Chris Young/The State Journal-Register, via Associated Press
The closest sighting of Asian silver carp on the Illinois River was 55 miles from Lake Michigan.
The most effective methods of keeping Asian carp from invading the Great Lakes via Chicago's web of waterways could cost up to $18.4 billion and take 25 years to put in place, the federal Army Corps of Engineers concluded in a study released Monday.
But a corps official cautioned in a telephone briefing for journalists that there was no guarantee that the carp or other unwanted species would not get into the lakes by then.
The agency's 210-page study, first ordered by Congress in 2007, laid out eight options to prevent the carp and other unwanted species from entering Lake Michigan, ranging from continuing existing efforts to building barriers that would seal the lake from the five Chicago-area streams that are linked to it.
Either blocking the lakefront waterways or blocking their two sources further inland would offer the greatest protection from invading species, the report said. But both options would prevent barges and other boats from using those routes, and would increase pollution in the lake and the waterways.
Most of the other options would be cheaper and would preserve some access to the lake, but would be somewhat less effective.The report arrived amid growing concern that some so-called nuisance species, led by two strains of the carp, may already have bypassed existing barriers and entered Lake Michigan. The carp, which multiply quickly and eat huge amounts of plankton, are seen as a threat to commercial and sport fish that feed on plankton during at least some stages of their lives.
A water sample collected last May near Green Bay, Wis., contained DNA fragments of silver carp. After that discovery, senators from Great Lakes states called for immediate action to block a carp invasion. In 2012, Congress ordered the secretary of the Army to start designing and preparing to build an effective barrier should it be deemed justified.
Corps officials said Monday that no barrier would be built without holding public hearings, consulting the many government agencies with a stake in the matter and getting Congress's blessing.
Conservation groups generally want carp and other invasive species to be blocked as quickly as possible, but commercial shippers and recreational boat organizations have expressed concerns about options that would seal off the lake.
Officials have taken a number of steps to keep the carp out of the lake, including physically removing scores of thousands of them from the Illinois River, the source of all five waterways, and installing an underwater electric barrier on the Chicago Sanitary and Ship Canal inland from the five streams.
The electric barrier was once believed to be effective, but a recent report by the Army Corps and the federal Fish and Wildlife Service said entire schools of fish frequently slip through it, swept along by barge wakes or in water beneath metal boat hulls where the electric current is weakened.
Federal officials say there is no evidence that carp have entered Lake Michigan, noting that the closest sighting of the fish in the Illinois River was still 55 miles from the lake.
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