Nothing was normal about this fishing expedition. The water—not a natural pond or stream. The temperature—well below freezing. We weren’t allowed on the fishing boats, our guides told us, because it wasn’t safe today. And the fishermen we watched were hoping not to catch the very fish we had all come here for—fish that, by the way, will neither strike at a worm, no matter how smelly, nor follow a lure, however realistic. Fish whose peculiar habits have created quite a problem—and whose same habits may offer hope for solutions.

Along with a dozen other reporters and cameramen, I was watching a government “carp corral.” A posse organized in February by five federal and state agencies was hunting two species of Asian carp—silver carp and bigheads—because these strange, gluttonous fish have upended the ecosystem of the Mississippi River and some of its tributaries, including the Illinois River, by devouring the base of the food chain and starving out native fish. If they reach Lake Michigan they could do the same to the Great Lakes.

The vast Mississippi and Great Lakes watersheds are naturally separated only by subtle features of the prairie, such as the nearly invisible ancient dune running along Ridgeland Avenue through Oak Park and southward. Aiming to profit by joining the two regions, Chicago in the mid 19th century stretched the little Illinois and Michigan Canal from Lake Michigan to the Illinois River, like a rope across a divide. The larger, deeper Sanitary and Ship Canal, dug just over a century ago, gave passage to modern boat traffic, and if other things passed through the new channel—the city’s sewage, for instance, or alien aquatic species—the voices of concern raised in central Illinois and around the lakes probably didn’t disturb the sleep of the Chicagoans who financed the connection.

But the northward spread of Asian carp—from an undetermined southern tributary of the Mississippi—is taking place in a changed landscape. Though many still depend on the economic connection provided by the canal, thousands of others, working on hundreds of charter fishing boats around the lakes, are demanding that the carp be quarantined away from Lake Michigan, even if this requires closing the locks that allow boats to pass between the canal and the lake.

February’s “carp corral” was an effort to see how far the contagion had spread, a test of whether radical surgery might be justified. Three olive green launches not much bigger than rowboats were “electrofishing” a stretch of the canal on the southern edge of Cicero, nine miles from the lake. Live lines dangling from their bows created electric currents used like cattle prods to herd fish toward underwater seines. Men with long-handled nets stood on deck snaring any stunned fish that floated to the surface. The fate of water people of various trades depended on what emerged from the murky water (I myself depend on the water for my livelihood as well, though my canoe rental business on the upper Chicago River would not likely be affected by anticarp measures). At about 10 AM a boat motored to the concrete ledge where we were all gathered to show the news cameras what the men had netted, just a small sample of what would come up in the seines later in the day. We saw a soup-pot’s portion of shiny gizzard shad, each with a daub of black above the eye like a mark of Ash Wednesday atonement, a single whiskered bullhead, and an enormous, ruddy gray common carp. So far, so good. No one will lose his job over a common carp.

A bottom-feeder, the common carp is what most of us think of when we think of carp. Like an aquatic pig, it eats by snuffling among the roots of water plants, sending up clouds of river muck that make it difficult for other fish to find food. Biologists call them ecosystem engineers for this tendency to take an underwater garden and leave it a muddy hog wallow. Fishermen attract them the way you might gather your swine, by scattering a handful of canned corn. All this has earned them an unwholesome reputation.

The bighead and silver carp, the real target of the corral, won’t scrabble at the bottom, and they couldn’t digest a kernel of corn if they accidentally sucked one in. Instead, they “filter feed” with internal rakes that strain tiny plants and critters from the water before their gills pull the oxygen from it. If we dispensed with eating because our lungs extracted all the nutrients we needed right from the air, we might approach the simplicity and efficiency of filter feeders.

Virtually everything that lives in our lakes and rivers depends on the same plankton that the Asian carp eat. Freshwater mussels filter plankton. Baby fish of all species slurp plankton till they’re big enough for adult fodder. The big, predatory fish that anglers love—salmon, sturgeon and walleye—prey on smaller species that eat plankton. Even zebra mussels filter plankton. The zebra mussel is another damaging alien, one that launched its invasion in the opposite direction, from the lake through the canal to the Mississippi basin.

But none of them eats plankton so voraciously or uses it so efficiently as Asian carp. They consume 40 percent of their weight in plankton daily—for every adult carp, which can grow to 100 pounds, that makes 30 to 40 pounds of algae a day unavailable to anything else in the water. And they quickly grow larger than the predators that might keep them in check. A stretch of water south of Peoria is thicker with them than any river in China, more than 4,000 per river mile. If you climb up Starved Rock for the river view, a half million pounds of fish could be staring back at you. An unexplained die-off three years ago sent tens of thousands of stinking carp carcasses floating toward Saint Louis without stalling their population growth.

The two species are closely related but they differ slightly in habit and diet—bigheads swim gape-mouthed at the surface in groups, vacuuming algae and tiny creatures that live in it the way a cow might consume grasshoppers with grass; silver carp target even smaller plankton and tend to swim a bit lower in the water, but when startled, as they are by motorboat engines, they flee by leaping, sometimes ten feet in the air. Turn your ski boat in the wrong direction and it’s like running a slalom course through a hail of rain-soaked Sunday papers. Flying silver carp have broken people’s arms, teeth, and even vertebrae.

The utility of the carp—its reason for coming to this hemisphere in the first place—is its power to consume algae and make water clearer, though not necessarily cleaner. The mystery of the carp’s initial escape into midwestern waters offers a blank canvas for our economic neuroses. Some say they fled a factory fish farm in Arkansas. Others blame lax government workers at a sewage treatment lagoon. Or was it a wealthy landowner who imported them to clear algae from his private pond? Yeah, and I bet he made his money in subprime mortgages.

At any rate, the economic threat they pose to the Great Lakes is very real, but it was thought to be distant until late last year. The big schools were still 50 miles from Lake Michigan, about ten miles on the Mississippi side of a $9 million electric carp barrier completed by the Army Corps of Engineers across the canal in 2005. But in November, Notre Dame notified the corps of environmental evidence based on DNA, or eDNA, that some carp might have reached and passed the barrier: carp DNA had been found as far from the barrier as the North Shore Channel leading into Wilmette. (See the next story for more on the test.)

Acting on the new information, the corps prepared for routine maintenance of the barrier by poisoning a 5.7-mile stretch of river just below it. Just to be sure. And to their dismay they found, amid thousands of pounds of dead fish, a dead Asian carp. Maybe the carp had already launched their invasion of the Great Lakes.

The news sent fishermen around the lakes into an uproar. “They could completely destroy the salmon and steelhead runs, the whole fishery,” said Kirk Novak, the owner of Northern Angler, a shop in Traverse City, Michigan, that offers rods, tackle, and classes in fly tying. His customers come to wrestle with salmon—they won’t come back for a carp that doesn’t strike a lure. “We’d like to see strong measures to close the door and keep them out of the lake,” Novak said, and he’s been broadcasting that message to his 800 Twitter followers.

Multiply Novak’s loss—which he didn’t want to guess at—by all the gear shops, marinas, and charter boats on the shores of the Great Lakes and you may or may not reach $7 billion, the U.S. Fish & Wildlife Service estimate of the impact of sport fishing on the region. But you do reach the desk of Michigan attorney general Mike Cox. Motivated by worried voters, and joined by attorneys general in five other Great Lakes states, Cox sued the state of Illinois, the Army Corps of Engineers, and the Metropolitan Water Reclamation District of Greater Chicago to force them to shut the Chicago locks against the carp. Relying on a 1929 Supreme Court decree that governs the use of the locks for water diversion, Cox went straight to the high court, asking for an emergency amendment to the decree to prevent looming “ecological and economic disaster.”

To boatmen on this side of the lake—barge hands who pass through the O’Brien Lock near Calumet Harbor and the staff of the dozen or so tour boat companies that use the downtown locks—the evidence didn’t look so solid. “They haven’t found a carp live or dead north of the barrier,” Craig Wenokur, operations director of Wendella Boats, told me. Wendella operates six boats and is purchasing a seventh, and employs about 150 people. “It’d be devastating if the lock closes,” Wenokur said. “Seventy percent of our tours pass through those locks. The electric barrier works, and the Corps of Engineers is doing their job. The locks weren’t even an issue till the attorney general in Michigan created this hysteria.”

Congressman Judy Biggert, whose district includes barge and freight companies near Joliet, echoed Wenokur’s comments. “What prompted the hysteria?” she asked. “What is eDNA? It could be fish feces?”

That’s true: DNA can come from intestinal cells sloughing off during digestion. But fish shit isn’t bullshit. With powerful interests on both sides, the branches of government responded cautiously. The Supreme Court deferred a decision on Michigan’s suit against Illinois. Agreeing with a lawyer for the Corps of Engineers that the DNA didn’t prove an ‘imminent threat,” the court denied Cox’s emergency request and asked for more evidence, but gave no indication how it would rule if something more than DNA were found.

Credit: Lloyd Degrane

On Monday, March 22, with DNA still the only evidence, it denied a second request.

The White House stepped into the fray and attempted to mollify everyone with money, perhaps fearing charges of favoritism if it did nothing and left the locks open to Illinois commerce and to the carp. At a news conference on February 8 the White House announced a $78.5 million plan to figure out exactly where the fish were, hunt them down north of the barrier, and then strengthen the canal defenses. The Corps of Engineers will also install an additional electric barrier just upriver from the existing one and build a 14-mile concrete and chain-link obstruction along the ridge between the Des Plaines River and the canal, which is often overtopped during heavy rain.

Meanwhile the state of Illinois will subsidize commercial fishermen to “overfish” the Illinois River near the barriers. To explain how overfishing works, Mike Hoff, a U.S. Fish and Wildlife Service biologist, pointed out that two-thirds of oceanic species are in trouble because trawlers have caught so many of the adults that the populations no longer reproduce. Of course, those are species we’d prefer to be healthy, but the principle with carp would be the same—catch so many adults that there are too few left to sustain population growth. (For the possible uses for a carp haul, see the adjacent story on chefs.)

There are reasons to believe the carp might not succeed in colonizing the lake even if they reach it: Since 2004, Kelly Baerwaldt of the Corps of Engineers has been sewing tiny transmitters into dozens of Asian carp to study when and where they move. As she expected, they’re most active in the early afternoon, moving to fresh blooms of algae in the peak sun. But she was surprised to find that they also move at night. Their spawning runs are triggered by small rises in water level in the spring and early summer. She calls them “broadcast spawners that breed in huge aggregates,” meaning they do best when a bunch of fish send clouds of eggs and sperm into roiling water—they’re most prolific when they engage in group sex. And they need turbulence to mix the materials, so they gather in predictable spots—at the mouths of tributaries or downstream from islands. This may make them vulnerable to being caught in the act, so to speak. It also may mean that just a few sneaking through the barrier might not be enough to take over the lakes.

So how many would it take to create a self-sustaining population? “It’s difficult to be sure,” Phil Willink, a fish biologist at the Field Museum, told me, then offered the wisdom of the ages. “At least two.”

Most elements of the White House plan have actually been under discussion for several years in regular meetings of a sort of carp council of federal and state agencies. At a raucous February hearing sponsored by this task force at Chicago’s Metcalfe Federal Building, tour boat staff demanded to know why more hadn’t been accomplished earlier. Major General John Peabody of the Corps of Engineers gave a laconic answer: “We have to assess resources against competing demands.” A less politic way of putting that is that no powerful bloc of voters demanded funding for a solution. The agencies knew a crisis was coming, but had no way to marshal the necessary resources.

Wenokur and the boatmen of Chicago may actually owe a debt of gratitude to the Michigan attorney general. Cox’s lawsuit did what years of accumulating data couldn’t—he attracted the TV cameras, instigated protests by angry fishermen, and provoked the downtown boat owners, all now pulling together to demand the resources to stop the carp. There’s even funding to study a permanent solution, “ecological separation”—which would disconnect the lake and river ecosystems while still allowing people and commerce to pass. After years of alien species using the canal—mussels invading southward and carp swarming north—we could put a stop to the invasions once and for all.

The carp corral has been fishing daily for a little over a month, and they’ve yet to land any Asian carp. Assuming we haven’t already lost the battle, it’s possible we can finally win it.