In 2009, Illinois treated a six-mile canal with more than 1,800 gallons of fish-killing poison — and found just one invasive carp

In December 2009, Illinois officials made a remarkable decision to protect the Great Lakes from an invasive fish threat: They treated roughly six miles of the Chicago Sanitary and Ship Canal with more than 1,800 gallons of rotenone, a chemical used to kill fish.

The operation was designed to stop Asian carp from using the canal as a gateway from the Mississippi River basin into the Great Lakes. What happened next was both dramatic and surprising. Hundreds of thousands of pounds of fish were recovered from the canal, yet crews found just one bighead carp.

The unusual operation took place while an electric fish barrier was temporarily shut down for maintenance. Officials were concerned that the interruption could give invasive carp an opportunity to move through the canal, particularly because Asian carp environmental DNA had already been detected in the area.


The Chicago Sanitary and Ship Canal is an artificial waterway connecting the Great Lakes drainage system with the Mississippi River basin. That connection created an ecological pathway for invasive aquatic species to move into waters where they could potentially disrupt native ecosystems.

Asian carp, particularly bighead and silver carp, were considered a serious threat. Bighead carp can grow extremely large and consume huge quantities of plankton, potentially competing with native fish. Silver carp are also notorious for their spectacular jumping behavior when startled by boats.

To slow their advance, authorities relied heavily on an electric barrier system. The barriers created an electrical field in the canal designed to discourage fish from moving upstream. But the system required periodic maintenance.

That created a problem in late 2009.

When Barrier IIA was scheduled for maintenance, officials faced a temporary vulnerability in the canal's defenses. Rather than leave the area unprotected, they decided to use rotenone.

The treatment began on December 2, 2009, covering approximately six miles of the waterway. More than 1,800 gallons of rotenone were injected at multiple locations. Some accounts put the quantity closer to 2,200 gallons, reflecting differences in how the treatment area and chemical use were reported.

Officials also used Rhodamine WT, a red dye, to track the movement of the chemical through the canal. Once the treatment was complete, the rotenone was neutralized to prevent it from spreading into connected waterways.

The result was an enormous fish kill.

More than 200,000 pounds of fish were recovered, with common carp making up a substantial share of the haul. But the number of dead fish was believed to be considerably higher because December's cold water caused many carcasses to sink before crews could collect them.

And then came the surprising part.

Despite the enormous number of fish killed, crews recovered only one bighead carp.

That single fish mattered enormously.

Environmental DNA testing had already detected genetic traces associated with Asian carp, but eDNA does not necessarily reveal exactly how many live fish are present or where they are located. Finding an actual bighead carp provided physical confirmation that the invasive species was indeed present in the canal.

The discovery also demonstrated just how difficult it could be to determine the size and location of an invasive fish population.

After the treatment, crews continued intensive searches using netting, electrofishing and additional eDNA sampling. Operations in other portions of the Chicago Area Waterway System produced hundreds of fish but did not uncover additional Asian carp.

The 2009 operation ultimately became an example of the extraordinary lengths authorities were willing to take to prevent Asian carp from reaching the Great Lakes.

It also exposed a major weakness in relying on a single defensive system. Electric barriers could slow fish movement, but they needed maintenance and could not simply be expected to operate without interruption.

The lesson was clear: protecting the Great Lakes required multiple layers of defense — physical barriers, environmental DNA surveillance, fishing operations and the ability to respond quickly when a potential invasion was detected.

In the end, Illinois used thousands of gallons of fish-killing chemical to treat miles of canal and recovered an enormous quantity of dead fish.

But among them, one bighead carp may have been the most important fish of all.