Procambarus clarkii, or red swamp crayfish, Louisiana red crayfish, crawfish, or crawdads. Photo by Sonja Nordahl

When Sonja Nordahl was walking her dogs in early August, she encountered a little gang of red crustaceans scuttling across the Canal Shores Golf Course in north Evanston. The creatures, resembling fun-sized lobsters, had apparently emerged from the North Shore Channel after a heavy rain. They marched across the street and onto neighborhood lawns, where the feisty things reared up and spread their claws if the dogs got too boisterous. When Ms. Nordahl plucked one out of the yard to show her neighbors, she says they “freaked out” at the sight of their surprising visitor: a crayfish. 

“We’ve lived near the canal for 10 years and this is the first year we’ve seen crayfish on the course and in our yard,” explains Ms. Nordahl. In addition to a handful of live ones, she says they saw a lot of shells that may have been the leftovers of crayfish devoured by coyotes living around the golf course. 

It turns out that of 375 crayfish species in the United States, 23 are native to Illinois, says Justin Vick, an aquatic biologist with the Metropolitan Water Reclamation District. Judging from photos that Ms. Nordahl snapped, Mr. Vick concluded they were likely Procambarus clarkii, or red swamp crayfish. Also known as Louisiana red crayfish, crawfish, or crawdads, they often wind up in a pot of boiling seasoned water and served with corn cobs as part of a cajun “boil.”

Procambarus clarkii are native to the Gulf Coastal Plain, from the Florida panhandle all the way over to Mexico,” explains Mr. Vick, with their territory continuing up the Mississippi River to St. Louis and into southern Illinois. Since at least 2000, P. clarkii have been found south of Chicago in the Calumet River and migrated north via Lake Michigan and the Chicago River.

Human activity is the most likely agent, or “vector,” that brought the non-indigenous red crayfish here, explains Mr. Vick.  They can be shipped live from Louisiana for boils, and fishermen often use live crayfish as bait. Pet shops have sold them as “freshwater lobsters.”  Any of those vectors, he says, can result in an accidental or intentional release of crayfish into local waters. The U.S. Fish and Wildlife Service (FWS) cites classroom and laboratory use as another vector, stating that crayfish are known to survive even after being flushed down the toilet.

Several states have banned or limited the import of P. clarkii, and the species has upset the ecosystems in several African countries including Kenya and South Africa.  In the Chicago area, red swamp crayfish are considered moderately invasive, meaning they have the potential to alter the existing food chain or “web” by consuming native plants and algae, preying on native species, and competing with native species for food or space, according to the FWS. Despite the risks, Mr. Vick knows of no eradication efforts and points out that crayfish have a number of native predators including herons, raccoons, possum, game fish, and turtles. 

“Since these guys are doing well, they are providing food for the things that eat them, so they are kind of in check,” explains Mr. Vick, noting that the crayfish population is not “exploding.”

Crayfish construct burrows along river banks, large enough for two adults and a number of newly hatched young.  The Evanston crayfish were doing just that when a group of City staffers spotted them during a canal canoe outing led by Matt Poole, program coordinator for the Evanston Ecology Center.

“We saw at least three of these large red Louisiana crayfish digging their burrows in the bank right by the shore,” says Mr. Poole.  The group moved in for a closer look and even had them grip a paddle.  Mr. Poole says they have not seen any crayfish in the water near the Ecology Center, less than half a mile west.

Crayfish eat plants, insects, fish, snails, other crayfish, and detritus (decaying organisms and fecal matter).  They typically mate in autumn, with females producing 100 to 500 eggs, and can survive two to five years in the wild.  Breeding males go through a “wandering phase” and have been known to travel as far as 10 miles in four days — “like a walkabout,” says Mr. Vick.  Though they normally range from 3 to 6 inches long, Evanston resident Rachel Sparrow said her husband saw one on the golf course that he described as “ginormous” — holding her hands at least 10 inches apart to demonstrate.

For seafood lovers, the real question is whether crayfish or any fish pulled from the Chicago River and North Shore Channel are safe to eat.  The Illinois Department of Public Health keeps an “Illinois Fish Advisory” web page listing which local fish contain elevated levels of chlordane, dioxin, methylmercury and polychlorinated biphenyls (PCBs), all of which are considered toxic environmental contaminants.  Although crayfish have no current advisories, Mr. Vick whose job it is to monitor water quality might take a pass.

“They’re kind of mucking around on the bottom there, where a lot of legacy contaminants may be,” says Mr. Vick.  He adds that prior to the Clean Water Act people could put whatever they wanted into the water, the mentality being “the solution to pollution is dilution.”  It’s something for local chefs to consider before heading to the canal with their crayfish traps and cajun seasoning.

Meg Evans has written science stories for the Evanston RoundTable since 2015, covering topics ranging from local crayfish, coyotes and cicadas to gravitational waves, medical cannabis, invasive garden...