The precipitous decline of Yellowstone Lake’s cutthroat trout population during the past 30 years following the illegal introduction of nonnative lake trout has had impacts on everything from zooplankton and water temperature to grizzly bears and otters, according to a newly published paper.
“Our study illustrates the potential impact of a single, invasive predatory species on otherwise pristine ecosystems,” the researchers wrote in the scientific journal “Science Advances.”
How steep was the cutthroat’s decline from its native stronghold? According to the park’s website, “The number of Yellowstone cutthroat trout spawning at Clear Creek peaked at more than 70,000 in 1978 and fell to 538 by 2007. The decline is attributed to predation by nonnative lake trout, low water during drought years, and the nonnative parasite that causes whirling disease.”
“By the mid- to late-2000s cutthroat trout were on their knees,” said Todd Koel, Yellowstone National Park fisheries supervisor, despite the park’s aggressive netting efforts to remove lake trout.
Koel said the best information he heard from a scientific panel convened in 2010 was that “all is not lost.”
“They said you’ve slowed them from growing, but told us you’ll have to double your efforts” to reduce the lake trout population, he explained.
The reproductive capability of the lake trout was staggering. Koel said between 2005 and 2012 their population was growing at the rate of about 92,600 fish a year.
Scientists from Yellowstone National Park and nearby Montana State University and the University of Wyoming collaboratively analyzed data from 1972-2017 to illustrate just how widespread the lake trout’s effect was on the park’s ecosystem. That research revealed several consequences.
The size of zooplankton in the lake increased, as fewer cutthroat trout were there to consume them. That brought about an increase in the lake’s water clarity, a likely factor in a slight rise in the lake’s surface water temperature during summer.
Fewer spawning cutthroat trout in tributary streams reduced the transport of nutrients such as ammonium into those streams.
The density and success of osprey greatly declined, as they’re unable to prey on lake trout, which live at inaccessible depths. In fact, while there were an average of 38 osprey nests at Yellowstone Lake from 1987-1991, only three were observed from 2013-2017. And those birds have been leaving the lake to forage in large lakes of the upper Snake River, at least five miles away. “That’s one of the most staggering changes,” Koel said.
Bald eagles shifted their diet to compensate for the loss of cutthroat trout. Even with that shift in diet, the average number of bald eagle nests on Yellowstone Lake dropped from 11 in 2004-2008 to eight in 2013-2017. And nesting success dropped from 56 percent in 1985-1989 to zero in 2009, rebounding to 70 percent during 2013-2017 as the eagles found alternative food sources. Bald eagles at Yellowstone Lake have been seen more frequently preying on common loons, trumpeter swan cygnets and young white pelicans, possibly contributing to declines in those bird numbers as well. “It’s unfortunate they’re feeding on other animals that are rare in the park, like loons and swans,” Koel said.
Otters, which relied on cutthroat trout as a primary food source, have shifted their diets to longnose suckers and amphibians. While there are no estimates on otter numbers in Yellowstone Lake and its tributaries before the introduction of lake trout, the estimate in 2008 — one otter for every eight miles of shoreline — is among the lowest ever reported for a river otter population.
Black bears and grizzly bears, for which spawning cutthroat trout had been an important food source, had to seek alternatives. The estimated number of spawning cutthroats consumed annually by grizzlies declined from 20,910 in the late 1980s to 2,266 in the late 1990s to only 302 in the late 2000s.
“The linkages between aquatic and terrestrial ecosystems have never been brought out in one paper at one time,” Koel said.
For the past 24 years, gillnetting has helped reduced the lake trout population as researchers have also explored other ways to kill their eggs before they hatch, another way to depress the population. Thanks to the extensive netting operation, the cutthroat trout population in Yellowstone Lake is slowly rebounding.
Last year in a sign that the lake trout population may be crashing, netting brought in 100,000 less fish than in 2017, Koel said, a 25 percent reduction.
“We still killed 297,000, but we killed 400,000 the year before,” he added.
“Juveniles are again recruiting to the cutthroat trout population,” the scientists wrote.”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.”
Although black and grizzly bears once exploited the small stream spawning habits of Yellowstone cutthroat trout as a tasty meal, research has shown they shifted their diet to other sources and their numbers remain stable.
“Grizzly and black bear frequency of occurrence on spawning tributaries and use of cutthroat trout as a food resource were greatly reduced following the lake trout invasion. However, this was localized displacement, and their populations were not otherwise affected, because only bears with home ranges neighboring Yellowstone Lake lost spawning cutthroat trout as a food resource,” the researchers wrote. “Since bears are omnivore generalists, they could make use of other foods.”
Two previous studies conducted in the park showed some of those other bear foods included elk calves.
“By 2007-2009, grizzly bears had shifted to alternative prey, and the proportion of cutthroat trout in their diet had declined to zero,” the researchers wrote. “Elk then accounted for 84 percent of all ungulates consumed by bears in the Yellowstone Lake area, suggesting lake trout had some level of indirect, negative impact on migratory elk using this area when spawning cutthroat trout were rare.”
The researchers discounted wolves, which were reintroduced to the park in 1995, as playing a role in the shifting diet of bears in the Yellowstone Lake area.
“While grizzlies prey upon elk calves primarily during the first few days after birth, wolves and other predators kill elk calves later in the summer and winter,” the University of Wyoming wrote in a press release.
In 1988, wildfires burned almost 800,000 acres in the park, but researchers could find no link between that natural disaster — which the area has adapted to withstand — and the decline of Yellowstone cutthroat trout.
“We do not attribute significant loss of cutthroat trout to wildfires because recruitment of young cutthroat trout from spawning streams, the process most likely to be impacted by wildfire, remained strong for more than a decade following 1988,” they wrote. “Cutthroat trout recruitment then severely declined concurrent with lake trout population growth. In addition, if wildfire were to have negatively impacted bears, bald eagles, ospreys or other cutthroat trout consumers, then those impacts would have also been documented elsewhere, outside of the Yellowstone Lake ecosystem, which has not been the case.”
Koel led the research using “data collected from trap nets for lake trout deployed throughout Yellowstone Lake every year from 1980 to 2017, as well as data from gill nets used to assess cutthroat trout populations for nearly the same time,” according to “Science Advances.” “The authors used statistical catch-at-age analyses to estimate both trout species’ abundance and biomass through time. Additionally, they assessed samples of zooplankton density, biomass and size before and after lake trout invasion. Koel et al. also visually surveyed spawning cutthroat trout, black bears, grizzly bears and birds to determine how lake trout may have affected those species.
“These findings point to possibilities of similar changes in many other ecosystems where nonnative species have been introduced or otherwise invaded.”
The other authors include Lusha Tronstad, research scientist with the Wyoming Natural Diversity Database at the University of Wyoming; National Park Service scientists Jeffrey Arnold, Kerry Gunther, Doug Smith and Patrick White; and John Syslo of the Montana Cooperative Fishery Research Unit at Montana State University.
The work to preserve and grow Yellowstone cutthroat trout in the lake is being complimented by efforts to remove nonnative fish from other streams and lakes in the park and replant those waters with genetically pure Yellowstone cutthroat.
The park’s goal is to maintain access for spawning cutthroat in “at least 45 of 59 of Yellowstone Lake’s historical spawning tributaries,” and “maintain or restore genetically pure cutthroat in streams occupied by pure or hybrid Yellowstone cutthroat.”
“Hopefully the next chapter is about the Yellowstone cutthroat trout’s recovery,” Koel said.