Colorado wildlife authorities to kill reservoir fish and restock with native trout in new restoration project

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A reservoir in Colorado is undergoing a restoration project that’ll bring native Rio Grande cutthroat trout back, but all other fish will be killed.

Colorado Parks and Wildlife (CPW) announced on Thursday, July 13, that the Rito Hondo Reservoir west of Creede, a statutory town in Mineral County, has been lowered for dam repair work and an eventual rotenone treatment that will "remove non-native brook trout."

Rotenone is a broad-spectrum insecticide and herbicide derived from jicama vine plants and many Fabaceae plants, which is also toxic to fish, according to the National Library of Medicine’s PubChem, a chemical glossary associated with the National Center for Biotechnology Information.

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The CPW plans to conduct the rotenone treatment beginning the week of July 24 and stock the Rito Hondo Reservoir with native Rio Grande cutthroats by summer 2024.

The Colorado wildlife agency is partnering with the U.S. Forest Service and the San Luis Valley Chapter of Trout Unlimited to carry out the restoration project.

Roads throughout the Divide Ranger District have been closed along with dam’s surrounding area, according to CPW.

The Rito Hondo Reservoir will remain fish-free through the winter and refilled "following spring runoff in 2024," the wildlife agency’s news release continued.

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"The reservoir being drained for repairs gives us the opportunity to reclaim the lake itself and the stream above it without having to treat the whole lake," Estevan Vigil, an aquatic biologist with the CPW, said in a statement. 

"The coolest thing about this project is that it’s popping up out of nowhere and providing another lake that can grow really good trout. We anticipate this being a great conservation success," he continued. "We hope to establish this as a broodstock lake to get more Rio Grande cutthroat eggs for stocking out across the San Luis Valley."

Colorado Parks and Wildlife Aquatic Biologist Estevan Vigil holds up a male Rio Grande cutthroat trout and shows off the trout's spawning colors. (Colorado Parks and Wildlife)

Vigil said it could take three to five years for cutthroat trout to grow to a "large, catchable" size.

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Removal of other fish is necessary because non-native trout compete, prey on and sometimes hybridize with Rio Grande cutthroat trout, according to the CPW.

The CPW says rotenone is a "registered piscicide" with the U.S. Environmental Protection Agency, it and has historically "been used by indigenous peoples to capture fish." The chemical has been "successfully used" by the CPW and other state fish and game agencies for more than 80 years, according to the CPW’s news release.

Rotenone is used around the world for fish management and reportedly "only affects gill breathing animals and invertebrates when used properly," according to the CPW.

The Colorado Parks and Wildlife is restoring the Rito Hondo Reservoir with Rio Grande cutthroat broodstock. (Colorado Parks and Wildlife)

The CPW noted that it’s also working on other fish restoration projects throughout the state.

Rio Grand cutthroat trout are one of three native trout species indigenous to Colorado. The other two species include Colorado River cutthroat trout, which are native to Colorado’s Western Slope and Greenback cutthroat, which are native to parts of Colorado’s Front Range – a mountain range that’s part of the Southern Rocky Mountains.

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Colorado residents and visitors who want to stay up to date on the Rito Hondo Reservoir and Rio Grande National Forest closures can visit the U.S. Department of Agriculture’s Forest Service website at fs.usda.gov.