Grizzly bear walks along Yellowstone Lake with a cutthroat trout/NPS

More than a quarter-million nonnative lake trout were removed from Yellowstone Lake this year, and while that seems like a lot, it reflects a nearly 30 percent decline in the harvest since 2017, according to Yellowstone National Park staff.

“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 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.” 

Lake trout were were noticed in the big lake back in 1994. They posed a serious threat to the native Yellowstone cutthroat trout, which are smaller and are eaten by the larger trout. Since cutthroat trout spawn in tributaries that feed into Yellowstone Lake, they are a food source for grizzlies, and also are consumed by osprey, otters, and other predators and scavengers.

Lake trout, however, don't use lake tributaries for spawning, but go deeper in the lake to lay their eggs, and so aren't as readily available to predators.

Native cutthroat trout are the park’s most ecologically important fish and the most highly regarded by visiting anglers. 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.

Back in 2017, crews removed 396,950 lake trout from the lake. This year, between May and October, 282,960 were taken out.

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. The number of lake trout caught in nets continues to steadily decline, from 4.4 per net in 2017, and 3.1 per net in 2018, to just 2.9 per net in 2019, according to park staff.

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 six 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. The models also indicate that the invasive species has been in decline since 2012.

Concurrent with the lake trout decline, 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. This July, fisheries staff found large numbers and sizes of cutthroat in these backcountry streams. Ten years ago, few cutthroat were present. This long-distance migration highlights the spatial extent to which the recovery of the cutthroat impacts Yellowstone. 

While models and monitoring point to positive trends, a panel of expert fishery scientists in May 2019 estimated that a minimum of five more years of effort is needed to reach the lake trout population goal of below 100,000. They also emphasized 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.  

Yellowstone has invested more than $20 million over the past two decades on this recovery effort. Much of that funding has come from the generosity of donations through Yellowstone Forever.

“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 Dr. Todd Koel, leader of the Native Fish Conservation Program.