Chicago-area water pollution may be stalling the spread of invasive carp

In a name-your-poison twist, a new study from the University of Illinois adds to the evidence that Chicago-area waterway pollution is slowing the relentless advance of the invasive silver carp.

Researchers turned their attention to a section of the Illinois River, about 50 miles from Chicago, where the silver carp's progress stalled out more than 10 years ago, with the formidable fish holding its ground but failing to continue on toward Lake Michigan.

The researchers took water from a more polluted section of the river — not far from where the carp stalled — and put young silver carp in it.

Instead of moving around and exploring, as they did in clean water, the fish slowed down almost immediately.

"It's sort of a coping mechanism," said study co-author Cory Suski, a professor of natural resources and environmental sciences at the University of Illinois at Urbana-Champaign. "You kind of hunker down and hope that whatever is bad in your environment is going to pass by."

The authors also found that the silver carp burned more energy in the polluted water, a sign that their bodies were fighting the effects of contaminants.

Suski co-authored a previous study in which silver carp from the Illinois River showed gene activity consistent with contaminant exposure.

"Now we've got a bit of a mechanism," Suski said. "If the fish do swim further on, the contaminant loads are probably going to be a little higher and it's going to be energetically costly to be there. It almost becomes like it's not worth it — they're spending more energy to keep themselves safe than they are possibly taking in."

Other factors that could be causing the Illinois River carp to stall include shoreline changes such as walled river banks closer to the city; downstream carp fishing; and locks and dams.

"There are a bunch of different things at play that could be preventing (the carp) from moving forward, but now some stimulus in the water should also be on the list," Suski said.

The study didn't identify which contaminants are affecting the carp.

Among the suspects: volatile organic compounds and substances not removed by wastewater treatment plants, such as pharmaceuticals.

It's possible that researchers could figure out which substances deter carp, Suski said, and that information could have practical benefits.

"Not to say that we need to start polluting," he said. "Pollution is not a good carp deterrent, but we might be able to say, '(There's) something that they don't seem to like. Is there some way we can use that?'"

Silver carp and bighead carp, which are both stalled about 47 miles from Lake Michigan in Channahon, outcompete native species. Government agencies are working to stop them from entering Lake Michigan, where they could harm native fish and the $7 billion Great Lakes fishing industry.

Invasive carp arrived in the U.S. from Asia in the 1970s, imported to eat algae in commercial fish ponds. They escaped and have been spreading through waterways since the 1980s.

"It's definitely one of the most challenging conservation issues of our time, in part because it's so widespread across the Mississippi River Basin, which reaches into 31 states," said U.S. Fish and Wildlife Service spokesperson Janet Lebson.

A few invasive carp have been spotted closer to Chicago than the stalling location, but those fish are viewed as isolated explorers rather than members of a population front on the move.

In 2022, a silver carp was found in Lake Calumet, about 7 miles from Lake Michigan; in 2017, a silver carp was found below T.J. O'Brien Lock and Dam, about 9 miles from Lake Michigan; and in 2010 a bighead carp was found in Lake Calumet, Lebson said.

The main line of defense in the Chicago area is an an electric current barrier system near Romeoville and about 30 miles from Lake Michigan, in the Chicago Sanitary and Ship Canal.

Construction of a major addition, the long-awaited $1.2 billion Brandon Road Interbasin Project deterrence system on the Des Plaines River near Joliet, is expected to begin early next year.

Another major initiative, invasive carp fishing by the Illinois Department of Natural Resources and its partners, removes about a million pounds of fish per year from the state's waterways.

Study co-author Amy Schneider noted that water quality in the Chicago area has been improving, and a lot of the native fish found in the Illinois River are doing fairly well in the Chicago River and its tributaries.

"It's possible we could see (silver carp) move upstream if the water quality improves," she said. "Keeping an eye on them, and trying to predict what aspect of the water quality is affecting them, might give us some insight as to when and why they might move upstream in the future."