There is a fish so feared in the United States that an electrified barrier has been used to keep them at bay, collateral damage be damned. It’s not an aggressive shark or an escaped piranha. It’s a bottom-feeder.

The common carp was introduced to the US in the 19th century and has since become a ubiquitous ecological nuisance. But several of its cousins—collectively referred to as “Asian carp”—have set the alarm bells ringing. These made it to the US in the 1960s and 1970s and were put to use in pond-cleaning duties on fish farms. They soon began to escape and establish themselves in the Mississippi River. One of the species—silver carp—is known for its bizarre leaping behavior when startled by passing boats. (It doesn’t seem quite as funny to boaters who have had bones broken by the huge, airborne fish.)

But that’s not what has biologists and fisheries managers worried. Asian carp can take over ecosystems, stealing all the food for themselves and muddying the water. As populations spread throughout the Mississippi River watershed, it has seemed inevitable that they'd spread into the Great Lakes. The people who have witnessed the incredible degradation of that ecosystem viewed Asian carp as the next big threat.

Wave after wave of invading species—some carried by international shipping vessels, others taking advantage of canals—has battered the Great Lakes. Sea lampreys and alewives transformed the food web. Filter-feeding zebra mussels (followed by their even more troublesome cousins the quagga mussels) have changed the lakes themselves, consuming so much plankton and algae that you can now see more than thirty feet deeper into Lake Michigan water.

The spread of Asian carp into Illinois put a spotlight on the canal connecting Lake Michigan to the Mississippi River watershed. This is where the electrified barrier comes in. The Army Corps of Engineers installed the first portion of the barrier, which is designed to prevent the passage of any fish, in 2002. Worries that the barrier may not be enough—stoked by detections of Asian carp DNA past the barrier—have led to demands that Chicago close the canal. Because of its economic value, Chicago has refused. This has culminated in lawsuits filed by five Great Lakes states against Chicago and the Army Corps.

Meanwhile, the DNA evidence hinting that Asian carp may have already made it into Lake Michigan has had biologists waiting for the seemingly inevitable confirmation of a population in the lake. But Lake Michigan isn’t the only Great Lake on the front line. Lake Erie has its own problems. There, a chain-link fence separates a flood-prone wetland between two rivers—one that flows toward the Mississippi and another that flows into Lake Erie.

Several grass carp (one of the Asian carp species) have been caught in or near Lake Erie in recent years. However, the ones that were discovered were sterile. Sterile fish are used to control plant growth, and they can't reproduce because they are triploid—having a triplicate set of chromosomes.

In October of last year, four smallish grass carp were caught in a river just a couple miles upstream from Lake Erie. The results of tests performed on the fish have now been published, and the news isn’t good. The fish weren’t sterile like the others, and they were only about a year old.

To determine where the fish had come from, researchers measured the amount of strontium in a bone inside the fish’s ears. The river in which the fish were found happens to have higher than average levels of strontium in it, and that’s what they found in the ear bones as well. In fact, the inner part of the bones, which formed early in life, contained significantly more strontium than the outer portion. That matched up with strontium in the river, which was high during a wet 2011 and low during the drought in 2012.

That means the grass carp appear to have hatched in the river, making this the first documented instance of Asian carp reproducing in the Lake Erie watershed.

The researchers write, “The implications that grass carp have spawned and recruited in the Great Lakes Basin are profound.” In habitats like Lake Erie, grass carp populations are capable of grazing lake-bottom plants basically down to nothing. If they can establish a population, it would mean trouble for the rest of the Lake Erie ecosystem, including the native fishes and birds that rely on those plants.

What’s more, it had been thought that grass carp required a longer river than this one for spawning. The apparent success in this instance means that grass carp—and the other species—might have an easier time establishing a foothold in Lake Erie than expected.

While no one is ready to wave the white flag just yet, it seems to simply be a matter of time before these carp join the long line of conquering invaders that have wreaked havoc in the diminished Great Lakes.

Journal of Great Lakes Research, 2013. DOI: 10.1016/j.jglr.2013.09.019 (About DOIs).