Nonnative fish cause harm to native fish in Yellowstone National Park by preying upon them, competing for food resources, or interbreeding (hybridizing) with them. Biologists use “selective removal” techniques, such as electrofishing, to reduce the abundance of nonnatives and minimize these impacts. Anglers can assist with these conservation efforts by harvesting the nonnative fish they catch. Angling regulations include no creel limits and (in some areas) a mandatory kill requirement for nonnatives, which have proven effective for selectively removing them.
In some instances selective removal methods cannot effectively suppress the invaders and the native species have been largely lost. In those cases, in-stream barriers that isolate headwater refuge areas followed by chemical treatments to completely remove the nonnatives are considered. Some natural waterfall barriers already exist; sometimes modifications to the natural waterfalls can form complete barriers to upstream movement by fish. In a few places, the barriers must be completely fabricated from concrete or other materials. Once the headwater refuge is protected from invasion by nonnative fish located downstream, a chemical (rotenone) treatment is used to eliminate nonnative species from the restoration area.
Piscicides are toxins which remove fish from habitats where nets, electrofishing, angling, traps, or other mechanical methods are impractical or ineffective. Fish removal projects in the United States use piscicide containing the natural compound rotenone. Biologists in Yellowstone National Park have used rotenone in formulations approved by the Environmental Protection Agency to remove nonnative fish species from High Lake and East Fork Specimen Creek (2006-2009), Goose Lake (2011), Elk Creek (2012-2014), Grayling Creek (2013-2014), Soda Butte Creek (2015-2016), and the upper Gibbon River including Wolf, Grebe, and Ice lakes (2017-2019).
Rotenone occurs in the roots, stems, and leaves of tropical plants in the pea family (Fabaceae). Ingestion has a relatively minor effect on land animals because the enzymes and acids of the digestive system break it down. Rotenone must be absorbed into the bloodstream, usually across the gill membrane of fish, to be harmful. It kills by inhibiting the biochemical reaction some cells use to turn nutrients into energy. Essentially, rotenone starves the cells, causing death.
To treat a section of stream, rotenone is dripped in at a rate determined by the volume, speed, and temperature of the water. At the downstream end of the restoration area, potassium permanganate is added to the water to neutralize the rotenone. Rotenone is quickly broken down in the environment by sunlight and readily binds to sediments or organic matter in the water. The rapid degradation and dissipation mean that managers have a short window of time to successfully remove nonnative fish.
Unfortunately, piscicides may impact other gill-breathing aquatic organisms, including non-target fish species (i.e., native fishes if present), larval amphibians, and macroinvertebrates. To reduce potential impacts on non-target organisms, specialists use a minimum dosage of rotenone for short periods of time. Biologists limit treatment areas and leave recovery intervals between treatments. All treatments in Yellowstone National Park have been conducted during late summer or fall to avoid impacts to amphibians in their early (larval, gill-breathing) developmental stages. Research conducted during these treatments provides strong evidence benthic macroinvertebrates and amphibians in Yellowstone have not been significantly impacted in the long-term. Following removal of the harmful species, stocking of the native fish restores their populations.
One Fish, Two Fish: Saving the Yellowstone Cutthroat Trout
Back in 1870, a member of the Washburn Expedition wrote in his diary about the Yellowstone cutthroat trout: "Two men could catch them faster than half a dozen could clean and get them ready for the frying pan.” Since then, things have become a lot more complicated. For this episode, a story of native fish conservation and what it takes to restore an ecosystem.
One Fish, Two Fish: Saving the Yellowstone Cutthroat Trout
[Lake water lapping sound]
Narrator: It’s a cold October morning on Yellowstone Lake. Heavy frost covers the dock the last few boats in the marina while fresh snow blankets the Absaroka mountains in the distance. [BOAT STARTING SOUND] Yellowstone National Park Fisheries Supervisor Todd Koel starts up a cold aluminum boat and scrapes frost off the windshield. [BOAT STARTING SOUND] We’re heading out to join fisheries crews working on the lake. Along the way Todd is thinking about the native Yellowstone Cutthroat Trout.
TODD - Really, the park and especially Yellowstone Lake, lies right at the core of the distribution, the range-wide distribution, of the Yellowstone Cutthroat Trout subspecies.
Narrator: Yellowstone Lake is huge, roughly twice the area of Washington DC. It the largest high-altitude lake in North America. And, it is thought to have held the single biggest population of Yellowstone Cutthroat trout in the world. TODD - estimates were in the range of about four million or so catchable Cutthroat Trout in Yellowstone Lake. That's a lot of Cutthroat in the system.
Narrator: This was in the early 1990s. And in those days, people used to say there were so many fish that you could walk on their backs across the Yellowstone River when they spawned. Tourists and anglers gathered by the thousands just to see the Narrator: fish moving up the spawning streams. One year over 300,000 people visited La Hardy Rapids and Fishing Bridge to see the brightly colored cutthroat trout.
BOB - Fishing Bridge got its name because there used to be so many people fishing from it when I worked there in 1969, you had to roll your windows up as you drove across there to keep from getting a lure in your ear.
Narrator: That's Bob Gresswell, Research Scientist with the U.S. Geological Survey in Bozeman, MT. Bob has studied Yellowstone Cutthroat trout in the park for over 40 years.
BOB - there would be thousands of cutthroat trout at the mouth of Yellowstone Lake that could be observed during the peak of the migration.
Narrator: Lots of fish everywhere. [Sounds of fishing reel spinning]
Narrator: But then, over one decade, everything changed dramatically. By the mid 2000s cutthroat had practically disappeared from Yellowstone Lake. Biologists estimate the Yellowstone Cutthroat population plummeted from over 3 million fish to less than 300,000. Some big new factor had to be causing the decline. Again, Todd Koel.
TODD - For probably 10 to 15 years we really saw very little recruitment of small Cutthroat Trout. Why? Because they were getting eaten by the Lake Trout, that's why.
Narrator: And here was this big problem. Lake trout had invaded Yellowstone Lake. A native of the Great Lakes and Canada, lake trout had been introduced to some large bodies of water in the Western US as a sport fish early in the last century. But not Yellowstone Lake. Yellowstone Lake was a high mountain stronghold of the cutthroat until…
TODD - The first Lake Trout was first verified in 1994. We know some were probably introduced in the mid to late 1980s and the early 1990s
Narrator: It was theorized that anglers illegally dumped the lake trout into the Lake, but the theory couldn’t be verified. No matter how they got there, Koel says the invasive lake trout were uniquely suited to have a huge impact on the native cutthroat. TODD - Lake Trout do great harm to cutthroat trout. Lake trout are predatory fish, first and foremost. Essentially just big, predatory fish-eating machines. They grow large. Live 30, 40 or more years. Grow 30, 40 or more pounds. They're highly prolific in the numbers of eggs that they can produce, so they've got a lot of built-in mechanisms that give them sort of an advantage in Yellowstone Lake, over and above the native Cutthroat Trout.
Narrator: And the problem didn’t stop with the drop in cutthroat. Over forty species in the park depended on the native Yellowstone cutthroat. And they suddenly had far fewer cutthroat to eat.
Narrator: Before lake trout showed up, enough bears fed on cutthroat in streams around the lake that the park enforced bear closures to protect bears and visitors. But by 2005, very few bears showed up in the closure areas. In the early 1990s over 100 osprey nested around the lake. By 2000 there were roughly six.
Narrator: And this highlights one of the basic problems with invasive species: in any ecosystem, an invasive species can quickly upset the natural balance and affect many other species.
Narrator: As the cutthroat declined, the lake trout population exploded. And the new lake trout? They were no replacement for the cutthroat. Behaving quite differently, they preferred deep water. They also didn’t leave the lake to reproduce in the tributary streams. All of which made them inaccessible as a food source.
It was an ecosystem at risk.
[BOAT REVVING UP AND GETTING UP ON PLANE SOUND]
Narrator: Todd Koel drives the work boat out of Bridge Bay Marina headed toward the southeast arm of the Lake, over fifteen miles away.
TODD – So we’re lucky that most of the boats are in the same general area of the lake today, so hopefully we won’t have to travel too far and wide to see gill netting boats, operating out there today, and then…[FADE IN NARRATION OVER TODD TALKING]
Narrator: The Park Service started trying to control the lake trout population soon after the fish were found in 1994. Taking a page from commercial fisherman, fisheries staff deployed gill netting boats to catch the invasive fish—an effort that still continues today. Currently, the Park has somewhere between 30 and 40 miles of nets set in the lake to catch lake trout every day during the fishing season. [SOUND OF A GILL NET PULLER STARTING UP, BINS RATTLING]
Narrator: Bouys mark the location the nets are set. Then gill netting boats haul up the nets full of lake trout. As the net comes in, crews pick the fish from the nets. They note their size, weight, and sex. And then they slit them open and dump them back in the lake.
[NETTING SOUNDS FADES OUT]
Narrator: And here is the next big challenge with invasive species. Once installed, it takes a ton of work to control them, they can also be nearly impossible to erradicate. In 2016 nearly 350,000 lake trout were caught in the gil nets—the highest catch yet in a single year. And this is the main effort to save the cutthroat—the effort to catch the invading lake trout until their population crashes. But, because of the massive amount of work that effort takes, biologists are looking at new ways to knock down the lake trout population.
Narrator: In the fall lake trout group up in large numbers, sometimes thousands of fish, to lay their eggs at specific locations—making both the lake trout and their eggs vulnerable. To find these sites, scientists have implanted radio telemetry tags in some lake trout. And by tracking the fish they’ve identified over a dozen lake trout spawning areas in Yellowstone Lake.
TODD - Then we're doing research on methods to kill the Lake Trout eggs on these spawning sites…
Narrator: …To stop the lake trout at the start. One experimental technique involved shocking the eggs with electricity. Another attempted sucking them up with a small suction mining dredge. But by far the most promising new method is surprisingly simple.
[GRINDER NOISE START] Narrator: Fisheries technician Mike Symiantowski stands on the deck of a gill netting boat with a large lake trout in his hands.
MIKE SYMIANTOWSKI - So we got all the lake trout we just caught in the last couple gangs of net. So right now we’re over one of our dumping sites. We're taking the lake trout and we’re putting them through this fish grinder here so that way we know that all the pieces of the fish are sinking all the way down to the bottom.
GRINDER SOUND FADE OUT
Narrator: Grinding is used over some sites, while on others whole dead lake trout are being dumped. Laboratory tests show that the decomposing fish carcasses kill the eggs by eating the oxygen in the water. Oxygen that the eggs need to survive.
TODD - We're moving forward pretty quickly with that method because it's so simple, because we're not taking anything foreign from the lake and bringing it in, like some kind of a chemical or something like that. We're just simply taking the dead fish that we're catching and putting them strategically on spawning areas.
Narrator: And these big efforts to keep the ecosystem healthy, They seem to be paying off. Fewer small young lake trout and fewer big, old lake trout are being caught in the nets.
TODD - That's telling us, even though our catches are still high each year, that's a strong signal that our suppression program is collapsing this population.
Narrator: And as a result the cutthroat are making a comeback.
TODD - For every Lake Trout we kill each summer, we're probably saving 40 to 50 or more Cutthroat Trout just in every given year.
Narrator: More cutthroat of different sizes are being found in annual population monitoring. Anglers are catching more cutthroat. Visitors are returning to watch the cutthroat spawn in the late spring. Nesting osprey numbers are growing around Yellowstone Lake. And finally, bears are coming back to the spawning streams. Koel has even seen this firsthand.
TODD - I was on that spawning stream trying to collect a water sample in June with some of my crew and a grizzly bear showed up. TODD - That was his stream, and those were his fish. Then watching him get after the Cutthroat Trout and bring them up on the bank, and chow them down. That's pretty amazing to see. Then on top of that, the hundreds of people that were gathered to watch that as well. That occurred for well over a month.
Narrator: All encouraging signs. For Yellowstone National Park, whose mission to preserve our natural resources unimpaired for future generations, the effort to repair the damage done by invasive lake trout is worth it. Yellowstone is one of our few remaining relatively intact ecosystems left in North America. And restoring the cutthroat? [Lake water lapping sound] It helps it stay that way. [Lake water lapping sound] From Yellowstone National Park, I’m Scott Christy. [Lake water lapping sound]
To maintain the natural biodiversity of the Yellowstone ecosystem, sometimes you have to start small. Fish biologist Todd Koel discusses efforts to restore native fish in Grayling Creek, a cup of eggs at a time.