Written BY: Trent Jones

Colorado Parks and Wildlife (CPW) is taking a groundbreaking step to save the native cutthroat trout In a couple small streams in middle park Colorado . On September 17, CPW stocked 480 Trojan male brook trout (also known as YY brook trout) into Bobtail and Steelman creeks in Grand County. This innovative technique is designed to eradicate non-native brook trout that have been decimating the cutthroat population for years.

Unlike typical male trout, these YY brook trout have two Y chromosomes, meaning when they reproduce with wild brook trout, they produce only male offspring. Without any females, the brook trout population will eventually die off, allowing cutthroat trout to thrive once again.

This method of creating YY brook trout involves manipulating the fish’s sex chromosomes to control reproduction. Typically, fish like brook trout have two sex chromosomes: XX for females and XY for males. However, by introducing estradiol (a female hormone) to young male brook trout, the treatment converts them into egg-producers while keeping their genetic makeup male (XY). These treated males are then bred with normal XY males, resulting in offspring where 25% are YY males.

To establish a broodstock of YY fish, some of these YY males are exposed again to estradiol, turning them into egg-producing YY males. Both egg- and sperm-producing YY males are then used for breeding, ensuring that all future offspring inherit only Y chromosomes, making them YY males without the need for further hormone treatments.

These YY brook trout are introduced into wild brook trout populations, where they breed and produce only male offspring. Without females, the population cannot sustain itself, leading to the eventual disappearance of the brook trout. This clears the way for native cutthroat trout to be restored to their natural habitats.

“This is a pretty historic moment for Colorado and native cutthroat trout restoration,” said CPW aquatic biologist Jon Ewert. The introduction of YY trout marks the first time this method has been used in Colorado, with plans to repeat the process in the future.

Aquatic Biologist Jon Ewert scans YY brook trout fitted with P.I.T tags before placing them into stocking bags during a historic stocking event in the headwaters of the Colorado River basin September 17, 2024. Photo by Rachael Gonzales/CPW.

The issue arose in 2010 when surveys showed an alarming number of brook trout had nearly wiped out the native cutthroat population in the headwaters of the Williams Fork River. Thanks to CPW's efforts, the cutthroat numbers in Bobtail and Steelman creeks have surged from just 123 fish in 2011 to over 1,400 today.

Though this strategy draws comparisons to the cautionary tale of "Jurassic Park," CPW is confident it will succeed in restoring one of Colorado's most valuable cutthroat populations. The agency plans to continue stocking Trojan male trout for several more years to ensure the brook trout population cannot rebound.

This innovative approach to fish breeding is helping restore native cutthroat trout populations by managing the reproductive capabilities of brook trout. If you want to read more, on this topic check out this article from: Colorado Outdoors.