Rusty crayfish, an 'aquatic nuisance,' discovered in northern Colorado river

Lauren Penington, The Denver Post on May 6, 2025

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DENVER — Rusty crayfish, an aquatic nuisance and an invasive species, has been discovered in a Larimer County river south of Estes Park, according to Colorado Parks and Wildlife.

This is the first time the crayfish has been confirmed in the agency’s northeast region and the second time it’s been found east of the Continental Divide in Colorado, state officials said in a news release.

Wildlife officials said the crayfish was reported April 7 in Larimer County’s West Fork Little Thompson River by a user on iNaturalist, a species identification app.

A team from Colorado Parks and Wildlife was able to confirm the aquatic nuisance’s presence with genetic testing later that month, according to the agency’s news release.

The rusty crayfish is native to the Ohio River basin and is one of Colorado wildlife managers’ top invasive concerns in the state. It is larger and more aggressive than Colorado’s native crayfish species, often outcompeting them for food and resources, state wildlife officials said.

The crayfish has been confirmed in five Colorado waterways since 2009, including Lake Granby in August 2023, according to the release. Wildlife managers believe they were introduced after anglers used them as bait.

State officials said the rusty crayfish is classified as a prohibited aquatic nuisance species “due to their voracious appetite on critical habitat vegetation and prolific reproduction habits.”

“Rusty crayfish are much more aggressive than our native crayfish species and are not an ideal food source for our sportfish,” said Robert Walters, CPW’s Invasive Species Program Manager, in the release. “They also eat aquatic vegetation, which native fish rely on for spawning, cover and food.”

The rusty crayfish has dark black rings near the tips of its claws and eponymous rusty patches on either side of its abdomen. Males can grow up to 5 inches long.

Colorado wildlife crews are working to conduct additional surveys of the Little Thompson River to determine the size and extent of the invasive population.

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