Asian Carp, Knocking at the Great Lakes’ Back Door

Michigan and Illinois are fighting over fish. Four fish, actually: a group of species collectively called Asian carp, the most recent high-profile Great Lakes invaders. They haven’t yet established a population in Lake Michigan, but researchers have detected carp DNA in the lake. The finding prompted the U.S. Supreme Court to re-assess Friday whether it will get involved in a battle over Chicago’s canals, a battle with billion-dollar economic implications. The court will likely announce on Monday, March 22 whether it will take up the case.

Over the past two centuries, human economic movement has increasingly allowed invasive species to move into the Great Lakes ecosystem. The fight over Asian carp is taking place in Chicago, which is pretty far from where the Great Lakes meet the ocean, at the mouth of the St. Lawrence River in Quebec. That distance hints at the heart of the Great Lakes’ invasive species problem: humans have created access points to an ecosystem that was cut off from the ocean for thousands of years by an ecological barrier. That barrier? Niagara Falls.

“They were the protectors of the lakes above,” David Jude, a researcher at the University of Michigan’s Great Lakes Fishery Laboratory, said of the falls. “Lake Ontario was the mixing pot since it had access to the ocean.”

At Niagara Falls, the honeymoon capital of the world, 700,000 gallons of water drop off a 200-foot cliff each second. In other words, Niagara Falls consumes water at more than twice the rate of all U.S. residents combined. It’s a formidable obstacle for any critters trying to make their way upstream from Lake Ontario into Lake Erie.

That barrier meant that after twelve thousand years of relative solitude, the ecosystems of lakes Erie, Huron, Michigan, and Superior weren’t ready for the canals and ships that enterprising North Americans have constructed in the past 200 years. As a result, zebra mussels, rusty crayfish, purple loosestrife and other invaders have made the Great Lakes their home at the expense of native species.

In general, invasive species are problematic because they exploit loopholes in native ecosystems.  The zebra mussel, for example, attaches more strongly to surfaces than other freshwater mussels, so they have been able to adhere themselves to just about everything in the Great Lakes — including the native mussels. With a dozen zebra mussels riding on its back, the native mussel is unable to move. And with so many zebra mussels around, there’s less delicious algae for those native mussels to eat.

As they adapt to a new environment, non-native species can multiply until they dominate the region. Their dominance reduces biological diversity, and a region that lacks biodiversity is more susceptible to disease in the long run.

In Chicago, canals that connect the Illinois River — a tributary of the Mississippi — with Lake Michigan serve as one of many human-made back doors to the Great Lakes. Asian carp are abundant in the Mississippi River system, where they are popular with sport fishermen who haul out 50-pound specimens.

Should we worry about a carp presence in the Great Lakes? For one, they would probably eat the plankton that more desirable native fishes would otherwise munch on. But there’s quite a bit of uncertainty about whether the fish could establish populations in the lakes, and exactly how much disruption they might cause once they invade, according to Duane Chapman, a biologist with the U.S. Geological Survey who wrote part of the Supreme Court opposition brief on the topic.

Chapman was also quick to point out that Asian carp probably won’t cause immediate catastrophic damage if they get a foothold in Lake Michigan, saying it would likely be a decade before the effects were felt.

Maybe the collective anxiety about carp populations, then, comes from the long history of invasive species taking up residence in the Great Lakes.

If the Supreme Court decides to hear the canal locks case and rules against Illinois, the state would have to close the locks on the Chicago Sanitary and Ship Canal, which separate the Mississippi and Great Lakes watersheds more solidly than the electric barriers the U.S. Army Corps of Engineers is building. Closing the locks would shut down a shipping industry that moves $1.5 billion through Chicago each year — so Chicago isn’t likely to comply without a fight.

The Asian carp battle wages on, one part of the war against Great Lakes eco-invaders.