Yellowstone Lake is a central feature of Yellowstone National Park, covering an area of 136 square miles. The lake’s great depth (394 feet) and elevation (7,732 feet) make it the ideal habitat for cold-water fish species. The lake is home to a unique subspecies of the cutthroat trout (Oncorhynchus clarkii) known as the Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri). The Yellowstone cutthroat trout evolved as a species in river systems draining to the Pacific Ocean. Still, they naturally traveled into the headwaters of rivers draining to the Atlantic Ocean. This migration brought them to Yellowstone Lake, in the headwaters of the Mississippi River.
Scientists estimate that Yellowstone cutthroat trout occupied 17,800 miles of streams across Wyoming, Montana, Idaho, Utah, and Nevada before European settlement. Yellowstone cutthroat trout were less common in lakes. Yellowstone Lake had the single largest lake dwelling population, making up two-thirds of all lake habitat.
Introduction of Lake Trout
Invasive species are a massive problem across the globe. Research estimates the total damage from invasive species from 1970 to 2017 to be $1.288 trillion. The Yellowstone cutthroat trout are feeling the adverse effects of invasive species as well. Though, the invasive species threatening them isn’t from Europe or Asia, where many invasive species in the United States come from. The Yellowstone cutthroat trout are impacted by lake trout (Salvelinus namaycush), a species native to Canada, Alaska, parts of Montana, the Great Lakes, and much of the northeastern United States. The irony is that lake trout are being threatened in their native range by climate change and pollution, making them a priority for conservation in their native range.
Lake trout were first found in Yellowstone Lake in 1994, though research indicates they were likely introduced sometime in the 1980s from nearby Lewis Lake. Lake trout are voracious predators that will eat as many as 41 cutthroat trout per year. More importantly, like many invasive species, lake trout occupy an ecological niche, or unique space in the ecosystem, that other organisms have not adapted to. Lake trout grow larger than Yellowstone cutthroat trout making it more challenging for some species to eat them. In addition, they occupy deeper waters of the lake, further distancing them from predators that rely on fish for food. Finally, lake trout spawn in the lake rather than in shallow tributaries, again evading many predators that evolved to rely on Yellowstone cutthroat trout as a source of food.
Introducing a voracious predator into an ecosystem that can also evade predation is the perfect recipe for trouble for the native fish species they prey on. This is precisely what happened to the Yellowstone cutthroat trout when lake trout were introduced to Yellowstone Lake. Estimates are that the Yellowstone cutthroat trout population declined by 90% over the first decade after the lake trout introduction. Before the introduction of lake trout, the population was estimated to be 3.5-4 million; a decade later, it was less than 400,000.
Bad News For Grizzly Bears and Bald Eagles
“If the land mechanism as a whole is good then every part is good, whether we understand it or not… To keep every cog and wheel is the first precaution of intelligent tinkering.”
Aldo Leopold
Ecosystems are incredibly complex, intricate, interdependent systems. Every push, pull, addition or subtraction to one part of the system will have a reverberation throughout. Adding lake trout to Yellowstone lake resulted in a substantial decline in the cutthroat trout population. The impacts didn’t stop there.
The eagles and osprey around Yellowstone Lake rely on the abundance of cutthroat trout in the shallow waters of Yellowstone Lake as a source of food. As the cutthroat trout population crashed in Yellowstone Lake, the bald eagles soaring high above its waters would have noticed. Lake trout were feasting from below as the eagles were growing hungry from above. It might be easy to assume a fish is a fish, and the bald eagles could eat the lake trout instead. But the lake trout grow too big and stay too deep to be grabbed by an eagle.
The eagles and osprey weren’t the only ones to feel the pain of losing an abundant food source. The grizzly bears and river otters up Yellowstone Lakes tributaries also have felt the loss. From spring through early summer, Yellowstone cutthroat trout move into the lake’s tributaries providing an essential source of high-calorie nutrient-rich food for the bears and otters. An abundant food source in the spring is essential for the otters and bears after a long winter. In one stream studied by scientists, there were 70,000 fish spawning in 1978, and in 2007, there were just 500. That is more than a 99% decline.
The numbers tell the tale well, but let’s consider a human-oriented analogy for a moment. Imagine if the food in all of the major grocery stores in your city declined by 99%. Panic would undoubtedly ensue, but people would also adapt. You might start shopping at your local gas station or patronizing a local restaurant. No matter how your city adapted to this abrupt change, one thing would be sure; the change would be felt by many and noticeable by all.
The Recovery of Cutthroat Trout
Following the discovery of lake trout in Yellowstone Lake, the National Park Service began exploring options to control the population in Yellowstone Lake. Scientists recommended that the National Park Service pursue gillnetting and trap netting as a means to control the lake trout population, with the clear expectation that this may be a perpetual effort. In the early years of this effort, as more effort was put into gillnetting, more fish were being caught; this suggested that even as the netting activities increased, so was the population of lake trout.
In 2009, the National Park Service began contracting with commercial fishing operations to manage the gillnetting of lake trout in Yellowstone Lake. Initially, the lake trout population continued to increase and peaked in 2012. Improvements in management and continual effort have resulted in a 32% decrease in the population across all age classes and a 72% decline in mature adults.
As the lake trout population has declined, the cutthroat population has been slower to respond. However, recent studies have shown an increase in the weight and condition of adult cutthroat trout, an indication that recovery efforts are headed in the right direction. Time will only tell how the rest of the Yellowstone ecosystem responds to the suppression of lake trout and recovery of the Yellowstone cutthroat trout.
Conclusion
The story of the impact of lake trout on the Yellowstone cutthroat trout and the entire Yellowstone ecosystem is a stark reminder of the importance of preventing the spread and introduction of non-native species. Similar impacts are seen in thousands of lakes across the country as the result of invasive species, but in many cases, the story isn’t as well known or ever even told. The impact of lake trout in such a notable ecosystem strongly tied to our national identity, plus the involvement of icon species like bald eagles, makes the story much easier to tell and for people to understand. The introduction of Eurasian watermilfoil or Asian clam in your local lake can also result in reverberations throughout the lake ecosystems, but they might be harder to see, and there may not be researchers studying your lake in enough detail to tease apart the impacts the way there has been for Yellowstone Lake. And so we should remember Leopold’s quote above and be cautious with introducing new species, including the intentional introduction of new fish species for angling purposes.
The next time you launch your boat in a new lake or pond, remember to follow the Clean, Drain, Dry protocols to prevent the introduction and spread of invasive species.
Sources
Yellowstone Cutthroat Trout – NPS
High and rising economic costs of biological invasions worldwide
Implications of Cutthroat Trout Declines for Breeding Ospreys and Bald Eagles at Yellowstone Lake
Cutthroat trout continue to rebound thanks to Yellowstone Forever’s members
Yellowstone Lake ecosystem restoration: A case study for invasive fish management
