Stocking a species of fish to help get rid of the same species of fish. Come again?
There is a new fish management tool where stock- ing a particular fish species — with some special characteristics — can help eliminate that same fish species from a waterbody over time. And, the Wyoming Game and Fish Department is getting in on that action.
The goal of stocking Trojan male brook trout is to have a population of all males. (Photo by Sean Landsman/Engbretson Underwater Photography)
In July 2024 and 2025, Game and Fish personnel stocked Trojan male brook trout in three streams — one in the Jackson Region and two in the Cody Region. These fish are often called YY brook trout because they have two Y chromosomes, unlike normally produced males with an X and Y chromosome. The hope is these fish will reproduce with wild female brook trout, which have XX chromosomes. Females would only be able to provide an X chromosome to offspring, and YY males could only produce a Y chromosome. The result is only male offspring. Over multiple years of stocking these fish and wild trout removal, a population of all males would be created, eliminating the population from that waterbody.
In the case with the three streams in Wyoming — Game Creek near Jackson and Pickett and Dick creeks in the Cody Region — this would help native cutthroat trout. Brook trout are currently found in all three streams, and outcompete native cutthroat for food and habitat.
YY brook trout were developed in Idaho, where the first successful use of YY brook trout in fisheries management occurred. YY brook trout were field tested in streams beginning in 2014. The goal was to reduce the wild population of brook trout using mechanical removal, such as electrofishing, by 50 percent each year and backfill it with YY brook trout. For example, if there were 100 fish in the stream, the goal was to remove 50 and stock 50 YY brook trout each year. Last year, the Idaho Department of Fish and Game reported its first success in a field trial. It took nine years to achieve that success. Since 2014, projects have been initiated in Arizona, Colorado, Idaho, Nevada, New Mexico, Oregon, Utah, Washington and Wyoming.
Game and Fish has conducted electrofishing removal of wild brook trout on Game Creek and will continue into the future. Electrofishing is effective, but it is nearly impossible to remove all unwanted fish.
“We’re really hopeful we can do it in a shorter span than nine years,” said Diana Miller, Game and Fish fisheries biologist in the Jackson Region.
“We are removing fish every year and have removed more than 95 percent of the wild population, but if you miss one wild female, your population can continue. It has to be at least one generation, more likely two or three generations, so for brook trout that is about a 5-7-year timeline.”
But why wait so long if mechanical removal, such as electrofishing or chemical treatments, can do the job faster? These techniques are important management tools, and are often preferred, but each stream has its own unique circumstances. Sometimes, some tools are more appropriate than others. And, the use of YY brook trout isn’t meant to be an end-all-be-all in terms of a fisheries management technique.
“It’s cool stuff, but it’s pretty counterintuitive, too. We’re trying to stock a fish in order to eradicate that fish,” said Jason Burckhardt, Game and Fish fisheries biologist in the Cody Region. “It’s still an emerging technology. The theory has been around for a while, but the application is finally getting to the point where we’re starting to see some success. It’s not going to solve all of our problems, but it is another tool we can use.”
The use of YY brook trout as a management tool is still an emerging technology. (Isaac Szabo/Engbretson Underwater Photography)
WYOMING SPECIFICS
Game Creek is south of Jackson and is about 6 miles long. In the 1950s, a fishing guide in the Jackson area wrote a book and noted Game Creek was a great place to fish for spawning Snake River cutthroat. However, the creek runs underneath U.S. Highway 89, and in 1960 work was done on the highway, and a culvert was placed that didn’t allow fish to pass. Miller said over the next 20 years, brook trout that were already present in the creek outcompeted the cutthroat, and Game and Fish identified it as a spot for Snake River cutthroat trout restoration in 2016. In 2019 and 2020, the department looked into chemically removing fish from the creek with rotenone. Although the creek is on public land, there is a subdivision nearby and homeowners were concerned about the safety of their drinking water, so that option was scrapped.
Game and Fish has been removing brook trout from Game Creek through electrofishing the last four years in preparation for stocking YY brook trout. Before that, there were 1,400 brook trout per mile in a stream that is about 21⁄2 feet wide.
“They were thick and they were the only species there. The stream is hard to fish and the fish were so small that it wasn’t providing much opportunity for anglers,” Miller said.
Game and Fish has reduced the brook trout population by 97 percent.
Game and Fish personnel stocked 3,600 YY brook trout in Game Creek in 2024 and 2025, and the same number of fish are planned for stocking in the future. With continued mechanical removal, along with the presence of YY brook trout, the hope is to eliminate all brook trout from Game Creek and remove the opportunity for brook trout to move into Flat Creek — an important Snake River cutthroat trout fishery.
Miller said Game and Fish didn’t see cutthroat in Game Creek from 1961-2021. In 2018, a new culvert was installed under the highway that allowed fish passage, and Game and Fish has recorded increased numbers of Snake River cutthroat in Game Creek from 2021-24 — five in 2021, 15 in 2022, 16 in 2023 and 72 in 2024. Miller said Game and Fish didn’t stock those cutts. They moved there on their own from Flat Creek.
“The reason Game Creek is a concern is not necessarily because we want cutthroat back in there, although that would be great, but it is more about the protection of the Flat Creek fishery,” Miller said.
In the Cody Region, Burckhardt said Pickett and Dick creeks have solid populations of Yellowstone cutthroat trout. Those two creeks are larger than Game Creek, but the number of brook trout is smaller.
“What we’re trying to do is remove the brook trout to avoid their expansion in the Greybull River drainage, one of our last strongholds for cutthroat trout,” Burckhardt said. “We don’t want to do chemical treatments because we’d be killing many more cutthroat than brook trout.”
“It’s integrated pest management — applying YY brook trout and removing wild fish in order to shift the sex ratio of those brook trout.”
Burckhardt added what’s being attempted in the Cody Region with YY brook trout “is a little more on edge” because much of the previous work and trials involved waters with no native cutthroat.
Game and Fish stocked 250 YY brook trout in Pickett Creek and 150 in Dick Creek in the summers of 2024 and 2025. Those stocking numbers are planned for the next few years, along with mechanical removal of wild brook trout.
It will take some time to see if the use of Trojan male brook trout will work as a management tool in Wyoming. (Photo by Isaac Szabo/Engbretson Underwater Photography)
“I’m hopeful over the next five years to see we are no longer catching any wild brook trout in Pickett Creek and Dick Creek, whether it is through mechanical removals alone or because of the addition of the YY males,” Burckhardt said.
FUTURE
Burckhardt and other Game and Fish fisheries personnel presented a report to the department in 2022 about the opportunities of using YY brook trout as a management tool in the state.
Wyoming Game and Fish Department biologists stock Trojan male brook trout, sometimes called YY brook trout, in Game Creek in the Jackson Region. (Photo by Raegin Akhtar/WGFD)
Today, the work has started, but it will take time to see the results. Game and Fish will collect genetic samples over time to see whether or not the population turns all male. In future years, Game and Fish will collect offspring following multiple YY stockings and do genetic samplings on those fish to determine sex and also see if those fish were born from wild parents or are offspring from YY fish.
“We will do offspring testing every year until we are 100 percent male, and maybe even test a few years after that,” Miller said.
There are no immediate plans to use YY brook trout in other streams in the state. Even if the results from Game, Pickett and Dick creeks are positive, the use of YY brook trout will not replace other management techniques such as mechanical removal or chemical treatments. All of those methods have their own set of challenges, and as of now, time is the biggest factor with YY brook trout.
However, the time is here for Game and Fish to see if YY brook trout will help with the ongoing challenge to preserve and manage fisheries for the state’s native cutthroat.
“YY brook trout is not a silver bullet. It’s not going to solve all of our problems, but it is another tool we can use,” Burckhardt said. “It’s hard to say what we’re going to be dealing with in 20 years, but it’s nice to have this tool available for us to ensure we have the cutthroat on the landscape that we currently do.”
— Robert Gagliardi is the associate editor of Wyoming Wildlife.