- Silver carp are famous for leaping out of the water and smacking into fishermen.
- They’re an invasive fish that can outcompete native species for fish and habitat.
- Officials fear they will one day reach the Great Lakes.
There may be an upside to pollution in Chicago’s waterways: A new study says a “toxic soup” might be blocking the invasive silver carp’s path to the Great Lakes.
The fish, which are famous for leaping out of the water and smacking into fishermen, threaten native species and the Great Lakes multi-billion-dollar fishing industry. They were imported from Southeast Asia in the 1970s to help control nuisance algae blooms and vegetation in aquaculture facilities, farm ponds and sewage lagoons, according to the U.S. Fish and Wildlife Service. Thanks to flooding and accidental releases, they escaped into the Mississippi River system. Since the 1980s, silver carp have made their way up through the Missouri and Illinois river basins.
But over the past decade, none have been found in any waterway past Kankakee, Illinois, about an hour south of Chicago and Lake Michigan, researchers say. That might be because of the pharmaceuticals, wastewater and other volatile compounds waiting for them upriver, according to a new study published last week.
“It’s a really toxic soup coming down from the Chicago Area Waterway, but a lot of those chemicals go away near Kankakee,” Cory Suski, a co-author of the study and an associate professor in the Department of Natural Resources and Environmental Sciences at the University of Illinois, said in a press release. “They might degrade or settle out, or the Kankakee River might dilute them. We don’t really know what happens, but there’s a stark change in water quality at that point. That’s right where the invading front stops.”
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The researchers formulated their hypothesis after reading a 2017 water quality report from the U.S. Geological Survey that tracked changes in water chemistry as it moved downstream from Chicago to the Illinois River. They then studied gene expression patterns in blood and liver samples from silver carp at Kankakee and two locations farther downstream from Chicago.
The results showed that the fish near Kankakee were working harder to clear toxins from their bodies.
“We saw huge differences in gene expression patterns between the Kankakee fish and the two downstream populations,” Suski said. “Fish near Kankakee were turning on genes associated with clearing out toxins and turning off genes related to DNA repair and protective measures. Basically, their livers are working overtime and detoxifying pathways are extremely active, which seem to be occurring at the cost of their own repair mechanisms. We didn’t see that in either of the downstream populations.”
All four types of Asian carp – silver, black, grass and bighead – are prolific eaters and reproducers, according to the FWS. One Asian carp can eat up to 40 percent of its body weight each day, and can females can produce as many as 1 million eggs each year.
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Silver carp are the smallest of the species, but can still grow up to 4 feet long and weigh as much as 100 pounds. They feast on phytoplankton and zooplankton, the food base for a number of native invertebrates, freshwater mussels, snails, juvenile and adult fish species. As they spread, they can outcompete those native species for food.
The invasive fish have been found in at least 17 states via the connecting Mississippi, Missouri and Illinois rivers. Officials fear they will one day reach the Great Lakes from the Illinois River via the Chicago Area Waterway System. The USGS study that launched the new research identified some 280 chemicals in the waterway system and downstream sites, according to the press release.
Suski made it clear his research didn’t aim to make a direct link between the pollution and the carps’ stall at Kankakee. But he said understanding why the fish haven’t moved past there could help future efforts to combat the spread of silver carp.
“We’re not saying we should pollute more to keep silver carp out of the Great Lakes. That’s not it,” Suski said. “Right now, things are stable, but that might not always be the case. There’s a lot of work in Chicago to clean up the Chicago Area Waterway. Already, water quality is improving, fish communities are getting healthier. Through the process of improving the water quality, which we should absolutely be doing, there’s a possibility that this chemical barrier could go away. We don’t need to hit the panic button yet, but at least we should be aware.”