On the morning of September 17, I met up with CPW Aquatic experts Jon Ewert, Kevin Rogers, Boyd Wright, and George Schisler for a fish stocking and restoration project of historic proportions for Colorado, at the headwaters of the Williams Fork River. While all of the fish restoration projects CPW aquatic biologists across Colorado do are important, this was a first-of-its-kind for CPW Aquatics.
Making the way up the windy dirt road of Jones Pass, I couldn’t help but notice the freshly fall-kissed foliage on the alpine tundra surrounding us. When we arrived at the top of the pass we stopped for a few minutes, taking in the iconic 360-degree views from 12,400 feet in elevation, where the Continental Divide splits Colorado’s western and eastern slopes. From this vantage point, we could see down into the Upper Williams Fork drainage in front of us. Jon pointed out roughly where the two creeks we’d be stocking were and approximately where they’d intersect with the Williams Fork River, eventually connecting with the Colorado River west of Hot Sulphur Springs.
During the hour-long trip up and over the pass to our meeting point on the west side, Jon shared the background of this project to help me understand why we were here today. To do this, we have to pause and go back to 2010, when Ewert noticed an alarming trend in the number of brook trout compared to cutthroat trout in the creeks. Understanding this helps to fully understand this moment and why it is a historic first for Colorado Aquatics and the restoration of native cutthroat trout.
Before we get to the history, it is important to note that this population of cutthroat trout found within Bobtail and Steelman creeks are some of the highest-valued native cutthroat populations in the headwaters of the Colorado River basin. Considered a species of special concern in Colorado, this subspecies of trout is genetically pure and naturally reproducing. The combination of CPW historic stocking records and modern genetic studies suggest that these populations have never been manipulated and are the true native cutthroat that originally inhabited these streams, which is a rarity in this area.
Time to sound the alarm
In 2010, CPW noticed an alarming number of non-native brook trout after completing a fish survey in the headwaters of the Williams Fork drainage. While it is unknown when brook trout invaded these creeks, it was clear that over time the thriving brook trout had nearly wiped out the native cutthroat population. While brook trout are an excellent sport fish in some areas, they can easily outcompete native trout as seen here.
After completing a survey in Bobtail Creek in 2010 Ewert found 100% brook trout at one survey site, over 80% in another, and less than 40 cutthroat trout overall in the creek. He and his colleagues knew that if they wanted any hope to preserve the cutthroat in this drainage, they would have to act fast.
The following year, CPW began removing brook trout from both creeks to restore the native cutthroat population. In a two-step process, aquatic biologists would collect and count the number of cutthroat trout through a common fish survey process called electrofishing. The trout would be measured and weighed before being released into the water. Any brook trout captured during the electrofishing process would be removed. Knowing they wouldn’t be able to capture and remove all of the fish, data collected from these surveys was used to generate population estimates. These estimates allow aquatic biologists working on this project to gauge the success rate of the removal effort.
For the next six years, with an access window of roughly six weeks each year and weather permitting, Jon, Kevin, and a team of aquatic experts from around the agency and the U.S. Forest Service would continue surveying the creeks and removing brook trout. In just a few short years the biologists would go from capturing less than 40 cutthroat in 2011 to 296 cutthroat in 2017 in Bobtail Creek, with similar results in Steelman Creek. Much like the increase in cutthroat, aquatic biologists saw a decline in the number of brook trout removed, meaning fewer brook trout were reproducing in the creeks. Although moving in the right direction, Jon knew this would be an ongoing effort that would never completely eradicate brook trout.
In 2020, During an already active wildfire year, Colorado would see the start of yet another fire on Aug. 14. Actively burning within the drainage, the Williams Fork fire would burn for nearly two and a half months, threatening the nearby towns of Fraser and Winter Park and keeping CPW Aquatic Biologists on edge. Knowing the importance of preserving the cutthroat trout in both creeks and the potential for the fire to move into either creek’s drainage, CPW, and the U.S. Forest Service created a plan to rescue the few remaining genetically pure native cutthroat trout.
The plan, depending on the direction the fire shifted, was to bring a group of people in by helicopter to remove as many cutthroat trout from the creek as possible and take them to a location where they would be kept until the fire was contained. Although the fire would ultimately burn over 14,500 acres, it never made the shift, and no rescue attempt was needed. However, because of the fire, CPW was unable to complete the survey and brook trout removal for the year.
While CPW aquatic biologists were closely monitoring the Williams Fork Fire, aquatic researchers at the Aquatic Research Hatchery in Fort Collins were busy making a breakthrough in Trojan Male brown trout research, after seeing success with similar research done in Idaho. This research would ultimately set the stage for critical conservation efforts in aquatic ecosystems not only in Colorado but also in other western states.
Since 2021, CPW aquatic biologists have continued to see positive results each year in Bobtail and Steelman creeks, catching ever higher numbers of cutthroat trout and removing fewer and fewer brook trout. Despite breaking a record in 2022, capturing over 400 cutthroat trout in Bobtail Creek, over a 900% increase in the number from when the project began in 2011, biologists knew just doing removals would not get them to the ultimate goal of removing all brook trout from the creeks.
In late 2023, over in Fort Collins, the CPW aquatic research team began spawning efforts of their broodstock of Trojan male or YY brook trout, the second in the nation.
“This was not the first time our research team worked to spawn a YY broodstock,” said George Schisler, CPW Aquatics Research Section Chief. “In 2020, our research team successfully created YY brown trout, a first in the nation. With this knowledge and lessons learned from our partners in Idaho who successfully created the first broodstock of YY brook trout, we were able to successfully spawn YY brook trout of our own here in Colorado.”
So, what is a Trojan Male or YY brook trout? The simplest way to describe them is a male brook trout with two Y chromosomes. Now, we all know science isn’t that simple, so let’s dive into the details of “Trojan Male” or “YY” fish technology a little more.
Like mammals, most fish species have two sex chromosomes, an X and Y. A female will have two X chromosomes (XX), and a male will have one X and one Y chromosome (XY). In the development of YY brook trout, very young brook trout are exposed to the female hormone estradiol in their feed at a specific level and for a specific length of time. The treatment makes the male fish become egg-producers, but they are still genetically male. These egg-producing males are then bred with normal XY males, which means 25% of the offspring will have two Y chromosomes, which is why they are referred to as YY males.
To create a broodstock of fish, some YY males produced in that generation are exposed to estradiol, which converts them into egg-producing YY males. This process allows the hatchery to have a group of egg-producing YY males and a group of sperm-producing YY males. After that, hormone treatment is unnecessary because only Y chromosomes are present in the eggs and sperm of the brood stock, and all of the offspring will be YY males.
These trout are stocked into wild brook trout populations and reproduce with the wild fish, producing only male offspring. Without a reproducing population (male and female fish), the brook trout will eventually die out, allowing for native cutthroat trout to be restored.
History in the making: the first introduction of YY Brook Trout in Colorado
Leading up to that day, just a week before, Ewert, Wright, and Rogers led a team of 20 biologists and volunteers made the trek up and over the pass as they had for the past 13 years to count the number of cutthroats and remove brook trout. In addition to the capture and removal, disease and genetic sampling were conducted to ensure the success of the upcoming stocking effort. The results were once again record-breaking, with over 1,400 cutthroat and less than 300 brook trout captured between Bobtail and Steelman creeks.
“It felt pretty good to see the results from the September 9th and 10th surveys. Essentially swapping the number of fish we were capturing and removing from when the project first started was the result of hard work and dedication by many biologists over the last 13 years,” said Ewert.
Two weeks before this project, Wright implanted all 480 fish to be stocked with electronic tags, making it possible to monitor and track each individual fish. With storm clouds building to the west, the rush was on to divide, scan, and ready the 480 YY fish for the trip to their new home. For this fish plant, CPW staff and Denver Water volunteers would be hiking the fish in large backpacks, transporting the fish from the holding tank on the hatchery truck to their new home in one of the two creeks. To help speed up the process, staff and volunteers split up into two teams. One group would be focused on scanning fish going into Bobtail Creek, while the other would focus on fish going into Steelman Creek.
“It’s a little unnerving when you are only relying on technology to track your data. When working with fish that have been tagged, we scan and write down the information as a failsafe. Knowing we’d only be scanning, we decided to create a backup by scanning each fish twice, just in case,” said Ewert.
After scanning each fish twice, they were then placed into stocking bags – tall, narrow, clear plastic bags filled halfway with water and a handful of ice to ensure the water didn’t warm up too much and the fish did not go into shock during the trip. Once the bags were filled with water and fish, the remainder of the bag was filled with oxygen before sealing them up. For this stocking event, the fish were relatively large, nearing about six inches on average. Because of the size of the fish, each person would be carrying two bags holding 16 brook trout each to specific points along each creek. Experience with stocking fish into small streams such as these shows very little movement, so to ensure even distribution, the fish were stocked into several predetermined spots along the creeks.
As a steady rain settled into the area the two teams set out to make history, one heading to Steelman Creek and the other to Bobtail Creek.
After an initial creek crossing, the group made its way up a fairly well-established trail along Bobtail Creek. Those making the trek up Steelman Creek would have to navigate through dense willows following less established game trails made by local wildlife. About a half-mile into the Bobtail Creek hike members of the group began splitting off, heading to their stocking point on the map. For some this was a quick hike just off the trail, for others, it required a little bushwhacking through some dense willows keeping a close eye for moose.
At the spots, the bags of fish would be placed in the creek for a short period. This would allow the water in the bag to cool down close enough to the water in the creek to prevent any further stress on the fish. After about five to eight minutes the bags would be opened and fish would slowly be released into the water. Each member of the group would wait a few minutes to make sure the fish were going to be ok. Once this was done, it was off to the next site.
The final stop on the creek was about 2.5 miles from the trailhead, just below a waterfall. The pools at the bottom of the waterfall are the perfect fish habitat and a place where biologists often find high numbers of brook trout in the creek. With all the fish successfully in the water it was time to head back down the trail in hopes of beating the next round of rain we could see slowly making its way toward us.
Back to the truck, just in time for another round of heavy rain, Jon, Kevin, Boyd, and George stop to reflect on years of hard work and celebrate the historic moment that just happened. Knowing this is not a one-time effort utilizing this new tool in our fish restoration toolbox, biologists across the agency are hopeful and excited to see the results as we continue restoring cutthroat trout populations. With everyone accounted for, the group took one last opportunity to celebrate before heading back up Jones Pass for the last time this year.
In addition to introducing YY brook trout, Denver Water is working to improve barriers on the creeks to prevent the spread of non-native fish species in this river system.
Colorado Parks and Wildlife will continue to stock both Bobtail and Steelman creeks with YY brook trout over the next several years to increase the male-only population, eliminating the potential for reproducing male and female brook trout in the creeks. This new method, in combination with the continuation of yearly removals, is one CPW aquatic biologists believe will be successful here and will be used in other cutthroat restoration projects across the state.
Written by Rachael Gonzales, Northwest Region public information officer for Colorado Parks and Wildlife.
