three years.

Lake trout are removed in an effort to preserve the native cutthroat trout population, the largest remaining concentration of inland cutthroat trout in existence.

Native cutthroat trout are the park’s most ecologically important fish, Yellowstone officials say, and the most highly regarded by visiting anglers. It is an important food source for grizzly bears, birds of prey, and other wildlife. The cutthroat decline resulted in several of these species being displaced from Yellowstone Lake or having to use alternate food sources during certain times of the year.

“The park will never completely eradicate lake trout but the return on investment is the ecological restoration of Yellowstone cutthroat trout, sustainable angling, and a chance to glimpse a river otter, osprey, or bear catching a cutthroat,” said Todd Koel, leader of the park’s Native Fish Conservation Program.

Yellowstone’s lake trout suppression program is one of the largest nonnative fish removal programs in the United States. Since lake trout were first discovered in 1994, more than 3.4 million have been removed from Yellowstone Lake through suppression gillnetting.

In order to predict the success of the removal effort and set benchmarks for gillnetting in the future, Yellowstone National Park and Michigan State University collaborate to generate statistical models of the lake trout population. The models suggest there are 73 percent less lake trout ages 6 and older in Yellowstone Lake now than were present at the population’s peak in 2011. This is critical because older, larger lake trout have the highest reproductive potential and consume the most cutthroat trout, park officials say. The models also indicate that the invasive species has been in decline since 2012.

Alongside the drop in lake trout, long-term monitoring indicates a substantial increase in the number of cutthroat trout in the Yellowstone Lake ecosystem.

Cutthroat that inhabit the lake migrate more than 30 miles up into the Upper Yellowstone and Thorofare streams where they spawn and then return to the lake. In July, fisheries staff found large numbers and sizes of cutthroat in these backcountry streams. That’s in contrast with 10 years ago, when few cutthroat were present.

While models and monitoring point to positive trends, expert fishery scientists estimate that a minimum of five more years of effort is needed to get the lake trout population below 100,000. They also emphasize that lake trout cannot be completely eradicated with current techniques and will continue to require annual removal and monitoring into the future.

Yellowstone fishery biologists continue to explore alternatives to gillnetting that will augment lake trout suppression, increase efficacy, and reduce costs. For example, over the last few years, biologists tested a method to suppress the population by killing lake trout eggs on spawning sites. One technique involved placing plant-based organic pellets on spawning sites to facilitate decomposition and loss of dissolved oxygen. This method killed eggs within two days. Since the results are promising, the park intends to expand the technique in the future.      

Much of the funding for the work over the years has come from the generosity of donations through Yellowstone Forever.

“I want to personally thank the National Park Service team, our partners, and the many people who have philanthropically supported this continuing conservation effort,” said Yellowstone Superintendent Cam Sholly. “There is a considerable amount of work yet to do to build on this progress. This will continue to be one of our conservation priorities.”