GENOA, Wis. — Any invasive carp trying to swim up the Mississippi River through the shipping lock here will get an earful.
Five underwater speakers blare out a racket equivalent to about 20 outboard motors — a sound unpleasant to humans, unnoticed by native fish and super-annoying to invasive bighead and silver carp, so much so that they turn tail and swim back downstream, researchers hope.
The “acoustic deterrent system,” designed to slow the upstream migration of the non-native fish often known as Asian carp, is activated every time the downstream gates of Lock and Dam No. 8 south of La Crosse open.
The experimental project — believed to be the largest underwater speaker system in the world — is the brainchild of University of Minnesota scientists and has been in operation for about a week, researchers said Monday.
“It produces a sound that we know, from experiments in the lab and observations in the field, they hate,” said Peter Sorensen, a professor and lead researcher at the U’s Aquatic Invasive Species Research Center. “This is why they jump.”
Silver carp are known for leaping into the air — sometimes by the hundreds — as motorboats speed through their river habitats.
Voracious feeders on plankton, which are at the base of the underwater food chain, the carp are seen as a major threat to fish and other aquatic life throughout the Upper Midwest.
In areas farther south where they have established themselves, Asian carp make up the majority of the biomass, to the detriment of native fish, mollusks and other life forms.
The possibility that invasive carp might crowd out walleye, northern pike and bass in the headwaters of the Mississippi River in the heart of Minnesota — harming the state’s freshwater fishing industry — has generated concern among state and federal officials.
Gov. Mark Dayton, for example, has convened summits on the issue, and the state’s congressional has proposed legislative action.
This year, Congress approved closing the Upper St. Anthony Falls lock in Minneapolis to help prevent the carp from migrating farther upstream.
The sound system at Lock and Dam No. 8 is part of a larger strategy to slow the advance of the fish.
“We’re trying to buy as much time as we can while we learn more about these fish and how to deal with them,” Sorensen said Monday as the gates opened for a motorboat passing through the lock.
Built during the Great Depression, the lock-and-dam system creates what is often called a gradual “staircase” for boats to ascend and descend the hundreds of feet of elevation difference between the Gulf of Mexico and Minneapolis, the farthest point upstream where commercial barges travel the river.
“It’s the Achilles’ heel of the system,” said Sorensen, referring to how the opening and closing of lock gates can allow the carp access to another part of the river.
Individual fish have been found as far north as the Ford Dam (Lock and Dam No. 1) in the Twin Cities, but no breeding populations are believed to exist above Lock and Dam No. 19 in Keokuk, Iowa, along the Illinois border.
Many scientists say they believe the fish could thrive upstream but simply haven’t invaded in large enough numbers to support a breeding population.
Last winter, scientists mistakenly announced the discovery of fertilized invasive carp eggs in the waters immediately below Lock and Dam No. 8. Although scientists later concluded the eggs were not fertilized eggs from invasive carp, the possibility added urgency to Sorensen’s plan to adapt locks and dams to reduce the likelihood of a carp invasion.
He and his team, including postdoctoral associate Dan Zielinski, designed a three-year plan for the lock and dam at Genoa, which is below the Mississippi’s confluence of many important tributaries, including the Minnesota and St. Croix rivers.
The $75,000 sound system, paid for with state lottery funds and nearly $7,000 in private donations, is part of that plan.
The other part involves adjusting the speed of waters flowing through the dam’s gates. Carp aren’t particularly strong swimmers, and the vast majority of them could be kept from passing through the dam gates by changing how the U.S. Army Corps of Engineers operates the dam, Zielinski said.
Such talk worries Mark Clements, who runs Clements Fishing Barge about 300 feet downstream from the dam. The business, which was started by his grandfather in 1936, operates piers where anglers can fish for a fee.
“The main concern for us is how is this going to affect the native fish,” Clements said Monday shortly before hailing Sorensen to discuss the project with him.
Among his questions: Will the underwater speakers frighten off native fish that benefit from being able to migrate through the locks? And will changes to the flow of water coming through the dam gates alter the locations of game fish his business and its clients rely on?
To both questions, Sorensen and Zielinski say no.
Zielinski said the changes to the flow rates coming through the dam gates, which are routinely raised and lowered for flood control and commercial shipping needs, would be “subtle” and within the ranges the Army Corps of Engineers already sets.
As for the racket from the underwater speakers, Sorensen said invasive carp have hearing thousands of times more sensitive than native fish species, which shouldn’t hear the sounds as well. Nonetheless, he hopes to team up with federal and state researchers to monitor how native fish behave.
Under normal operation, the racket from the underwater speakers is hard to hear above the water. And unlike the blare beneath the surface, the sound is similar to crickets chirping.
At one point during Monday’s demonstration to the media, Zielinski cranked up the volume to broadcast a more recognizable — and ominous — sound. It was composer John Williams’ “The Imperial March” from the movie “Star Wars.”
