MAMMOTH HOT SPRINGS -- The 20-year and $20 million effort to remove nonnative lake trout from Yellowstone Lake continues, but it may never come to a conclusion, according to a news release from Yellowstone National Park.

The park recently finished its annual removal of the invasive species to preserve the native cutthroat population.

"I want to personally thank the National Park Service team, our partners, and the many people who have philanthropically supported this continuing conservation effort,” Park Superintendent Cam Sholly said in the news release.

"There is a considerable amount of work yet to do to build on this progress," Sholly said. "This will continue to be one of our conservation priorities."

Native cutthroat are the park's most ecologically important fish because it is a food source for grizzly bears, birds of prey and other wildlife.

Their decline resulted in some of those species being displaced from the lake or having to use alternate food sources during certain times of the year.

Between May and October, park staff and contract crews removed 282,960 lake trout compared to 297,110 in 2018, and 396,950 in 2017 for a 29% decline over three years.

This 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 further efforts, Yellowstone National Park and Michigan State University have generated statistical models that suggest there are 73% less lake trout older than six years -- the ones with the highest reproductive potential and the ones that eat the most cutthroat -- in Yellowstone Lake at the population's peak in 2011.

Likewise, long-term monitoring indicates a substantial increase in the number of cutthroat trout in backcountry streams over the past 10 years.

Cutthroat migrate more than 30 miles into the Upper Yellowstone and Thorofare streams where they spawn and then return to the lake.

However, fishery scientists in May estimated the park will need at least five years of annual removal to reduce the lake trout population to the goal of less that 100,000, and the fish cannot be eradicated with current techniques.

So fishery biologists are looking at alternatives to gillnetting, such as a suppression method by killing lake trout eggs on spawning sites with plant-based organic pellets on spawning sites. The pellets facilitate decomposition and loss of dissolved oxygen that can kill eggs within two days.

"The park will never completely eradicate lake trout," said Dr. Todd Koel of the Native Fish Conservation Program. "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."

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