An Underwater Fence to Stop Invasive Species
Monday, July 9, 2007
A century ago, engineers blasted the Chicago Sanitary and Ship Canal out of limestone to reverse the Chicago River, an unprecedented engineering feat that linked Lake Michigan with the Mississippi River.
The project served its purpose, sucking water from the lake to flush out the growing city's waste, but there was an unintended consequence: It created a pathway between two previously distinct ecosystems and a route for invasive species traveling in both directions. Until recent years, though, heavy pollution in the canal prevented fish from swimming through it.
Today, the canal is cleaner, and the infamous Asian carp and the round goby are both threatening to take advantage. But the government is stepping up its efforts to stop them.
The carp, prolific filter feeders that wipe out food supplies for other fish, have been slowly making their way up the Mississippi since escaping into the wild after they were imported in the 1960s and '70s to control algae in Southern fish farms.
The voracious carp, which can grow to 50 pounds, are within 50 miles of the lake, and the only thing stopping them from completing the journey is an underwater "electric fence," which the U.S. Army Corps of Engineers constructed under the canal. It was activated in 2002 -- the first such barrier ever installed in a navigable waterway.
The U.S. Fish and Wildlife Service monitors the advance of the Asian carp with an annual "carp corral," in which workers count the carp netted at checkpoints along a stretch of the Illinois River.
During the corral last month, at a spot in the river about 100 miles from Chicago, scores of bighead and silver carp rocketed into the air, stirred by vibrations from boats.
"We've had lots of broken noses and damaged boats," said Pam Thiel, a project director of the U.S. Fish and Wildlife Service.
The survey netted more than twice as many Asian carp at that location as last year: 241, compared with 110. "They haven't moved much upstream since 2002, but their numbers are increasing," Thiel said.
The underwater barrier has outlived its predicted five-year life span; the bundles of steel cables, which electrify a 54-foot-long stretch of canal with a one-volt-per-square-inch charge, are still working.
People feel just a tingle, or nothing at all, if they stick their hands in, Corps engineers say. But the effects could be more serious if someone were to fall into the canal. The Corps is working with the U.S. Navy to figure out, through modeling, how the electricity would affect a person who falls overboard.
"If someone were to fall in the water, what would happen?" asked Commander Paul Mehler III, commanding officer of the U.S. Coast Guard Marine Safety Unit Chicago. "We really don't know."