After spending decades hiding away in isolated habitats, the recently discovered San Juan cutthroat trout are on their way to being introduced in the headwaters of Wolf Creek in the San Juan National Forest.
But first, other non-native fish have to go.
Multiple research studies show that San Juan cutthroat trout die out in the presence of non-native trout, which compete, prey on and hybridize with the cutthroat trout, according to Colorado Parks and Wildlife.
The state agency will start an eradication effort Monday on Wolf Creek, which will result in a weeklong closure of a 3-mile portion of the creek. It’s part of a long-term plan to restore the native San Juan cutthroat to its historic habitat.
“Restoring native species is a high priority for Colorado Parks and Wildlife, and Wolf Creek is an ideal location for pure San Juan cutthroat trout,” said Jim White, CPW aquatic biologist in Durango who has coordinated the project, in a news release.
Since the 1970s, CPW aquatic biologists, like White, have searched backcountry streams looking for isolated populations of cutthroats – Colorado’s native trout.
In the 1980s and 1990s, biologists found cutthroat populations suspected of having a unique genetic marker in Southwest Colorado.
In the 2010s, scientists compared the genetic markers of the wild cutthroat to preserved fish specimens taken from the San Juan River near Pagosa Springs in 1874.
By 2018, they were able to confirm the discovery of San Juan cutthroat, previously thought to be extinct.
CPW has set a goal of conserving the state’s native trout species.
Colorado River cutthroat trout currently occupy about 11% of their historic habitat. They face threats such as disease, other non-native fish, habitat loss and over-harvest.
In 2018, a team captured just over 50 wild cutthroats to save them from possible ash and sediment runoff from the 416 Fire. By May 2020, the fish spawned a new lineage at the Durango Fish Hatchery.
It was enough for a limited amount of stocking in the wild.
CPW is trying to expand the populations of San Juan cutthroat into headwater streams that are pristine and free of whirling disease, which is caused by an invasive parasite.
The Wolf Creek restoration project will establish a population of the unique cutthroat from the creek’s headwaters down to a 200-foot waterfall downstream of Forest Road 39 (Fall Creek Road) off U.S. Highway 160.
First, the competing non-native fish have to be killed so the San Juan cutthroat can thrive in the creek.
The only fish population in the treatment area is a rainbow trout-cutthroat trout hybrid, White said.
CPW will use a piscicide, called rotenone, to remove the fish. Rotenone, an Environmental Protection Agency-registered organic chemical, affects gill-breathing animals and invertebrates.
The chemical comes from a tropical legume root and was used by Indigenous peoples to capture fish. CPW has used it for 80 years to remove fish in controlled areas, according to the news release.
At the end of the treatment area, potassium permanganate, commonly used in water treatment plants, will neutralize the rotenone over time. Some incidental fish mortality will occur below the Forest Road 725 waterfall, the news release said.
White expected impacts on other species to be minimal.
“There’s really not a significant natural predator, other than human beings. So we’re not too worried about that,” he said.
Invertebrate populations affected by the rotenone should bounce back quickly.
“In these high-elevation, cold-water streams, we don’t typically see an (ecosystem) imbalance,” White said.
If someone is fishing downstream of the treatment area, “it’s absolutely fine” to eat their catches, he said.
The Wolf Creek project will be done in three phases, according to the CPW news release. The first phase, completed in 2018, removed non-native trout from the south fork of Wolf Creek up toward Treasure Mountain.
The second phase, which starts Monday, will be conducted in the 3-mile section of the headwaters from the top of Wolf Creek Pass down to Forest Road 725 (Wolf Creek Road), according to CPW.
The 3-mile stretch of the creek will be closed to the public for the weeklong treatment. The public may access the creek below Forest Road 725 during this time. This closure has no impact on traffic on U.S. Highway 160.
The treated areas will be void of fish until summer 2022. After the spring runoff, CPW biologists will check the streams to ensure non-native trout have been eliminated. If none are found, San Juan cutthroats will be stocked next summer.