Stocking of ‘Trojan’ male brook trout in Colorado helping restore native Colorado River cutthroat

Grand Junction, Colo. — Colorado Parks and Wildlife has seen positive results in restoring native Colorado River cutthroat trout in Colorado through the introduction of Trojan male brook trout.

Also known as YY brook trout, Trojan males have two Y chromosomes, unlike the wild males that have one X and one Y chromosome.

 When introduced into wild fish populations of the same species, Trojan males only produce male offspring when they spawn with wild fish. As a result, without a breeding population of both male and female fish, the invasive species will eventually die out, allowing for the restoration of native species.

In September 2025, aquatic biologists and researchers from CPW conducted a survey of Bobtail and Steelman creeks, located just below the summit of Jones Pass in a remote corner of Grand County. Over the course of two days, the teams surveyed the creeks to look for cutthroat trout, wild brook trout and YY brook trout, as well as the offspring from YY brook trout stocked in 2024.

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Between the two creeks, staff recaptured and released over 130 YY brook trout, and removed around the same number of wild brook trout. In addition to YY and wild brook trout, staff documented over 1,300 cutthroat trout from the two creeks.

“We continue to see positive results for our native cutthroat trout population from the brook trout removal efforts in these two creeks,” said Jon Ewert, Hot Sulphur Springs Area Aquatic Biologist. “It has been really encouraging to see how far we have come in restoring the native cutthroat trout since 2011, when we only observed 123 cutthroat trout in these creeks.”

The following week, CPW aquatic biologists and researchers stocked over 450 YY brook trout, something CPW will continue to do over the next several years, eventually leading to the complete elimination of the brook trout populations in the two creeks.

“In 2024, we did something historic for fish restoration in Colorado when we first stocked YY brook trout into Bobtail and Steelman creeks,” said Dr. George Schisler, CPW Aquatics Research Section Chief. “The findings from this year’s sampling strengthen the growing body of evidence for the beneficial use of YY fish in these situations.”

These early results have CPW aquatic researchers and other scientists investigating new methods to use YY fish to help eliminate non-native species, such as common carp, in areas where they have become problematic. 

Check out the latest article in Colorado Outdoors Online Magazine to learn more about CPW’s latest efforts and the 2025 results of the cutthroat trout restoration project in the Upper Williams Fork drainage.