a rusty crayfish held by its tail
Colorado Parks and Wildlife discovered rusty crayfish in Lake Granby during routine aquatic nuisance species sampling on Aug. 17, 2023. Biologists are worried because the species is larger and more aggressive than native crayfish. The rusty crayfish, found in the Yampa River and Catamount Reservoir in 2009, are native to the Ohio River basin. They are believed to have been illegally introduced to Colorado by anglers as bait. (Colorado Parks and Wildlife photo)

With oversized claws unholstered, and sporting a mean appetite, the invasive rusty crayfish has arrived at a new Western shootout announcing there ain’t enough room in this reservoir for anyone else. 

State wildlife officials last week announced the startling discovery of rusty crayfish — Faxonius rusticus for those keeping score in Roman numerals — in Lake Granby, one of the primary headwaters reservoirs feeding the entire Colorado River system. 

The aggressive, insatiable invasive species from the Ohio River Valley has been found in Colorado before. But never on the big river. 

Biologists now fear the critters will munch their way through the eggs, minnows, algae, water plants and insects, from Grand Lake to the Gulf of Mexico. And there’s not a lot they can do about it. 

“We have never been successful in eradicating rusty crayfish from any of the lakes or reservoirs in the state locations where they have been previously detected,” said Robert Walters, Colorado Parks and Wildlife’s invasive species program manager. 

People examining crayfish
ANS Sampling and Monitoring team members separating and identifying crayfish from Lake Granby during trapping efforts the week of Sept. 11. (Colorado Parks and Wildlife photo)

And unlike the other invasive crayfish species that made their way to Colorado in the past, rusty crayfish are too feisty to serve as meal for trout or the other usual suspect crayfish predators. 

“Once they get in, nothing eats them,” Walters said. Except humans, apparently, who find them among the tastiest of the armored species. But if you capture some for a home boil, don’t drop them anywhere else along the way. State regs don’t say much about cooking, but live transport is absolutely illegal, so you have to pull the heads off the body before leaving the lake shore. 

A wildlife officer first found the Lake Granby infestation of rusty crayfish while walking the shoreline on a routine patrol in early September. State biologists then set traps on the lake, and about 10% of the crayfish that turned up were the dreaded rusty version. 

Rusty crayfish protect their eggs in a sticky mass underneath their tails, spawning hundreds a year. (University of Wisconsin Stevens Point)

Previously, the wildlife division had confirmed rusty crayfish in Catamount Reservoir, in the Yampa River and Stagecoach Reservoir, all near Steamboat Springs; and Sanchez Reservoir in the San Luis Valley. But it’s been 10 years since the last new detection, and a jump into the Colorado River headwaters area opens up thousands of miles of new worries. 

With rusty crayfish not being known for self-propelled intermountain treks, Walters figures they arrived at Lake Granby as bait in a thoughtless fisherman’s tool kit. Other crayfish may have arrived in Colorado waters as bait or clinging to boating equipment, or in bilge water. When wildlife officers check boats before put-in at Colorado reservoirs, they look for crayfish along with zebra mussels and other conquering klingons. 

A dark red mark like a blotch of spray paint reveals the rusty crayfish. (University of Wisconsin Stevens Point)

The survival-prepped rusty crayfish moms hold hundreds of eggs underneath their tails in a sticky bundle and can spawn thousands of voracious copycats in a couple of seasons. Some fish will eat the youngest, tenderest crayfish, but once they become fight-ready adults, only concentrated trapping can cut into their numbers, and even then only with an all-claws-on-deck capture effort. 

Wisconsin says it is researching a parasite that could potentially become a predator against rusty and other crayfish, one invasive species against another. What could possibly go wrong?

For all the angst about the rusty crayfish making its way north into Grand Lake, or south over the Granby dam into the river, what Walters doesn’t do is blame the opportunistic crustaceans. 

“It’s not their fault,” Walters said. “They’re just following evolution.”

Michael Booth is The Sun’s environment writer, and co-author of The Sun’s weekly climate and health newsletter The Temperature. He and John Ingold host the weekly Sun-Up podcast on The Temperature topics every Thursday. He is co-author with...