The splash of one fish ripples through an ecosystem

In 2007, biologist Arthur Middleton was studying the Clark’s Fork elk herd, which migrates between Yellowstone National Park and the foothills northwest of Cody, when he made a perplexing discovery.

Elk cow and calf. Photo by Charlie Reinertsen.
Elk cow and calf. Photo by Charlie Reinertsen.

Surveys documented about half as many calves as historically observed on the elks’ winter range. At first, biologists speculated that wolves were targeting vulnerable elk calves during the winter. However, when the elk returned in the fall from Yellowstone National Park, there was a surprise—the calves were already missing. The wolves on the winter range were not the culprit. “Something was going on up in Yellowstone,” Middleton described, and he set out to solve the mystery.

Middleton, who received his doctorate in 2012 from the University of Wyoming studying elk in northwestern Wyoming, had an idea of where to start. As a biologist, he knew that grizzly bears were the primary predator of newborn elk calves, and that recent research was documenting a shift in grizzly bear diet. This spurred him to dig into historic research on grizzlies to find out if this shift in diet could explain the missing elk calves. “I found myself fascinated by the interactions. I read through dozens and dozens of old papers, going back into the 70’s and the 80’s and the 90’s, and digging through the library for the stuff that wasn’t online, and really getting deep into the history of studies that compare diets. I exhaustively used everything we knew about bear diet so that I could compare historical diets to contemporary diets.”

Cutthroat trout in Yellowstone. Photo by Jay Fleming.
Cutthroat trout in Yellowstone. Photo by Jay Fleming.

Through his investigation, Middleton started to piece together an intricate food web of interactions. Grizzly bears eat a wide variety of foods, from moths and berries to full-grown elk and moose. Their diet is limited seasonally as different foods become available. In the spring, the bears around Yellowstone Lake focus their foraging efforts on spawning Yellowstone cutthroat trout. That is, they did until the cutthroat population plummeted in the mid-1990s.

In 1994, scientists discovered lake trout, a non-native fish species, in Yellowstone Lake. It is widely thought that rogue fishermen introduced them to the lake in the mid 1980s for sport fishing. Adult lake trout primarily eat other fish, and in Yellowstone Lake, they eat Yellowstone cutthroat trout. Since the invasion, lake trout have wreaked havoc on the native trout. As the lake trout population grew, cutthroats started to disappear. When combined with the effects of drought and whirling disease, which is caused by a parasite that has decimated trout throughout the west, the cutthroat population has reached dangerously low numbers.

Cutthroat trout are a crucial component of the surrounding ecosystem, and lake trout do not fit into the food web in the same way. In the spring, cutthroats migrate up tributaries to spawn, where they are exposed to grizzly bears, osprey, otters, and over 30 other native predators. Unlike cutthroat, lake trout do not spawn in streams. Instead, they spawn in Yellowstone Lake in the fall, out of reach of bears and other predators.

As cutthroat numbers dwindle, the many species that once relied on them have lost a major spring food source. The grizzlies have turned to newborn elk calves to supplement their diet. The most recent research Middleton uncovered, published by the Interagency Grizzly Bear Study Team, indicated that grizzly bears near Yellowstone Lake are eating more elk calves than ever, and that this shift is due to the decline of Yellowstone cutthroat trout.

Middleton had something to add to the conversation. “What the people who documented [this shift] didn’t know, that I knew, is that if grizzly bears are switching from cutthroat trout to elk calves in the watershed of Yellowstone Lake, … they can only be eating the calves of migratory elk.” Middleton said. “I sort of came into the picture and said, wow, you know, this effect, this change in the grizzly bear diet, is something that could be transmitting all the way out to places like Cody and Jackson and Paradise Valley, because these are migratory elk herds.”

Middleton wanted to find out what the shift in grizzly bear diet would mean for the migratory elk. To do this, he used his detective work with historic research papers to estimate how many trout and elk calves the bears used to eat, and compared that number to how many trout and elk calves the bears eat today. To compare the number of elk calves from year to year, Middleton looked at what biologists call the “cow/calf ratio,” that is, the number of calves per 100 cows, which indicates the reproductive success of a herd. Historically, biologists documented 30 to 35 elk calves for every 100 cows in the Clark’s Fork herd.

Middleton, working with a University of Wyoming postdoctoral researcher Tom Morrison, used this information to create a computer model that could calculate whether the bears’ shift in diet was enough to actually impact the elk herd. The surveys starting in 2007 had documented a drop to 10 to 15 elk calves per 100 cows. Of the 15 to 20 missing elk calves per 100 cows, Middleton’s model predicted that the observed shift in bear diet accounted for 3 to 4 of them. This may sound like a small number, but to an ecologist, it matters. Middleton described the implications with excitement: “You know, wow, trout 70 miles away can affect how many elk calves there are. It is just kind of crazy.”

The remaining missing elk calves may be due to drought (which reduces the available forage), increased grizzly bear populations, and wolf predation. Although a shift in grizzly bear diet does not entirely explain the observed decline in elk calves, it is playing a role.

In April of 2013, Middleton published his research, adding elk to the list of species affected by the lake trout invasion. Meanwhile, contract fishermen were gearing up for yet another season of lake trout removal on Yellowstone Lake, as they had been doing for the past four years.


The ice on Yellowstone Lake typically goes out in late May, and contract fishing operations start as soon as the water is open. Three fishing boats with four fishermen each set out from shore at dawn. The boats are equipped with hydraulic lifts to raise miles of gillnet that have been set in the lake overnight. As the fishermen draw the nets out of the water, sun flashes off slabs of silver—a good haul. The fishermen grasp the frigid, slime-covered fish and remove

Commercial fishermen use gillnets to pull lake trout out of Yellowstone Lake. Photo by Jay Fleming.
Commercial fishermen use gillnets to pull lake trout out of Yellowstone Lake. Photo by Jay Fleming.

them from the nets. This is not your typical contract fishing operation—these fishermen are in the heart of the nation’s first National Park, and these fish will never see a dinner plate. Instead, their carcasses are destined for the bottom of Yellowstone Lake.

“Part of the Park Service mission is to conserve natural resources, and conserving the Yellowstone cutthroat population within Yellowstone Lake is a huge portion of our natural resources, especially in the fisheries,” explains Patricia Bigelow, fisheries biologist for Yellowstone National Park who has been part of the lake trout control program on Yellowstone Lake for the past fifteen years.

“The problem is the lake trout predation, so if you want to help the cutthroat trout population, that is what you need to address,” Bigelow continues, “The goal is to suppress the lake trout population to the point that the cutthroat population can rebound.”

Yellowstone cutthroat trout, named for the red gashes of color slicing underneath their jaws, are highly valued fish. Anglers come from all over the world to fish for this black-spotted, golden-red trout that can only be found in the Greater Yellowstone Ecosystem. Hundreds of thousands of Yellowstone National Park visitors once watched from Fishing Bridge on the Yellowstone River as teams of spawning trout jockeyed for position. And as researchers have shown, cutthroat trout are a crucial component of the food web. For these reasons, the Park Service has gone to war against the lake trout invasion.

In 1995, the Park Service invited a panel of scientists to assess the severity of the situation and advise the control efforts. The panel advised the park to remove lake trout, and the park started the efforts immediately. In 2008, the science panel reconvened and advised the park that it was on the right track, and it needed to do even more to stop the lake trout population growth. In response, the park tried hiring contract fishermen to harvest lake trout, and has continued to increase the control effort every year. By 2012, the park reached control levels recommended by the science panel. Middleton published his research in 2013, providing further fuel for lake trout control efforts and inspiring bumper stickers proclaiming “Lake Trout Kill Elk.”

Today, the park contracts the Hickey Brothers from Wisconsin to net lake trout every day from ice off until early October, roughly 130 days of fishing every year. On average, the fleet of three boats catches over two thousand fish every day. The fishermen kill the lake trout (if they aren’t already dead), count them to monitor the population, pop the air bladder so that the carcass will sink, and dump the fish in the deepest sections of the lake.

Grizzly and cub in Yellowstone. Photo by Mark Gocke.
Grizzly and cub in Yellowstone. Photo by Mark Gocke.

The park, with the help of the Hickey Brothers, has deposited metric tons of fish in Yellowstone Lake in the hopes that these carcasses will keep nutrients in the system. Bob Gresswell, a research biologist who served as chair on both science panels and has been studying Yellowstone cutthroat trout with the park and the US Geological Survey since 1974, says there is no evidence that dumping the fish into Yellowstone Lake is harmful; however, some scientists are concerned that it may be altering the ecosystem in unpredictable ways. Further research is needed to understand the impacts of this “nutrient loading.”

Gillnetting removes fish efficiently, but not without consequences, including by-catch of Yellowstone cutthroat trout. The fishermen reduce the amount of by-catch by adjusting the gillnet mesh size. They also focus on areas of the lake that have high densities of lake trout and very few cutthroat trout. The science panel closely monitors by-catch to ensure that it does not exceed levels that could negatively impact the cutthroat trout population. Additionally, if a cutthroat is caught in a net, the fishermen attempt to return it safely to the lake. Even so, over half of the cutthroats caught in the nets will die.

Gillnetting is also expensive. Lake trout control costs the park (as well as other donors and organizations) over two million dollars annually, of which gillnetting claims 1.7 million. And if the park ends the program today, all of the progress will be lost. “All of our modeling shows that if you cut back at any point in time, the lake trout will begin to increase again,” Gresswell explains. “The suppression of lake trout is something that will always have to occur in Yellowstone Lake. It has to become something that is just part of the working budget of Yellowstone Lake, just like plowing the roads, or cleaning the bathrooms and the campgrounds.”

The park is testing cheaper and more efficient ways to control lake trout. One alternative uses large grids of electric wires to zap embryos and emerging fry in lake trout spawning areas. Another uses a mining dredge to suck the embryos out of spawning beds. These alternatives show promise by eliminating by-catch, but they still cannot completely remove lake trout from the system.

Future technologies may provide the solution. Gresswell suggests that “emerging genetic technologies” could help eradicate lake trout from Yellowstone Lake. One solution scientists are developing is to genetically alter male lake trout so that they can only produce male offspring. The idea is that the park would continually stock Yellowstone Lake with these genetically altered males while maintaining gillnetting efforts to control the overall population. With several decades of stocking altered males and gillnetting, the park may be able to eradicate lake trout.

Even without new technology, “There are lots of very positive signs that this suppression activity is beginning to accomplish its goal,” explains Gresswell. Bigelow states that “it is really exciting that population modeling is showing a decline in the lake trout population overall,” and Gresswell is hopeful that “we should see a relatively rapid, within decades, resurgence of cutthroat trout.”


When irresponsible fishermen introduced lake trout in Yellowstone Lake, they could never imagine the consequences of their actions. Over thirty years later, the lake trout effect is still rippling through the ecosystem in surprising ways. “These cutthroat trout are not just important because they’re native fish,” Middleton said, “they are important because they are at the heart of a … web of interactions that reaches all the way to elk in the outskirts of the Greater Yellowstone Ecosystem that are economically, recreationally, and culturally immensely valuable.” But even with cutthroat trout rebounding, this web of interactions may never be the same again.

Bringing cutthroats back to historic levels would be a huge accomplishment for the park. But whether or not the resurgence of cutthroats will restore the ecosystem is uncertain. “We can’t go around complacently thinking that we have restored Yellowstone. We are not going back to a baseline; we are not fixing it to where it was. We are all going somewhere new,” explained Middleton, “And that’s as sad as it is exhilarating.”

By Charlie Reinertsen

Charlie Reinertsen is the 2014-2015 Western Confluence editorial fellow.

See a graphic showing Yellowstone Lake’s ecological web in The Trout Effect.


Further Reading

Arthur Middleton, Thomas Morrison, Jennifer Fortin, Charles Robbins, Kelly Proffitt, P. J. White, Douglas McWhirter, Todd Koel, Douglas Brimeyer, W. Sue Fairbanks, and Matthew Kauffman, “Grizzly bear predation links the loss of native trout to the demography of migratory elk in Yellowstone,” Proceedings of the Royal Society B: Biological Sciences, 280 (2013): doi:10.1098/rspb.2013.0870.

Patricia Bigelow, Todd Koel, Dan Mahoney, Brian Ertel, Barb Rowdon, and S. Thomas Olliff, “Protection of Native Yellowstone Cutthroat Trout in Yellowstone Lake, Yellowstone National Park, Wyoming,” Water Resources Division, National Park Service, United States Department of Interior (2003).


  1. I enjoyed reading this. Nice article, it is very informative. I love reading articles about plants and animals.

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