YY brook trout make a splash in Bobtail and Steelman creeks just one year after release

On a crisp September day in 2025, Colorado Parks and Wildlife (CPW) aquatic biologists and researchers made the hour-long journey up and over Jones Pass, as they’ve done for the past 14 years. Their destination, Bobtail and Steelman creeks, are located on the other side of the pass. With a narrow window of only a few weeks to complete the project, the crews began their work, conducting the annual native cutthroat trout survey and brook trout removal project for the two creeks.  Once established, brook trout outcompete cutthroat trout, eventually replacing them entirely in small populations. As a result, these removals become necessary for native cutthroat trout survival.

In addition to documenting the number of cutthroat and removal of brook trout, this year, they were also surveying for Trojan male brook trout stocked in September of 2024 and their offspring. Trojan males, more commonly known as YY brook trout, have two Y chromosomes, unlike the wild males that have one X and one Y chromosome. Because they only have Y chromosomes, they can only produce male offspring, and when they spawn with wild fish, increasingly more males and fewer females are born in that population. Without a viable breeding population of both males and females, this non-native species population will eventually die out, allowing for the restoration of native species.

Splitting up into teams, staff surveyed sections of each creek using backpack electrofishers. This battery-powered device, worn like a backpack, generates a temporary electric field in the water through a handheld pole with a metal anode. This process temporarily stuns the fish in the immediate vicinity, allowing them to be collected with a net.

For the next two days, crews surveyed the creeks, recording lengths and weights of wild brook trout and previously stocked YY trout.  Crew members also collected data from the YY trout’s Passive Integrated Transponder (PIT) tags. These small microchips, about the size of a grain of rice, are implanted to record fish’s movements and growth. All captured wild brook trout were removed permanently from the creeks, and some were submitted for genetic testing to determine the number of male and female offspring, as well as the count of YY brook trout offspring.

In Steelman Creek, staff recaptured and released 41 YY brook trout, and removed 33 wild brook trout.

In Bobtail Creek, staff recaptured and released 98 YY brook trout, and removed 105 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 from our brook trout removal efforts,” said Jon Ewert, Hot Sulphur Springs Area Aquatic Biologist. “Like 2024, the number of cutthroat trout significantly outnumbered brook trout in both creeks. It has been encouraging to see how far we have come in restoring the native cutthroat trout since 2011, when we observed only 123 cutthroat trout in these creeks.” 
“We were extremely encouraged by the survey results,” said Boyd Wright, Native Aquatic Species Coordinator. “The recaptured YY we stocked into the creeks in 2024 were in good condition, and outnumbered wild-born brook trout. Seeing these results early on in the process has us hopeful about completely eliminating brook trout from these two creeks sooner than expected.”

Round two: Another year of YY brook trout stocking

The following week, surrounded by freshly fall-kissed foliage, CPW aquatic biologists and researchers, and Denver Water volunteers once again headed up and over the pass, this time teaming up to stock this year’s YY brook trout. 

With the arrival of the hatchery truck and the 480 YY brook trout, staff and volunteers got to work. Dividing up into two teams, Bobtail Creek and Steelman Creek, the already PIT tagged fish were moved from the hatchery truck to buckets filled with water where they’d wait to be counted and scanned.

After each fish was scanned twice, it was placed into a numbered stocking bag. These tall, narrow, clear plastic bags are filled halfway with water and a handful of ice or cold water, depending on what creek they would be stocked in. This helps ensure that the water remains cool and prevents the fish from going into shock during the trip. Once the bags were filled with water and fish, the remaining space in each bag was filled with oxygen before sealing them. This process was repeated 40 times (20 bags per creek), with each bag containing 12 YY brook trout.

With fish secured in their backpacks, the group of over 40 crew members set off to take this year’s batch of fish to their new home in Bobtail or Steelman creeks. The hikes into their designated stocking spot would take them between approximately a half-mile to two miles to the farthest upstream point at each creek from the trailheads.

After arriving at their designated stocking point, the bags of fish were placed in the creek for approximately 10 minutes. This allowed the water inside the bags to cool down to a temperature close to that of the creek water, minimizing stress on the fish. After this initial cooling period, a small amount of creek water was added to the bags to help further acclimate the fish to the creek. After a few more minutes, the bags were then opened, and the fish gently released into the water. After all the fish had been released from the bag, each person waited a few extra minutes to ensure the fish were adjusting well to their new surroundings.

Monitoring movement

At the lower end of each creek, stationary antennas were placed to help track movement of the newly stocked YY brook trout. As the PIT tagged fish swims by the antennas, the date and time of passage, along with each fish’s unique tag number, are detected by the antenna receiver, recorded and stored. Each week, information from the antennas is downloaded until they are removed for the season, before the water freezes and winter conditions make the site inaccessible.

In 2024, data collected at Bobtail Creek after the release of the YY brook trout indicated that approximately 27% (around 64) of the 240 fish stocked left the creek, moving downstream outside of the restoration area . In Steelman Creek, the data showed that approximately 39% (around 93) of the 240 fish also left the creek, moving downstream outside of the restoration area.

“This type of movement was to be expected,” said Wright. “Despite the initial wave of fish leaving the creeks, we still observed a significant number of fish remaining in each creek.” 

Using this information, CPW aquatic biologists adjusted the stocking strategy for this year by focusing on a more “top-loaded” approach. Meaning, fish stocking was generally more concentrated in the upper reaches of both streams to minimize the number of fish moving out of the creeks.

Although early in data collection for the second year of the project, staff are generally very pleased with the movement data from the stocking effort in 2025 and all early indications show survival was high through fish transport and release.

What’s next for YY fish in Colorado

Colorado Parks and Wildlife will keep adding YY brook trout to both streams over the next few years. This will help ensure that there are more YY males in the population. Over time, as more and more YY brook trout occupy the creeks, monitoring will shift to tracking the proportion of male brook trout in each creek. As that proportion increases and females become increasingly rare, the likelihood of spawning also decreases. This will eventually lead to the complete elimination of brook trout from the upper Williams Fork Drainage, securing the future of native Colorado River cutthroat trout in these creeks. 

Though CPW is not the only state conservation agency currently using Trojan male fish technology, in 2020, our Aquatic Research section became the first conservation team in the nation to create YY brown trout.  The efforts at  Bobtail and Steelman creeks mark the first instance of stocking YY brook trout in Colorado. CPW researchers and biologists are encouraged by these positive results observed so far, which reinforce the growing body of evidence supporting the use of YY fish. Currently, CPW aquatic researchers and other scientists are investigating additional methods to utilize YY technology to help eliminate other non-native species, such as common carp, in areas where they have become problematic.

Rachael Gonzales, Northwest Region Public Information Officer