Game And Fish Genetically Modify Brook Trout To Make Them All Male

The Wyoming Game and Fish Department might have come up with a seemingly sci-fi way to combat brook trout that have been taking over habitat and food sources from native cutthroats.

Turn the brookies all male by mixing them with genetically-altered “Trojan” fish in hopes that the brookies will eventually die off. 

“This is a long game. Although we don’t know the exact timeframe, we expect it to take a minimum of 5 to 7 years and may take longer than that depending on the survival of the trojan fish and our ability to remove wild fish,” Game and Fish Jackson Fisheries Biologist Diana Miller told Cowboy State Daily.

The Game and Fish’s “Trojan” fish are genetically-altered male brook trout that can mate with wild brook trout but the progeny produced can only be male. Years of interbreeding with Trojan fish should produce an all-male population, unable to reproduce, killing off the brookie population.

Game and Fish this month released 3,600 Trojan male brook trout into three streams in the Game Creek area, part of the upper Snake River drainage.

It's a first for Wyoming, Miller added. Using genetically-altered fish to change the course of an aquatic population has never been tried here before.

And if things go well in Game Creek, Game and Fish might consider using that method for population control in other fisheries.

Altered Chromosomes

The Trojan Fish method is still relatively new. It was developed in Idaho, and has been used in a few other states, such as New Mexico and Washington, Miller said. 

Trojan males are sometimes called “YY brook trout,” according to Game and Fish. They have two Y-chromosomes, unlike normal males, with X and Y chromosomes.

When they’re introduced into a fishery, they can reproduce with wild fish. But because of their altered chromosomes, only male offspring are produced.

“The stocking will continue until the population is 100% male and the trout no longer have the ability to reproduce. Without a reproducing population, the population will eventually die out, allowing for native cutthroat trout to be restored,” according to Game and Fish.

The Trojan brook trout dumped into Game Creek came from a hatchery in Colorado, Miller said.

Better Than Poison

Game and Fish has used more low-tech and lethal methods to rid waters of unwanted fish.

In November 2022, Game and Fish dumped seven tons of rotenone (poison) into Saratoga Lake, which at that time was overrun with illegally-stocked yellow perch. 

But the rotenone didn’t kill only the perch. It wiped out every living thing in the lake, paving the way for trout and other species to be re-stocked there the following spring.  

Such a sledgehammer approach wouldn’t have worked in Game Creek, so Game and Fish opted to take a subtler tactic, using the brookies’ chromosomes against them. 

“We decided to use this method (Trojan males) in Game Creek because it was a good candidate for this approach.  It is relatively small, can be easily accessed, and wasn’t a good candidate for other removal methods such as rotenone,” Miller said.

Brookies Outbreed Cutthroats  

Non-native brook trout out-competing native cutthroats is an ongoing concern in Wyoming.

Game and Fish previously tried giving cutthroats an edge by stocking them in the Game Creek drainage, but it didn’t work, Miller said.

“The brook trout in Game Creek were stocked in the 1960s and 1980’s. Cutthroat trout were also stocked, on a much more frequent basis, but were still outcompeted by the brook trout present,” she said. 

Brook trout prevailed because they can breed faster.

“Brook trout are highly reproductive, reproduce at an early age —ages 1 to 2 years, compared to a cutthroat at 3 years — and are fall spawners, giving them a growth advantage over cutthroat trout fry.

In addition to using the brookies’ prolific breeding against them, Game and Fish hopes to continue stocking cutthroats in Game Creek.

But the agency wants to first make sure the brookie population is tipping towards all-male.

“Fisheries biologists in the Jackson Region will begin genetic testing of brook trout in Game Creek in 2026 to determine sex ratios,” according to Game and Fish.