ROMEOVILLE, Ill. — The now two-decades-old battle to keep invasive Asian carp out of the Great Lakes, being waged at numerous sites around the Midwest, has evolved into an uneasy standoff of sorts here on the Chicago waterway that is universally considered the front line in this biological conflict.

All indications are that bighead and silver carp, the most feared of the four Asian carp species because of the devastating impact they would be expected to have on the food web in the lakes, have not made their way past the electrical barriers intended to halt their Sherman’s March up the Mississippi River watershed.

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These two aggressive and prolific filter feeders that rob key nutrients from the young of native species are not yet pressing up against the barriers that are maintained by the U.S. Army Corps of Engineers in the canal system that corrupts the continental divide and creates an artificial link between the Great Lakes and Mississippi River watersheds.

“We’re holding our ground — that’s a good way to describe it,” said Charlie Wooley, U.S. Fish and Wildlife Service Midwest Regional director, employing the not infrequent use of military-type terms that this epic encounter seems to elicit. 

“With all of the extensive sampling and testing being done there in the Chicago area, we’ve seen things become more or less a stalemate,” Mr. Wooley said.

These exotic carp, brought to the United States more than 40 years ago to help control algae in southern fish farms and wastewater treatment ponds, escaped into the Mississippi River system during floods and have been surging throughout this vast waterway. Bighead and silver carp, which can produce a million eggs a year and reach massive size, are now found in the Mississippi and its myriad tributaries from New Orleans to Minnesota, and in the Ohio River and Missouri River, but it is their charge up the Illinois River and its feeder waterways that has put them ominously close to Lake Michigan.

Two other invasive Asian carp species — grass carp and black carp — also pose threats to the ecosystems of the U.S. rivers and lakes where they might become established. Black carp feed primarily on mussels and snails, and where present compete with native wildlife for these mollusks. Grass carp, sterile and fertile specimens of which have been found in small numbers in several of the Great Lakes, including Lake Erie, consume large amounts of vegetation, thus altering the aquatic environment and destroying habitat needed by native fish and waterfowl.

“They are all a big concern, because any invasive species has the potential to disrupt the environment in the lakes and displace native fish, but I believe it’s fair to say the bighead and the silvers present the biggest threat to the Great Lakes because they impact the bottom of the food chain,” said Travis Hartman, the Lake Erie Program administrator for the Ohio Department of Natural Resources.

“The prospect of bighead and the silver carp reaching the Great Lakes is a very serious thing, since they have a track record of dominating large sections of the Mississippi.”

A crude canal linking the two watersheds was carved into the Chicago-area landscape shortly after the Civil War, but it was dramatically expanded about 1900 to carry Chicago’s sewage overflows away from Lake Michigan and into the Mississippi River system, via the Des Plaines River. The construction of the Chicago Sanitary and Ship Canal included reversing the flow of the Chicago River so it no longer dumped into Lake Michigan, but it instead connected with the Des Plaines and, eventually, the Illinois River and the Mississippi.

“That canal is a big problem, but realistically we’ll never see it closed or a reversal of the flow — there’s way too much political and financial clout involved,” said Dave Spangler, a veteran Lake Erie charter boat captain and president of the Lake Erie Waterkeeper conservation group.

“It’s kind of scary because these fish have been on the move for decades, and they can out-feed and out-breed any native fish. What the biologists have told us all along is that if Asian carp reach the Great Lakes, then they will find Lake Erie and all of its nutrients as their version of seventh heaven. I hope we’re doing everything conceivable to stopping their movement in that canal and preventing that from ever happening.”

This canal system, later expanded with a series of dams and locks so that it could accommodate large commercial barges, provided the carp with a veritable highway to Lake Michigan. The first of the series of electrical barriers in the shipping canal that hopefully would halt the advancing horde went online in 2002. Three of the electrical barriers are now operational, while a fourth is under construction.

“When that initial barrier went up, it was the first of its kind, and at the time we were really concerned that the carp would soon be pushing right up against it,” said Marc Gaden, the communications chief and legislative liaison for the Great Lakes Fishery Commission.

“But here we are in 2017, and they are not there yet. The locks seem to have slowed them down, plus the management actions the state of Illinois has taken have removed millions of pounds of these fish from the Illinois River system. This is good news, because we certainly don’t want them challenging the barrier in great numbers.”

Biologists are much better equipped to fight Asian carp today than when the threat was still in the assessment stage. Mr. Gaden said dozens of agencies — public and private, state, federal, and provincial, and in both the United States and Canada — have joined the battle, and he said he is encouraged by what he calls an “unprecedented” level of cooperation.

“A war is what we view this as,” he said, “so we have to use any weapons we can. We want to keep these invading fish out of our territory, and push them back, if possible.”

Besides intensive study of the fish, their habits and tendencies, and a probe for genetic weaknesses, the consortium of biologists have displayed a heightened sense of urgency when dealing with Asian carp.

“Those of us around the Great Lakes have learned some very bitter lessons from dealing with previous invasives, such as sea lamprey and zebra mussels,” Mr. Gaden said. “Once they get in, your options for dealing with them are greatly diminished.”

He said the overall approach with Asian carp involves intense monitoring, mostly conducted by the Illinois DNR, active management of the existing Illinois River population of Asian carp, a series of well-mapped-out contingency plans if the fish do breach the barrier or show up in greater numbers closer to the barrier, and the development of long-term solutions to the crisis.

Netting and electro-fishing teams probe the waters below the barriers on an almost daily basis, looking for new concentrations of the carp. Farther downstream, contract fishermen, working with the Illinois DNR, have used huge nets to remove more than 5 million pounds of fish from the upper Illinois River in a project that started in 2010. 

The fish are harvested for nonhuman consumptive purposes — primarily fertilizer and pet foods. Commercial ventures farther downstream remove more of the carp from the waterway.

Kevin Irons, the aquaculture and aquatic nuisance species program manager for the Illinois DNR, said these extensive removal efforts have depleted the ranks of breeding fish in the sections of river closest to the barriers.

“We are continuing the good fight,” said Mr. Irons, who spent 10 days studying these fish in their native China, where the carp are farmed and a diet staple. “The leading edge of these fish in the Illinois River has not advanced at all. Working together, we are keeping them away from the electric barrier system and out of Lake Michigan.”

He added that the netting efforts have decreased the adult Asian carp numbers by about 70 percent in the Dresden Pool, about 50 miles from Lake Michigan. “Our goal is to maximize removal,” he said. “Certainly, we believe a heightened fishery downstream will further help reduce the population upstream, as well.”

Some studies have questioned the effectiveness of the electrical barrier on juvenile fish, and there is concern about Asian carp eggs and larval fish slipping past, hidden in voids in barges. 

Many biologists favor a system under development that would treat the water around barges as they move through the locks to kill any Asian carp larvae or eggs that could be present.

“That is the best way, in many people’s eyes, to move toward a permanent solution,” Mr. Gaden said.

Mr. Irons said seining efforts and underwater cameras are on the lookout for those smaller fish, and part of the contingency options call for the use of the poison rotenone to kill everything in sections of the waterway nearest the barriers, if needed.

“We have plans in place to go in and extensively work to get them out,” he said. “As a last resort, we have rotenone stockpiled, if we need it.”

Across Lake Michigan, on the banks for the Grand River, charter boat captain Willis Kerridge worries about the Asian carp threat, and the devastating impact these invasive fish could have on his business, which includes a bait and tackle shop in Nunica, near where the river dumps into the big lake.

“It’s been a concern for a long time, because from what we’ve learned these things could do a lot of harm in a hurry,” he said recently. “We don’t hear a lot of new information, but it’s still a worry for all of us fishermen.”

For Mr. Spangler, optimism is still elusive despite all of the progress made on the Chicago-area battlefront, given what is at stake in this war.

“Asian carp are the enemy we know all too well, since we’ve been aware of them for so long,” he said. “We can only hope that we’re firing everything we have at them, and that this approach will work and keep them out of the Great Lakes, because with invasive species you really only get one opportunity. 

“Once they’re in, there’s no second chance.”

Contact Blade outdoors editor Matt Markey at: mmarkey@theblade.com or 419-724-6068.

First Published February 26, 2017, 5:40 a.m.