Pollution from the Chicago Area Waterway System might be stopping an invasion of silver carp in its tracks. Recent research has found that heavily polluted water flowing south from Chicago might be overloading the fish’s ability to process the toxins.
“Invasive silver carp at the leading edge of their invasion front in the Illinois River seem to be exhibiting responses consistent with their exposure to increased environmental contaminants,” lead researcher Jennifer Jeffrey, a biologist at the University of Manitoba in Winnipeg, Canada, told Eos.
“This provides us with some important clues regarding what might be limiting this invasive species from continuing its progression towards the Great Lakes of North America, when little else has seemed to affect its pervasive spread,” she said.
A Stalled Invasion
Silver carp were introduced into the Mississippi River Basin in the 1970s to combat algae growth. They aggressively spread upriver and were classified as an invasive species in 2007. But the invasion stalled about a decade ago in the Illinois River just north of the junction with the Kankakee River, about 65 kilometers downstream from Lake Michigan.
“There’s a stark change in water quality at that point,” coauthor Cory Suski, an ecologist at the University of Illinois at Urbana-Champaign, said in a statement. “That’s right where the invading front stops.”
This kind of abrupt halt is not typical for an invasive species. “Some invasive species thrive in degraded or disturbed habitats because in these situations they are less likely to encounter resistance from native species,” explained Anthony Ricciardi, an aquatic ecologist at McGill University who was not involved with the study. “Native competitors may be better adapted to natural conditions, so when these conditions change, the natives lose their ‘home field advantage.’”
“This fish never stops for anything,” Suski said. The researchers wanted to determine whether something in the fish’s biology could explain the coincidence.
Fish Versus Pollution
In 2016, the team captured fish from three locations along the Illinois River: one site at the leading edge of the invasion front and two downriver sites with well-entrenched populations. They collected blood and liver samples from the fish to see if specimens from different areas showed any physiological or genetic differences.
“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,” he said. “We didn’t see that in either of the downstream populations.” These results were published in Comparative Biochemistry and Physiology, Part D: Genomics and Proteomics in July.
When the Answer to a Problem Is Its Own Problem
Jeffrey said that to her knowledge, this is the only situation we know of where human activity has accidentally halted the spread of an invasive species. U.S. Geological Survey hydrologist William Battaglin, who was not involved with the research, agreed that this scenario might be unprecedented.
The researchers cautioned that they haven’t definitively linked the water pollution to the changes in the carp’s biology. It might yet be a coincidence that the water quality and the fish’s biology show changes at the same location.
“Currently, we don’t know how sensitive silver carp are to the contaminants that are coming out of the Chicago area,” Jeffrey said. The team continues to research the possible connection between the toxins and the genetic changes.
“We’re not saying we should pollute more to keep silver carp out of the Great Lakes. That’s not it,” Suski said. But Chicago area water managers should be aware of the potential connection as the region continues to clean up its waters. “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,” he said.
“Bottom line,” said Ricciardi, is that “invasive species can exploit a major change in environmental conditions, regardless of whether such a change is considered an improvement or a deterioration in overall habitat quality.”
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer