By Juanpablo Ramirez-Franco, WBEZ
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For close to five decades, invasive carp have overrun huge swaths of the Mississippi River Basin. Yet the hardy, oversized and voracious silver carp and bighead carp stopped about 40 miles southeast of Chicago and haven’t gone any farther. Now, scientists may have discovered the reason why: Chicago’s polluted water.
A new study published Friday in the journal Scientific Reports found the silver carp may have stopped advancing upriver due to contaminants in the Chicago Area Waterway System, the network of canals and rivers that make barge traffic possible between Lake Michigan and the Mississippi River and absorb the outflow of the city’s combined sewer system.
The invasive carp is a catchall term for a family of four fish formerly referred to as Asian carp: bighead carp, black carp, grass carp and silver carp. They are all native to China but currently dominate much of the Mississippi River Basin, where they have devastated the food web in the freshwater ecosystem. In fact, there are currently more silver carp in the Illinois River than in any other waterway in the world.
Legislators, environmentalists and business leaders have long worried that if the carp make it past Chicago and into Lake Michigan, they could upend the multibillion-dollar recreational and commercial fisheries in and around the Great Lakes.
But about a decade ago, at a spot along the Illinois River, the carp hit an invisible wall. They simply stopped advancing. It was a first for the invasive species. Until then, no dam or lock had managed to permanently halt the carps’ advance. Scientists have long theorized that contaminants in Chicago’s water could be the reason why the prolific fishes — particularly the silver and bighead carp — stopped swimming toward the city. But the theory was never directly tested, until now.
“We were not able to move carp farther upstream than where they were, but we wanted to simulate that,” said Cory Suski, a co-author of the study and professor of aquatic resources at the University of Illinois Urbana-Champaign. “And so, what we did was move the water.”
The researchers at UIUC partnered with scientists at the University of Texas and the U.S. Geological Survey to conduct their research. They collected water from just north of where the carp seem to stop, along the Des Plaines River, just before it flows into the Kankakee River. The confluence of the two waterways is what forms the Illinois River, and according to Suski, this is where the water quality notably changes.
To run the experiment, they dropped lab-hatched silver carp into two tanks: one containing water sourced from near the CAWS and another with water from the lab, said Suski: “Just drop the fish in there, put a video camera over top of it to video their behavior and watch how they swim.”
In CAWS water, the carp swam slowly. Eventually, they stopped moving altogether. All the while, they burned more energy to stay afloat than the fish in the clean control water. The same tests were run on golden shiners, a fish species native to the area and found in the CAWS. Those fish showed no major behavioral differences when exposed to either water type.
Although their research did not look into exactly which contaminants might be responsible for causing the phenomenon, Suski said traces of pharmaceuticals and volatile organic compounds like gasoline have been previously detected upstream of the location.
If the pollution in CAWS is, in fact, deterring the carp’s migration, there are questions about how long that might be the case. The waterways are cleaner now than they have been for more than a century, with fewer carp-paralyzing contaminants washing down the CAWS than ever before.
“The CAWS has changed from something that would have been described as an open sewer of industrial waste to being an ecosystem-supporting waterway,” said Rachel Havrelock, director of the University of Illinois at Chicago’s Fresh Water Lab.
Havrelock points out that transformation of the waterway once consisting of approximately 70% treated wastewater is the combined result of the Clean Water Act, several Supreme Court decisions and consistent local environmental activism over the years. Recent studies have found more than 70 species of fish thriving in the city’s once-troubled waters due to the continued improvements in water quality, up from just 10 species in 1974.
“The water quality is only going to get better, and it’s going to become what it should be, which is the best possible habitat for fish, wildlife and people that it can be,” said Joel Brammeier, president and CEO of Alliance for the Great Lakes, a nonprofit environmental organization based in Chicago.
The trouble is that a better habitat for native fish is also a better habitat for invasive fish. Brammeier says that’s why it is imperative that the U.S. Army Corps of Engineers construct the Brandon Road Interbasin Project — a billion-dollar, high-tech, multistep barrier about 10 miles north of the carp’s leading edge that many hope will keep the fish out of Chicago for good. The states of Illinois and Michigan signed an agreement with the Corps earlier this summer to unlock funds to begin the first phase of the project.
“The Chicago Area Waterway getting cleaned up is a good thing,” said Suski. “That’s got to continue.” That’s why, he says, he is hoping that his team can soon embark upon a study to try to pinpoint exactly which contaminants are flooring the carp.
Catch more news at Great Lakes Now:
Great Lakes most unwanted: Top 10 invasive species
Featured image: One of the ways the Illinois Department of Natural Resources is fighting the carp invasion into the Great Lakes is by catching them. Photo from Great Lakes Now episode 1006.
