On the Prowl for Invasive Carp in Chicago

We’ve all heard of looking for a needle in a haystack. But what about looking through a haystack for a needle that may or may not be there? For the Seasonal Intensive Monitoring project, that’s exactly what it’s like to seek out invasive carp in Chicago.

Chicago is home to the Chicago Area Waterway System (CAWS), which represents the only direct connection between the Mississippi River basin and the Great Lakes basin. The connection is man-made, the result of canals constructed for shipping and wastewater management. This makes the CAWS a vulnerable pathway for invasive bighead and silver carp (hereafter for simplicity, invasive carp) to assault the Great Lakes (see map). These fish, introduced to the United States in the 1970s, are filter feeders with the capability to impact the planktonic base that supports the rest of the aquatic food web. Capable of producing millions of eggs a year, silver carp especially are notorious for being easily startled, springing out of the water in huge schools at the slightest disturbance. After escaping from flooded aquaculture ponds in the southern U.S., invasive carp have steadily expanded their range through the Mississippi River basin. The invasion front in northeastern Illinois has remained static around the town of Channahon, in the Des Plaines River, approximately 47 miles from Lake Michigan.

One of the primary lines of defense against invasive carp is the Electric Dispersal Barrier System. Located on the Chicago Sanitary and Ship Canal approximately 37 miles from Lake Michigan in Romeoville, the barriers send a strong electric current through the river that prevents fish from moving upstream towards Lake Michigan while still allowing for commercial and recreational navigation of the waterway. These barriers, the first one becoming operational in 2002, are just one part of a suite of detection and control projects aimed at preventing invasive carp from moving into the Great Lakes. Notably, the Illinois Department of Natural Resources (IDNR) contracts commercial fishers to hunt down and remove all the invasive carp they can find, reducing the threat of invasive carp approaching the electric barriers.

While carp management efforts are highly effective, no deterrent system is 100 percent foolproof. A live carp could be transported accidentally through the electric barriers on a shipping barge, an unwitting fisherman could dump a bait bucket contaminated with invasive carp upstream of the barriers, and other unlikely but not impossible scenarios exist that would allow invasive carp to circumvent existing defenses despite our best efforts. Therefore, it is critical that a detection system is in place to identify and remove any invasive carp that are present upstream of the electric barriers.

Enter Seasonal Intensive Monitoring, or SIM for short. For two weeks each spring and fall, a veritable armada of sampling vessels descends upon the more than 77 miles of navigable waterway upstream of the electric barriers with the goal of seeking out any invasive carp that may be present. In a prime example of interagency collaboration, these vessels, and the biologists that crew them, come from the IDNR, Illinois Natural History Survey, U.S. Fish and Wildlife Service, and U.S. Army Corps of Engineers. Also participating is the same team of IDNR-contracted commercial fishers that work to detect and remove carp downstream of the electric barriers. These crews work together to scour the Chicago Area Waterway System looking for any hint of invasive carp. Their primary tools of the trade are boat electrofishing and gill netting, but commercial seines, tandem trap nets, hoop nets, fyke nets, and pound nets have also been deployed as part of the search. In boat electrofishing, electrified droppers are deployed in front of the boat to create an electric field which causes fish to rise to the surface of the water for collection by dippers with nets. Gill nets consist of wide panels of mesh and are deployed in the water column by contracted fishers, who then create noise by revving their motors or banging their boats to drive fish into the nets, which are hauled back into the boat and examined. With seining, a long net is set in a circle or arc and slowly tightened, scooping up the fish inside. All fish are identified and counted, and all native species are returned to the water where they were sampled.

Sampling crews divide their time between fixed sites, which are at the same location every time, and random sites, which vary year to year. Crews work in all kinds of habitat, ranging from more natural sections of the CAWS with vegetated banks to the sheet-piling walls of downtown Chicago and everything in between. This sampling methodology, combined with close examination of any captured fish, ensures that the CAWS receives thorough coverage and is the best way to find invasive carp. In 2022 alone, sampling crews completed 148 hours of electrofishing, set 98 miles of gill net, and set 2 miles of commercial seine. More than 37,000 fish were sampled representing 65 species and three hybrid groups. Since 2010, sampling crews have conducted 1,605 hours of electrofishing, set 992 miles of gill net, and set 16 miles of commercial seine. Between 2010 and 2022, a total of 550,706 fish representing 89 species and nine hybrid groups were sampled in the CAWS. The fish species diversity represented in the SIM samples helps provide an important reference for researchers and managers in a river system that often is considered highly impacted by urbanization.

In the more than half a million fish that have been sampled in the CAWS using SIM since 2010, only two live invasive carp have been collected: a bighead carp in Lake Calumet in 2010, and a silver carp in the Little Calumet River in 2017. An additional live silver carp was collected in Lake Calumet in 2022 outside of SIM sampling. Those three fish represent about 0.0005 percent of all SIM fish captures upstream of the electric barriers, a very low rate. In the vast majority of SIM events, not a single bighead or silver carp was captured or observed. It is extremely difficult to prove the absence of a species, but SIM provides the best possible assurance that invasive carp have not become established upstream of the electric barriers. The lack of invasive carp is an important notation for those concerned about the health of the Great Lakes. In the extremely rare event that a live invasive carp is detected upstream of the electric barriers, detailed contingency response plans are immediately activated that involve high level collaboration at several agencies as well as coordinated on-the-ground efforts to contain the threat. The response varies depending on the exact circumstance of capture; most recently, swarms of sampling teams have exhaustively targeted the vicinity of capture to determine whether any additional carp are present. To date, rigorous contingency response actions have not resulted in the capture of any additional bighead or silver carp upstream of the electric barriers.

Is it possible to identify the pathways that these three invasive carps used to cross the electric barriers? Structures called otoliths, or ear-stones, that are harvested from inside the carp’s skulls, can help provide some information on their life history. These otoliths hold trace chemical deposits, which can be analyzed to help determine where the fish may have previously inhabited. Analysis of the carp captured upstream of the electric barriers showed chemicals consistent with residency in the Illinois River downstream of the electric barrier, suggesting that the invasive carp were not born upstream in the CAWS, but rather moved up. But whether the fish moved themselves by crossing the electric barrier or whether they were transported some other way like on a barge is impossible to determine. And the possibility, however slim, does exist that other invasive carp could follow the same pathway. As long as the threat of invasive carp persists, SIM will continue to provide the scientific expertise to determine any changes to invasive carp population abundance upstream of the electric barriers as well as first responder capabilities to remove any invasive carp that are detected. Whether the needle in the haystack is present at any given time or not, SIM crews will still be out conducting meticulous sampling to ensure the Great Lakes are kept safe.

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Claire Snyder is a Natural Resources Coordinator with the Illinois Department of Natural Resources. She has been with the department since 2020 and is based out of the Yorkville office in Silver Springs State Park. This office is responsible for detection and removal efforts of invasive carp in northeastern Illinois, as well as conducting standardized fish community sampling to inform on the impacts of invasive carp. She previously worked out of the same office as part of the Illinois History Survey, and currently also helps manage the state’s aquaculture program. She obtained her Master’s of Science in Zoology from Southern Illinois University, researching native fish passage in northeastern Illinois.