How Fish Autopsies Help in the Fight Against the Invasive Asian Carp

Dead fish tell many tales—but can they persuade authorities to strengthen Lake Michigan’s defenses against these ecological saboteurs?

Silver carp captured in the Illinois waterway, June 2017

Credit: Courtesy of Illinois Department of Natural Resources

Scalpel in hand, Greg Whitledge cuts into an Asian carp’s skull. Just hours before, the silver carp lay in an ice-packed cooler in the back of an Illinois Department of Natural Resources vehicle. An IDNR employee rushed the dead fish from just outside Chicago to a point about halfway to Whitledge’s lab at Southern Illinois University in Carbondale. At the designated meeting place, the handoff was made to one of Whitledge’s grad students. What was the hurry? State officials wanted this fish seen as quickly as possible.

Whitledge is a CSI of sorts. At the lab, he and his team perform fish autopsies to figure out where the animals once swam. To get their results, they carefully remove three sets of ear bones (yes, fish have ears), which, like tree rings, grow as the fish age. The researchers then drill into the bones, analyze their chemical composition at various life stages, and compare them with the compositions of local water bodies. In all, his lab analyzes hundreds of piscine ear bones each year.

But this dead fish was a special specimen. A commercial fisherman had caught the invasive Asian carp in the Little Calumet River just nine miles from Lake Michigan, a discovery that likely sent chills down the spines of public officials and conservationists. Illinois and the federal government have spent many years and almost $400 million to keep this fish and its ilk away from the Great Lakes, the largest freshwater ecosystem in the world.

This wasn’t the first Asian carp seen in the waterway. A fisherman caught one even closer to Lake Michigan in 2010 (Whitledge autopsied that one, too), but the latest catch (earlier this June) was the first since the Army Corp of Engineers completed installing a system of electric barriers to keep these fish out.

Why all the fuss? Asian carp aren’t your average invaders—they grow quickly, eat aggressively, and easily adapt to different aquatic environments. (Silver carp, one of the four invasive subspecies, can also jump forcefully out of the water.) If Asian carp get into the Great Lakes and reproduce, they could outcompete local fish stocks of whitefish, perch, and minnows for plankton, the invaders’ favorite meal.

Jumping silver carp in Illinois’ Fox River
Credit: Ryan Hagerty/USFWS

Back in the 1970s, fish farmers imported Asian carp to the southern United States to filter water on catfish farms. Soon, the carp found their way to the Mississippi River and its tributaries. A 2012 report published by researchers from Southern Illinois University found that in the lower 150 miles of the Illinois River, Asian carp now account for 60 percent of the fish biomass.

The fish on Whitledge’s autopsy table turned out to be a fertile male silver carp that spawned in either the Mississippi or the Illinois River. From there, it swam into the Des Plaines River before entering the Little Calumet. Whitledge’s analysis showed that the carp had likely been in the Little Calumet for a number of weeks or months. (After a two-week search for more carp nearby, IDNR concluded that this silver seemed to be solo.)

The journey it most likely took is remarkable. Traveling from its birthplace to the Little Calumet River would have required bypassing three electric barriers in addition to a lock at Brandon Road, a crucial intersection where Mississippi River water mixes with water originally from Lake Michigan. And it is at Brandon Road that the Army Corps of Engineers recently proposed adding new barriers, a suggestion that has sparked a battle between those who see the existing defenses as sufficient and those who want more to be done.

The Brandon Road Lock and Dam, an important north–south shipping corridor in Illinois where engineers want to add new barriers to block Asian carp
Credit: Dave Wethington/U.S. Army Photo

The shipping industry—or, more specifically, American Waterways Operators, a trade group for the tugboat, towboat, and barge industries, and representatives from the Ports of Indiana, which operates steel-carrying ships through the Brandon Road Lock and Dam—say additional measures are unnecessary. More work on the river allows fewer ships to pass through during construction, and due to the importance of this north–south shipping corridor, delays could have regional and national implications for some commodities. Industry figures also point out that in the six years that all three electric barriers have been in place, only one carp has gotten through.

Meanwhile, pushing for more carp-blocking infrastructure are mayors from Great Lakes communities, local citizens, and environmental groups, including NRDC. A fishing industry valued at $7 billion a year is at stake, and some towns along the lake are already spending significant amounts of money and resources trying to keep their waters free of other destructive foreign species, like zebra and quagga mussels.

The Army Corps’ current plan includes building an additional electric barrier as well as a noise barrier and water jets, which Mark Cornish, a Corps biologist who evaluated various alternatives, says are the best technologies scientists have to date. Noise, he says, could scare Asian carp away as long as they don’t get accustomed to it. The project—which would cost $275 million to construct over the next five years—is part of the Great Lakes and Mississippi River Interbasin Study, a multiagency effort launched in 2007 at the behest of Congress to prevent invasive species from gaining new ground in the region.

Installing electric barriers to block Asian carp in Chicago Sanitary and Ship Canal
Credit: Sarah Gross/U.S. Army Photo

“The goal is not to have a project that does nothing and costs nothing; it’s to have a project where the benefits outweigh the costs,” said Joel Brammeier, president and CEO of the Alliance for the Great Lakes, at a public meeting on the proposal in September. Speaker after speaker agreed with him, though many believe the new barriers still won’t be enough.

Many stakeholders in the effort to address Asian carp continue to believe the answer is to return the rivers to their natural flow, and so once again physically separate the Great Lakes from the Mississippi. As a compromise with the waterway operators who want the waters to continue flowing between the basins, some argue that a lock treatment system, in which boats enter kill chambers where water is chemically treated to eliminate Asian carp and other invasives, might be a better option. Cornish says this technology hasn’t been proven yet. If and when it is, he says, there’s no foreseeable reason why it wouldn’t work alongside his agency’s current suggestions.

But learning just how Whitledge’s silver carp got into the Little Calumet would certainly help guide the Army Corps’ efforts. “I wish we’d had a little radio transmitter that followed it all the way up through the system so we’d know where the chinks in our armor are,” says Cornish.

For now, thanks to CSI: Asian carp, all we do know is that a lone fish appears to have breached our defenses. And if one can, others might, too.

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