John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
When the tasty critters invade, they can undermine the integrity of dams and levees.
Wildlife authorities in Michigan received the two reports just days apart in July, one from a lake near Vicksburg and another 100 miles away in a retention pond outside of Detroit. Both tips concerned the same critters—red swamp crawfish, or Procambarus clarkii, pint-sized crustaceans with a bloodletting pinch native to Southern states like Louisiana.
When state workers visited the Detroit-adjacent pond in Novi, they found the land around the water shot through with what looked like cannon fire. “You wouldn’t be able to walk through without probably sinking your shoes in,” says Michelle Crook, a senior project engineer for Michigan’s Department of Natural Resources. “On better than half of the pond, I’d say there’s probably a two-foot width of the shoreline where there are chunks with holes and areas falling in.”
The shocking state of erosion was due to the crawfish, which dig burrows roughly four inches in diameter extending two to three feet into the soil. That’s not a good thing for this particular pond, as it catches storm runoff and is near two major roads, a mall, and a heavily used parking lot—all things that wouldn’t fare well should the banks collapse and release a wave of floodwater. It’s also not a good thing for Michigan as a whole, as the state has 2,000-plus dams in rural and urban regions. Many of them are earthen or include earthen portions, which would be similarly vulnerable to crawfish digging. “They can start eroding pieces, and eventually the dam isn’t going to hold the water back,” says Crook.
“We know from their native environments, like for example where they build crayfish farms, they’ll build levees to keep them in,” says Nick Popoff, who oversees the DNR’s aquatic invasive-species program and prefers (may the South forgive him) to use crayfish rather than crawfish. “The burrows will be so thick they’ll blow the levees out. That’s concerning to us from an infrastructure perspective.”
If this sounds like Northern worrywartness, just ask a Southerner who also happens to be one of the nation’s few crawfish-focused researchers. “Unfortunately, this is true,” says W. Ray McClain, an aquaculture professor at Louisiana State University. “The Louisiana red swamp crawfish could pose a threat to sensitive ecosystems, certain native aquatic species, as well as aspects of farming and civil water-retention structures, just as there are potential threats with any nonnative invasive species.”
Where the Michigan mudbugs came from is anybody’s guess. Until a couple of years ago, anglers were buying them alive from local seafood markets to bait their lines for bass. (The state has since banned the possession of live red swamp crawfish.) They could’ve found their way into Northern ecosystems from people dumping aquariums, or teachers purchasing them from biological-supply companies for students who then released them into yards and streams. But where Michigan wants the invasive species to go is clear: straight to crawdad hell.
Like the xenomorphs from Prometheus, red swamp crawfish seem almost biologically engineered for domination and destruction. The U.S. Geological Survey considers them “both a shredder and a predator” with the “potential to act as a keystone species and dominate energy flow.” They are more aggressive than other crawfish species and out-reproduce them, with females producing up to 500 eggs as frequently as three times a year. They’re ravenous consumers of vegetation, insects, snails, baby fish, fish eggs, and other crawfish. There have been cases where crawfish ate all the aquatic plants in a lake, turning the water nasty and turbid. And when they’re not finding a spot to their satisfaction, they’ll simply seek out another one.
“All crayfish can live out of water [for a while], but these crayfish will actively walk across land,” says Popoff. “So they’ve been documented walking like three kilometers over land to find new habitats.”
The red swamp crawfish is considered the most invasive species of crawfish. Since escaping from its native Southern haunts, they’ve built thriving populations on every continent save for Australia and Antarctica. They recently landed in Berlin. They’re a bane to rice farmers in California because they eat the plants and drill holes through sensitive levee systems. They’ve decimated a local fishing economy in Kenya’s Lake Naivasha by eating all the underwater vegetation that shelters fish, eating fish caught in nets, and destroying those nets by getting tangled up in them.
In the 1960s, Spain tried breeding Louisianian crawfish as a culinary treat, but quickly regretted it after they skittered off into the wild. A delightful 1991 Associated Press report relates the aftermath:
“The Louisiana crawfish was imported here with the hope it would prove a gastronomic success. But the spiny creatures have played a nasty trick: Instead of nestling beside beds of rice on Spanish plates, they are chewing up the country’s rice fields. In the northeastern Ebro River Delta, the fast-breeding visitors have been declared crustaceans non grata. ‘I don’t know exactly where Louisiana is, but I wish the people there would come and take them back,’ said Juan Tiron, a rice farmer.... In winter, [one biologist] says, the crawfish burrow into fields and levees, draining water from the fields and canals. Undermined levees collapse under the weight of tractors.”
It’s understandable, then, that Michigan is taking the threat seriously. Already, wildlife managers have confirmed roughly half-a-dozen other locations in the Detroit area harboring P. clarkii. The state is asking citizens to report red swamp crawfish and laying traps to catch the pests. At Sunset Lake near Vicksburg, the devices are snaring only handfuls of crawdads, but there’s concern over a nearby dam and the species’ ability to explosively breed. At the Novi retention pond, it’s a totally different situation.
“It’s only a one-acre pond, but we’ve already pulled out over 2,600 crayfish,” says Popoff. “It sort of speaks to the density they can achieve.” And though it’s just a retention pond, it’s connected to other ponds through various structures that eventually lead to the River Rouge flowing into the Detroit River. (Detroit press officers did not respond to requests for comment.)
Michigan plans to slow the march of the crawfish through containment because, as with many invasive species, it’s incredibly hard to eradicate them. Crook, the DNR engineer, is thinking about tweaking dam inspections to specifically look for crawfish damage. She’s also exploring ways to design future infrastructure that is burrow-proof, perhaps by using tough membranes on retention ponds and whatnot, because retrofitting the existing infrastructure would require a huge amount of money.
Looming control efforts might include expanded trapping, oxygen-deprivation techniques, environmentally safe piscicides that kill crawfish—maybe Michigan might even do what Wisconsin did when it discovered crawfish in a pond in 2009 and filled it with 4,000 gallons of bleach. Just don’t tell the state’s wildlife experts to look on the bright side of a new food source and more backyard barbecues featuring delicious, Cajun-style crawfish boils.
“These critters have certainly caused havoc everywhere else they’ve been, so there’s no upside to these,” says Popoff. “I know they’re important culturally to some of our Southern states and I totally respect that.... But we don’t want them here, and we’re going to do what we can to contain them.”