Colorado began removing non-native brook trout from two mountain creeks in 2011; 13 years later, rare native cutthroat trout had increased more than 11-fold

Somewhere below the Continental Divide, in a stretch of Grand County, Colorado, most hikers won't get close to, a fish barely anyone has heard of just made a notable conservation victory in the American West. In 2011, biologists counted 123 green lineage cutthroat trout in two small streams, Bobtail and Steelman Creek. By 2024, that number had grown to 1,425, more than an 11-fold increase over 13 years.

According to Denver Water, the utility that draws up drinking water for 1.5 million people from this same watershed, the turnaround came from one not very glamorous, decade-long intervention: physically removing an invasive fish one stream at a time. The reversal is real, but it covers only two creeks; researchers continue to warn that invasive trout and warming streams are closing in on hundreds of other isolated cutthroat populations across the region. It’s a story about what decade-long, multi-agency persistence looks like.

The fish that was about to blink out of existence


Green lineages cutthroat trout are a subspecies of Colorado River cutthroat trout, and the ones in Bobtail, McQueary and Steelman Creek are among the most genetically pure native fish in the Colorado River headwaters. Their decline tracks almost with Colorado's settlement history, according to biologists. Mining, logging, and water diversion in the 1860s began chipping away at their habitat, but the bigger threat came later, when brook trout were introduced and began outcompeting the natives for the same spawning grounds.

An invasive species without a villain origin story

Brook trout aren't native to Colorado. They were brought west from the eastern US and Canada in the late 1800s and early 1900s, mostly as a food source and sport fish for new settlers. The biological timing is problematic: brook trout spawn in fall, so their young are already established when cutthroat fry hatch in spring. The bigger, earlier brook trout outcompete, and sometimes eat, the native fish. Left alone, Colorado Parks and Wildlife biologists have said, the outcome in brook-trout streams is often only a matter of time.

The larger pattern isn't limited to these two creeks. A 2017 peer-reviewed study titled “Nonnative trout invasions combined with climate change threaten persistence of isolated cutthroat trout populations in the southern Rocky Mountains,” published in the North American Journal of Fisheries Management, found that nonnative trout invasions are already squeezing isolated Colorado River cutthroat populations across the southern Rockies and are expected to intensify in the coming decades due to climate change. In other words, Bobtail and Steelman Creek aren't an example of the trend reversing. They're a rare outlier showing it has.

Enter the "Trojan trout"

Since 2011, Colorado Parks and Wildlife and the US Forest Service have used electrofishing: stunning fish with a mild electric current so brook trout can be pulled out while cutthroat are counted and released, conducting this removal in most years since the project began. Denver Water has supported that effort with infrastructure: new culverts and modified dam channels engineered specifically to stop brook trout from swimming upstream during spring runoff.

The less common tool is genetic. In 2024, biologists released 240 "Trojan trout," lab-bred male brook trout that are genetically unable to produce female offspring, into each creek. The technique, formally called YY-male stocking, was pioneered in Idaho. According to a 2022 Idaho Department of Fish and Game research paper titled “The use of MYY fish to eradicate non-native Brook Trout populations in Idaho,” by Jennifer Vincent and colleagues, it is still experimental. Complete eradication of a wild brook trout population using this approach has not been documented, which is why Colorado is combining it with continued electrofishing rather than relying on it alone.

What the numbers show and what they don't

Cutthroat trout now outnumber brook trout roughly 5 to 1 in these two creeks, a genuine reversal. But it is worth being precise about its scope: this result applies to two creeks within a single watershed. Elsewhere, researchers continue to warn that invasive trout and a warming climate are still closing in on hundreds of other isolated cutthroat populations. What the Bobtail and Steelman Creek numbers do show is that this kind of recovery is slow, procedural, and dependent on sustained coordination between multiple agencies over more than a decade, not a one-time fix.