Colorado Parks and Wildlife (CPW) has confirmed the presence of rusty crayfish (Faxonius rusticus) — an aggressive and invasive aquatic species — in the West Fork Little Thompson River, located in Larimer County, just south of Estes Park, Colorado.
First Rusty Crayfish Sighting in Northeast Colorado Region
This marks the first confirmed detection of rusty crayfish in northeast Colorado and only the second time the species has been found east of the Continental Divide, according to CPW officials. The sighting was initially reported on April 7, 2025, by a user on iNaturalist, a popular species identification app. CPW later verified the report through genetic testing.
Why Rusty Crayfish Are a Major Aquatic Threat
Rusty crayfish are native to the Ohio River Basin, but have become a top aquatic invasive species of concern in Colorado. They are known for their aggressive behavior, larger size, and competitive dominance over native crayfish, often disrupting freshwater ecosystems.
State officials emphasized that rusty crayfish:
- Have a voracious appetite, feeding heavily on aquatic vegetation.
- Compete with native species for food and habitat.
- Are poor prey for sportfish, which disrupts the local food chain.
- Reproduce quickly, making control and eradication difficult.
“Rusty crayfish are much more aggressive than our native crayfish and are not an ideal food source for our sportfish,” said Robert Walters, CPW’s Invasive Species Program Manager. “They also eat aquatic vegetation, which is critical for fish spawning, cover, and food.”
Identification of Rusty Crayfish
Rusty crayfish can be identified by:
- Dark black rings near the tips of their claws.
- Distinctive rust-colored patches on both sides of their abdomen.
- Males can reach sizes of up to 5 inches long.
Previous Sightings and Spread
Since 2009, rusty crayfish have been detected in five Colorado waterways, including a 2023 discovery in Lake Granby. Wildlife officials suspect the species was introduced via live bait used by anglers.
What’s Next?
Colorado wildlife teams will conduct additional surveys in the Little Thompson River to assess the size and extent of the rusty crayfish population. CPW urges the public to report any suspected sightings of aquatic invasive species through platforms like iNaturalist or directly to the agency.
Protecting Colorado Waters from Aquatic Invasive Species
The rusty crayfish is officially listed as a prohibited aquatic nuisance species in Colorado. To help protect Colorado’s rivers, lakes, and native biodiversity, anglers and outdoor enthusiasts are reminded to:
- Never release live bait into the water.
- Clean, drain, and dry boats and equipment after use.
- Report sightings of suspected invasive species.