During summer mornings at Yellowstone Lake, workers in rubber boots and life jackets pull up gillnets in an effort to reverse years of ecological damage. By October, the seasonal fishing operation typically winds down after deploying thousands of miles of gillnets, according to the National Park Service (NPS). The target is an invasive fish first detected in the lake in 1994: lake trout.
The effects extended beyond the fishery. By the mid-to-late 2000s, the number of cutthroat trout spawning in streams had fallen to just 5% to 10% of late-1970s levels, according to the NPS. Anglers caught fewer fish, park rangers saw fewer animals around the lake, and some fishing guides reduced tours in affected areas. The decline threatened a vital food source for several native wildlife species.
Why lake trout became a problem
Yellowstone cutthroat trout are native to the lake and an important food source for grizzly bears, ospreys, bald eagles, and river otters. Lake trout, by contrast, were introduced to Yellowstone Lake in the 1980s, with their presence first confirmed in 1994. They altered the food web because they inhabit deeper waters, where many predators that feed on cutthroat trout cannot easily reach them. The NPS estimates that a single lake trout can consume up to 41 cutthroat trout annually.
Deep Dive
Why did lake trout cause Yellowstone cutthroat trout numbers to decline?
How has removing lake trout affected Yellowstone cutthroat trout and wildlife?
Can spreading organic pellets stop lake trout from reproducing in Yellowstone Lake?
A 2020 study published in Fishes by Todd Koel and colleagues also documented the invasive fish's reproductive potential. A female lake trout weighing 5 kilograms can produce 6,000 to 8,000 eggs in a single spawning event. The researchers also found that young lake trout in Yellowstone Lake appeared to survive at rates four to six times higher than those of young fish in their native range.
{{/usCountry}}A 2020 study published in Fishes by Todd Koel and colleagues also documented the invasive fish's reproductive potential. A female lake trout weighing 5 kilograms can produce 6,000 to 8,000 eggs in a single spawning event. The researchers also found that young lake trout in Yellowstone Lake appeared to survive at rates four to six times higher than those of young fish in their native range.
{{/usCountry}}The researchers reported that annual gillnetting effort exceeded 46,800 100-meter net-nights in 2012, meeting a benchmark recommended by a scientific review panel. As suppression intensified, estimated lake trout biomass declined from 432,017 kilograms in 2012 to 196,675 kilograms in 2019. The decrease was driven primarily by a 79% reduction in adult fish.
Progress and limits of the recovery
Large-scale removal efforts have continued. The NPS reported that crews caught 264,000 lake trout in 2024, bringing the cumulative number removed to 4.9 million. The agency also estimated that the adult lake trout population had declined by about 90% from its peak. Koel, who leads Yellowstone's Native Fish Conservation Program, has reported that cutthroat trout are returning to spawning streams and that bears and bald eagles are feeding near them again. Ospreys have also begun appearing in the area.
However, recovery remains incomplete. In the Fishes study, cutthroat trout abundance was still below management targets in 2019. Researchers found that larger fish, exceeding 400 millimeters (about 16 inches), had become more common and that individual fish weights had doubled. They suggested that reduced competition for food and other resources could explain these changes, pointing to improved conditions for surviving trout even though the population had not fully recovered.
The study also examined efforts to reduce lake trout reproduction. In October 2019, researchers used a helicopter to spread 18,000 kilograms of organic pellets over a lake trout spawning site near Carrington Island. The treatment was intended to reduce oxygen available to developing eggs. Traps set the following spring caught no lake trout fry. The researchers interpreted the result as evidence that the treatment likely killed embryos at the site, but the finding did not establish whether the method would be effective across the entire lake.
Another potential threat emerged in 2019, when crews caught a cisco, a fish not native to Yellowstone Lake. Researchers suspected it had been illegally introduced. If cisco become established, they could compete with cutthroat trout for food while also providing another prey source for lake trout.
The suppression program continues to rely on funding, fieldwork, and monitoring. Yellowstone Forever contributes approximately $1 million annually to native fish conservation, according to the NPS, while volunteer anglers help researchers collect fish and record data. Koel has compared lake trout control with maintaining roads and repairing boilers, describing it as essential work that requires ongoing attention.
Despite the removal of millions of invasive fish, cutthroat trout abundance remained below management targets in the latest year assessed by the 2020 study. Monitoring and subsequent population estimates will be needed to determine how closely the native trout are approaching those targets.