In the middle of the Chicago River, Lindsay Chadderton leans over the side of an 18-foot aluminum fishing boat and collects another two-liter water sample. Like a crime scene investigator, he's searching for evidence of invaders.
Chadderton, The Nature Conservancy's Great Lakes aquatic invasive species director, collects these freshwater samples to test for the presence of Asian carp, an invasive species that has left a trail of destruction during its 30-year migration up the Mississippi and Illinois rivers. Working with a team of scientists from the University of Notre Dame, Chadderton and his colleagues amplify any environmental DNA (eDNA) present in the water by filtering out DNA likely associated with scales, feces or mucus shed by the fish. Through this detective work, they are able to determine if Asian carp (bighead or silver) are present. Unfortunately, the results have been positive.
Asian carp eDNA has been found in eight locations throughout the Chicago waterway system north of underwater electric barriers constructed to keep the fish out of Lake Michigan. In June 2010, an adult bighead carp was captured in Lake Calumet upstream of the last potential barrier — the O'Brien shipping lock, just six miles from Lake Michigan. This is the first fish captured above that barrier, and its presence is consistent with the detection of DNA a few miles downstream of Lake Calumet in 2009. Chadderton and his collaborators at Notre Dame have pioneered the use of eDNA as a detection method for Asian carp.
"The eDNA detection method has shown we have a problem — some fish have clearly gone above the barriers and have likely entered Lake Michigan," Chadderton says. "Without our results, the management agencies would probably still be assuming the carp were 15-20 miles downstream of the electric barriers."
Asian carp are just one type of many aquatic invasive species making their way into Illinois' freshwater systems. Illinois waterways are vulnerable to invasion from many directions. The extensive canal and waterway system that connects the Mississippi River to the Illinois River and the Great Lakes enables the passage of invasive species from either basin. For example, maritime cargo ships entering the Great Lakes were responsible for introducing zebra mussels, which subsequently invaded the Mississippi basin via the Chicago canal system.
Recognizing how Illinois' freshwater systems are connected, the Conservancy works to both prevent and manage aquatic invasive species. Director of Science for the Conservancy in Illinois Dr. Jeff Walk identified reed canary grass as one of the most pervasive aquatic invasive species the staff faces at Conservancy preserves. The plant produces seeds that germinate for years, requiring staff to treat infested areas with herbicide year after year, Walk says.
At the Emiquon and Spunky Bottoms wetland restorations, the Conservancy is working to control Eurasian milfoil, a plant with feathery underwater foliage that was once commonly sold as an aquarium plant, and common carp, which can destroy aquatic vegetation by stirring up sediment and reducing water clarity when their numbers are too high.
Additional tactics employed by the Conservancy and its partners include educating boaters and enacting an "early detection, rapid response" approach. Scientists are currently testing one method of biological control at Emiquon to help manage common carp. High populations of largemouth bass and other native predatory fish such as bowfin are being maintained to help reduce the number of common carp. If this approach is successful, Walk said the population can be maintained at a low level as to not cause other problems in the system.
"Aside from the direct destruction of natural areas, invasive species are the most serious threat to biodiversity worldwide," Walk says.