A new tool to suppress an invasive fish shows promise
By Isabella Sadler
In October 2019 and 2020, helicopters hovered above the pristine waters of Yellowstone Lake, surrounded by an autumn landscape of yellowing aspen trees. The helicopters carried a weight equivalent to 14 small cars—17,000 kilograms of circular, brown pellets—which they released near a small, rocky island in the lake’s West Thumb. The pellets rained down, sinking to the lake’s bottom, where managers hoped they would suppress the thousands of invasive lake trout born in Yellowstone Lake annually. Years in the making, this novel technique targets a life stage that past efforts have been unsuccessful at controlling, and shows promise as an effective, low-cost way to eradicate invasive fish.
Yellowstone Lake is home to the largest population of genetically pure Yellowstone cutthroat trout, a spotted, golden-colored trout native to the western US. This culturally and ecologically significant fish attracts anglers from across the country and serves as a valuable food source for many land mammals and birds in the area. But lake trout—an invasive, predatory trout species first discovered in Yellowstone Lake in 1994—threaten cutthroat trout and the animals that rely on them.
Lake trout eat cutthroat, which led to a severe decline in the cutthroat population after the lake trout population expanded. Lake trout also do not occupy the same ecological role as cutthroat, which has implications for the Greater Yellowstone Ecosystem as a whole. Cutthroat are medium-bodied trout that reproduce in streams connected to Yellowstone Lake, making them available as food sources for many land animals that pass by streams. Lake trout, however, are much larger and do not access the streams, making it nearly impossible for land predators to catch them. Because of this, fewer cutthroat means less food for brown bears, black bears, eagles, osprey, and more.
To combat this problem, the National Park Service fisheries program began removing lake trout in 1995 with gillnets, which are large nets that entangle fish as they attempt to swim through. While lake trout numbers in Yellowstone Lake have decreased since 2012, the invasive trout persist in large numbers and pose a substantial threat to the cutthroat. In addition, gillnetting is very expensive, and young fish, typically two years old and below, are small enough to slip through the gillnets.
Thus, park service biologists sought methods to kill young lake trout before and just after they’ve hatched. To do this, they targeted where lake trout lay their eggs, attempting to make these spawning grounds inhospitable to the developing fish. After years of research, they developed pellets that mimic the way a decomposing lake trout carcass removes oxygen from the water. Releasing these organic, “carcass-analog” pellets in the water around the spawning grounds reduces oxygen concentrations to lethal levels, smothering lake trout eggs. This only harms the lake trout young because cutthroat spawn in streams far away from Yellowstone Lake.
It’s also cost-effective. More than $2 million is spent gillnetting lake trout in Yellowstone Lake each year, while it’s estimated that applying pellet treatments to all known spawning sites would cost just $250,000 annually. Gillnetting would still be needed to target adult lake trout, but fewer fish hatching each year would slow their reproduction and reduce overall costs.
Park biologists piloted this new method in 2019 and 2020, dropping the pellets on Carrington Island spawning reef. In the two years following pellet treatments, biologists didn’t catch a single lake trout hatchling in traps surrounding Carrington Island, implying that nearly 100% of lake trout eggs died at this location. While these results are very promising for controlling young fish, Carrington Island is just one of 14 known spawning sites in Yellowstone Lake. Researchers do not yet know how suppressing hatchlings at one site will impact the lake-wide invasive trout population and would need to treat more spawning locations to determine the pellets’ overall efficiency and impact on the lake trout.
Biologists must also consider potential negative side effects of this treatment. Two studies are currently evaluating the effect of pellet treatments on Yellowstone Lake. One project collects tissue samples of algae, zooplankton, macroinvertebrates, and fish to measure the extent to which pellets are incorporated into the food web. The other project evaluates the impacts of pellet treatments on water quality and nutrient dynamics. Because pellets contain nitrogen and phosphorus—nutrients that can stimulate the growth of algae—there is a chance that the pellets reduce water quality and affect other organisms in the lake. While they don’t expect unintended effects, biologists want to be certain before expanding the treatment to more spawning sites.
Overall, Yellowstone National Park is moving forward with cautious optimism. The cutthroat trout population has greatly recovered due to these efforts, but lake trout control will need to continue into the foreseeable future. Carrington Island will be treated again in autumn 2024 and 2025, and the pellets’ initial success has inspired further development and research. Not only is this new method a milestone in the park’s 30-year battle against lake trout, but the work in Yellowstone Lake is paving the way for management in other large, deep lakes where controlling invasive species has been extremely difficult.
Isabella Sadler is a PhD student in the Program in Ecology at the University of Wyoming. Her research interests involve how invasive species and disturbance alter freshwater ecosystems.
Header Image: The angular rock surrounding Carrington Island in the West Thumb is prime lake trout spawning habitat in Yellowstone Lake (Koel et al., CC BY 4.0).
