GRAND LAKE, Colo. — A section of the western side of Rocky Mountain National Park will close for a week to remove all non-native fish from the Grand Ditch and its tributaries within the park.
This will allow for the reintroduction of greenback cutthroat trout — Colorado's state fish — to their native range in the headwaters of the Cache la Poudre River, the National Park Service (NPS) said. Non-native trout, including brook trout, are currently in those waters.
"They pose threats to native greenback cutthroat trout if not removed as they could move into the greenback recovery area," NPS said.
This is part of the Poudre Headwaters Project. It is a keystone undertaking for the Rocky Mountain Flycasters Chapter of Trout Unlimited and will be led by biologists with the U.S. Forest Service.
The project will run the final week of August.
To remove the fish that are not native to that ecosystem, aquatic biologists will apply a piscicide — a substance only poisonous to fish — called rotenone to the Grand Ditch system, which runs about 14.3 miles, as well as its tributaries upstream and downstream.
The Colorado River may appear reddish or brown due to this treatment, the NPS said.
So, what exactly is rotenone?
The comment sections of RMNP's social media posts about the project prompted many questions about the compound and its impacts.
The NPS responded to several of the inquiries, explaining that rotenone is an EPA-approved pesticide used at the state level and federal level to remove non-native fish and restore native populations, and naturally breaks down within a few days of application.
It was recently used by CPW aquatic biologists on South Mesa Creek in Montrose County last year.
Rotenone poses "minimal risk" to people and other wildlife, the NPS said.
"At the concentration that will be used during this project, rotenone will not harm birds or mammals," the NPS responded to one person online. "This product is a piscicide that is designed to specifically kill fish."
It also does not harm any birds or mammals that consume the fish, the NPS said. However, they warned people to not pick up, consume fish or drink water that was treated for rotenone, noting that these areas will have clear signage in RMNP during the project.
In regard to a question about impacts to amphibians, the NPS said the treatments target cold-flowing water that do not typically support amphibians. By the time this project begins later this month, most amphibians will have completed metamorphosis to their terrestrial life stages, NPS said.
The NPS said using rotenone is the most effective way to remove the fish due to the steep and remote locations in the Cache la Poudre headwaters. Other alternative methods are not feasible.
It is a naturally occurring chemical found in the roots of some tropical plants.
This project is scheduled for Aug. 25-31 and will require some areas on the west side of the park to close hiking and fishing access, including the Grand Ditch area and the full Colorado River Trail to La Poudre Pass.
The following trails and areas will remain open:
- East Inlet Trail
- North Inlet/Tonohutu Trail
- Green Mountain Trail
- Coyote Valley Trail
- Onahu Trail
- Timber Lake Trail
- Holzwarth Historic Site
- Harbison Meadows Picnic Area
- Fishing access along the Colorado River south of the Colorado River Trailhead (including near the Holzwarth Historic Site and Coyote Valley)
Other partners in this project include Colorado Parks and Wildlife and U.S. Fish and Wildlife Service.
The Poudre Headwaters Project spans about 40 miles of connected waterways. Once complete, it will be the largest interconnected restoration area in the state, according to Rocky Mountain Flycasters Chapter of Trout Unlimited.
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