Invasive Crustacean Discovered in Northern Colorado River

When I was a kid, you could always find me at the creek near my house in the summer months. I was constantly on the hunt for crayfish. I even got so into it that I concocted traps to catch them.

Of course, I always let them go, but I wanted to see how big a crayfish I could catch. There was one point in time that I believe I caught at least two dozen in one short afternoon. From the mountains to the waters of Colorado, the wildlife is fascinating.

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Colorado Parks and Wildlife and many advocates try very hard to protect our state's animals. Try as they might, invasive species still seem to find a way into Colorado.

The Aggressive Rust Crayfish Found in the Big Thompson River

Recently, Colorado Parks and Wildlife has confirmed the presence of an invasive species known as the rusty crayfish. This crustacean is labeled as a nuisance in Colorado and threatens the native crayfish species.

READ MORE: How High Elevation Lakes are Stocked with Fish in Colorado

Rusty crayfish originate from the Ohio River Basin, have dark black rings near the tips of their claws, rusty patches on their sides, and are more aggressive than Colorado's crayfish. The rusty crayfish is also larger, has a much larger appetite, and can easily outcompete Colorado's crayfish when it comes to food and other resources they need to survive.

The discovery of the invasive species happened on April 7, 2025, and was confirmed by Colorado Parks and Wildlife by genetics on April 18 in the West Fork Little Thompson River, located in Larimer County, Colorado.

The discovery of the rusty crayfish is only the second time the species has been found in Colorado and the first instance of being recorded on the east side of Colorado's Continental Divide.

READ MORE: Creepiest Water Creatures You May Encounter in Colorado

Read more on how you can stop the spread of invasive species in the state of Colorado at cpw.state.co.us.