When silver carp invade a waterway, they’re hard to miss. The large, filter-feeding fish regularly grow more than three feet long and easily exceed 20 pounds. They loiter near the surface, where the plankton they feed on is most abundant. And, crucially, they’re easily startled. When boats go by, silver carp are known to catapult themselves multiple feet into the air, posing a projectile threat to both equipment, and people. Imagine dozens of 20+ pound fish jumping 10 feet out of the water in chaotic unison, causing a piscine maelstrom so thick it’s hard to navigate through it, let alone relax in its midst. Or, don’t imagine it– just watch a video of it.
Obviously, they disrupt recreational boating and other water-based activities. But from an ecological perspective, that’s far from the worst part. Silver carp grow so large, reproduce so numerously, and eat so much plankton that they outcompete native fish species, many of which are filter feeders, reliant on phyto- and zooplankton during their early life stages.
“They consume so much food and can exist in such great numbers that they can really reduce the amount of [resources] for resident species of fish,” says Peter Alsip, an ecologist with the NOAA Great Lakes Environmental Research Lab. Where they’ve invaded, research studies have noted changes in the plankton community and corresponding declines in native fish, he says. “They can have indirect effects on the whole ecosystem because [silver carp] are consuming phytoplankton and zooplankton, which are essentially the base of the food web,” adds Alsip.
Silver carp were initially introduced to the U.S. intentionally to help manage algae in aquaculture and wastewater facilities with their voracious appetites in the 1970s. Yet decades on, the fish have made their way into major U.S. watersheds and at least 20 states. They’re found in the Mississippi, Missouri, and Ohio rivers and their tributaries, according to the U.S. Fish and Wildlife Service. For many years, they’ve been on the cusp of entering the Great Lakes–the largest freshwater system on Earth. If the fish were to enter the lakes, they’d be poised to spread into all of the adjoining river systems–scaling up their takeover to new heights.
But something, somehow has been holding them back. Despite making it far up the Illinois River, silver carp still haven’t established a population in Lake Michigan. The “leading edge” or front of their progress has stalled south of Chicago for about a decade, says Cory Suski, a fisheries biologist and professor of natural resources and environmental science at the University of Illinois Urbana-Champaign. There are a few theories as to why.
Dams and locks might be impeding their progression (though research indicates the fish can overcome such physical barriers), says Suski. Active culling efforts along the Illinois River may be successfully helping to keep population in place, by removing the competition that would otherwise incentivize the carp to find new territory, suggests Alsip. The Illinois river is increasingly channelized and armored–with concrete walls replacing natural banks–as it nears Chicago, which is potentially a deterrent, says Suski. There’s the electric barrier at Joliet along the river, purposely intended to hold back invasive species, notes Alsip (though at least one has recently made it beyond that electric fish fence). Then there’s a less visible, possible impediment: pollution.
One long-standing hypothesis posits that silver carp in the Illinois River have largely stayed well south of Chicago because the water quality degrades closer to the city. Recent research, including newly published experimental evidence from Suski and his colleagues, supports the theory. Silver carp show signs of stress, including physical and behavioral changes, when exposed to river water collected from the Chicago area, according to a study published October 25 in the journal Scientific Reports.
The findings indicate that, in this one instance, humans have inadvertently pitted two environmental disasters against one another. If the waterway were cleaned up enough (and Illinois River water quality has been improving for decades), silver carp would be more likely to enter the Great Lakes. If it’s not, then everything else living in and alongside the river continues to suffer from the pollution.
“We’re not promoting pollution as a carp control,” says Suski. But, if the water quality continues to dramatically improve, carp may start to move, he explains. “It would be nice to get ahead of that… and at least be ready for it.” Perhaps, with more awareness of the water quality conundrum, wildlife managers can be prepared to ramp up fish removal efforts or invest in other, more targeted, less toxic, intentional barriers.
Previous work from Suski’s lab has hinted that contaminated water outside Chicago may be keeping the carp at bay, as in a 2019 study that showed wild silver carp living closer to the city show signs of their livers working overtime to filter toxins and genetic signatures of toxin-exposure. The new research bolsters those findings, demonstrating that there’s a clear link between water source and carp physiology, even in a lab experiment.
Over multiple experiments, the researchers compared the response of juvenile silver carp with that of native golden shiners, when exposed to Chicago-area Illinois River water. Each trial involved about 10 fish of each species, and water from the aquaculture facility the fish were reared in constituted a control condition.
Suski and his co-authors found that silver carp become far less active (i.e. freeze up) when exposed to the urban water. At the same time, their metabolisms shift into high gear, as measured via respiration rate. “They are moving less, but they’re burning more energy,” says Suski. In contrast, the golden shiners don’t display any significant change, indicating the carp are particularly sensitive to whatever contaminants are present in the water, he adds.
All together, it “shows that the water quality component could be a serious part of the equation,” says Alsip, who was uninvolved in the new research. “It’s another line of evidence towards this poor water quality theory.”
The research does have some limitations. Not all of the experiments panned out–a set of trials involving two-choice shuttle boxes didn’t go as planned, so the research failed to prove that carp actively avoid more polluted water, notes Suski. The scientists only tested the effects of short-term exposure, and longer duration exposures might alter the fish response. Plus, the sample size is limited, and the study only used juvenile fish–adults might react differently. Finally, the research doesn’t identify what toxins or contaminants are at play. Future work is needed to home in on the exact compound that’s stressing carp out.
And in reality, multiple factors likely contribute to keeping silver carp out of the Great Lakes, say both Suski and Alsip. Pollution is just one variable, and things like carp removals, the electric barrier, and habitat changes probably also have an effect.
Yet still, the study exemplifies how complicated human impacts on ecosystems can be. “I definitely wouldn’t say pollution is good,” says Alsip. “It is interesting, though, how one mess can help prevent or clean up another, in a way.”