Lake trout in Yellowstone impacting entire ecosystem

Invasive lake trout have impacted the entire Yellowstone ecosystem. Photo by Chris Hunt

A new report published by Science Advances and written by fisheries biologists—including the head fisheries manager in Yellowstone National Park—details the far-reaching impact invasive lake trout have on the Yellowstone Lake drainage, and on neighboring watersheds, as well. 

"Predatory fish introduction can cause cascading changes within recipient freshwater ecosystems," the report's abstract reads. "Linkages to avian and terrestrial food webs may occur, but effects are thought to attenuate across ecosystem boundaries."

The report, authored by Dr. Todd Koel, the fisheries director at Yellowstone, and a host of researchers and biologists hailing from the National Park Service, the University of Wyoming and Montana State University, details how deep-dwelling lake trout have not only had a lasting impact on native Yellowstone cutthroat trout populations within the Yellowstone Lake watershed, but how the loss of those native trout has impacted other Yellowstone wildlife throughout the park, not just the lake's drainage. Some of those impacts are being seen at microscopic levels—zooplanktons in Yellowstone Lake are larger thanks to reduced predation by native cutthroat trout (which provide a large prey base for non-native lake trout), which has resulted in increased clarity in the lake and warmer surface temperatures. Additionally, because fewer cutthroat trout are migrating into streams to spawn each spring, the transport of important nutrients, like ammonium, into tributaries has declined greatly. 

Invasive lake trout live in deep water and spawn in the lake. They are not an available source for most park predators. Native cutthroats migrate into streams to spawn and occupy shallower water in the lake—when they were thriving in Yellowstone Lake, they were a foundational food source for many park predators.

Documented impacts of the lake trout invasion (and cutthroat trout decline) include the loss of native trout as a prey base for everything from grizzly and black bears to otters, osprey and bald eagles. For instance, the estimated number of cutthroat trout eaten by grizzly bears in the late 1980s was nearly 30,000. By the late 2000s, that number was approximately 300. By the 2000s, grizzlies were forced to find alternative food sources—mostly elk calves. By 2009, cutthroat trout were no longer a measurable part of grizzly bear diets—elk, on the other hand, made up 84 percent of a grizzly's menu. 

Predatory birds, like bald eagles, have also altered their diets thanks to the decline of Yellowstone cutthroat trout, the report notes. Eagles now are more likely to prey on waterfowl like trumpeter swan cignets, young white pelicans and cormorants than they are on fish. Additionally, nesting success for bald eagles along Yellowstone Lake has declined. Ospreys, once very common during the summer months around Yellowstone Lake, have all but abandoned the region. Of the ospreys that still nest along Yellowstone Lake, none have been seen foraging on cutthroat trout. Instead, they travel to other drainages where cutthroats are available—some as far as five miles away. 

While the impacts of the lake trout invasion have been severe and far reaching, the report credits the National Park Service's 2010 Native Fish Recovery Plan for starting to reverse some of the trends caused by the cutthroat crash. Years of box-netting and gillnetting have reduced the numbers of lake trout in Yellowstone Lake, and native cutthroats are making a comeback. 

"Juveniles are again recruiting to the cutthroat trout population. After being absent for many years, spawning adult cutthroat trout are returning to some of the smaller tributaries, and bear use of these streams has increased as a result," the report reads. Ospreys, on the other hand, the report claims, have not responded significantly to the increased numbers of cutthroat trout, and the impacts of the ongoing cutthroat recovery have yet to be seen. 

"Lake trout suppression will be maintained in the interim ... to further reduce their abundance, thereby allowing the potential for further cutthroat trout recovery to a level where they regain their ecological importance and once again support natural processes and biodiversity in Yellowstone National Park," the report concludes. 

— Chris Hunt