Conservationists and wildlife officials have been policing the porous border between the Mississippi watershed and the Great Lakes for years. Their main concern: preventing Asian carp, an invasive species that has established itself in the Mississippi River system, from invading the Great Lakes, where the voracious eaters could cause havoc with natives. (Not to mention knocking a few recreational boaters out cold. Some Asian carp have a habit of launching themselves like missiles out of the water when they hear the roar of an approaching motorboat, as I learned in this TIME video from 2010.)
Officials haven’t had a whole lot of luck with their containment effort, despite spending tens of millions of dollars on the battle. A study in October found for the first time that a subspecies of Asian carp—grass carp—had successfully reproduced within the Great Lakes watershed, in a tributary of a river that feeds into Lake Erie.
Now it turns out that there may be reason to worry about another invasive species—going in the other direction. Scientists with the Nature Conservancy, the University of Notre Dame and Central Michigan University have for the first time discovered DNA from the Eurasian ruffe in two water samples taken in July from Lake Michigan’s Calumet Harbor in Chicago. While a Eurasian ruffe might sound like some kind of Siberian motorcycle gang, it’s actually an invasive species of small perch native to central and western Europe that established itself in Lake Superior in the mid-1980s after hitching a ride in a European freighter’s ballast water tanks. Since then it’s been slowly spreading around the southern shore of Lake Superior, before invading northern Lakes Michigan and Huron.
If Eurasian ruffes do establish themselves in the Mississippi watershed, the consequences would not be good. The ruffes have been identified by the U.S. Army Corps of Engineers as one of the 29 species that have the potential to transfer between the Great Lakes and the Mississippi River basin. Ruffes are comfortable in large rivers, and the Mississippi and its tributaries have twice the number of native fish as the Great Lakes basin does. That’s a lot of local fish that could be displaced if the ruffes move in.
“The Eurasian ruffe is a relatively small fish that produces a lot of eggs and reaches maturity very quickly,” says Lindsay Chadderton, the Aquatic Invasive Species Director for The Nature Conservancy’s Great Lakes Project. “They feed from the bottom of the food chain, and they’re going to compete with native and introduced species dependent on the same fauna.”
Still, Illinois officials note that no live ruffe have yet been captured in Calumet Harbor, and it’s possible the water containing the genetic material could have come from a bait bucket or ballast tanks, not from an actual fish. But even the possibility that ruffe could be poised to continue their invasion underscores how vulnerable the Great Lakes and the Mississippi basin are to invasive species—and the need for some kind of two-way species barrier in the artificial canal in Chicago that connects the two systems. Invasive species might be inevitable—nature has a way of going where we don’t want it to go—but we don’t have to make it easy for them.
(VIDEO: Holy Carp! Flying Fish in Illinois)