Across the Greater Yellowstone Ecosystem, state fisheries managers are doing everything they can to save the area’s native trout. Wednesday, Cody Region fisheries biologists stocked brook trout, which are a huge part of the problem, in an effort to save native cutthroat trout.

But these aren’t your average brookies. They are known as “trojan” male brook trout, or sometimes called YY brook trout or supermales, because they have two Y-chromosomes. These trout can be stocked into wild brook trout populations and will reproduce with the wild fish, but only produce male offspring. The stocking will continue until the population is 100% male and the trout no longer have the ability to reproduce, said Jason Burckhardt, Cody Region fisheries biologist.

“We’re actually trying to get rid of a fish [species] by stocking fish, which sounds counterintuitive,” Burckhardt said in an interview with the Tribune.

Without a reproducing population, the species will eventually die out, allowing for native cutthroat trout to be restored. The department had previously attempted to mechanically remove brook trout — through electrofishing — from waterways affected, but “you can’t get them all,” Burckhardt said.

Electrofishing is a labor intensive job, requiring several people hours to move along the tributaries and sometimes a few will get away and quickly reproduce.

Brook trout take over and displace cutthroats, which are native to the state. The trojan fish have been stocked in Dick Creek, a tributary of Wood River, and Pickett Creek, a tributary of the Greybull River. The area is one of the last remaining strongholds for the cutthroat trout, Burckhardt said.

Fisheries biologists in the Jackson Region last week also released 3,600 trojan male brook trout in the upper Snake River drainage.

“This is a huge step for our long-term goals in Game Creek,” said Diana Miller, Game and Fish fisheries biologist in the Jackson Region. “All the work that so many people have put into this project is contributing directly to the continued persistence of Snake River cutthroat trout in Flat Creek and the Snake River, and is something we all should be very proud of.”

Chemical treatments have also been used to rid brook trout from some area waterways, including near the Pitchfork Ranch recently. Yet in Dick and Pickett creeks, a chemical treatment would kill thousands of cutthroat trout to rid the tributaries of significantly fewer brook trout.

“We’re doing everything we can to stop the threat to cutthroat trout,” he said.

According to the Idaho Department of Fish and Game, the first YY male brook trout were created by the department during a period of years from 2008-2016. To create a supermale brood stock, normal XY males are feminized by exposing them to estrogen. The resulting XY female fish are crossed with normal XY males and one-quarter of the subsequent progeny are YY males.

Their research showed stocked hatchery male YY brook trout survive and produce viable YY offspring in streams, although reproductive fitness appeared to have been lower than wild brook trout.

“Even if reduced fitness is the norm in both streams and alpine lakes, our population simulations suggest that eradication can be achieved in reasonable time periods under some M YY stocking scenarios,” the team said in their report predictably called “Production and Evaluation of YY-Male Brook Trout to Eradicate Nonnative Wild Brook Trout Populations,” and unveiled at the Wild Trout Symposium XII in 2017.

According to the report, multiple stockings will be necessary to complete the transition in the selected habitats to 100% male populations unable to reproduce. Burckhardt hopes it will work within five years of stocking.

“This is not a one time deal,” he said. “It’s still experimental.”