Electric currents, underwater speakers scare up invasive Asian carp in Mississippi River near La Crosse

LA CROSSE – A fishing expedition for a highly invasive species of carp is underway in the waters of the Mississippi River. 

Armed with underwater speakers, electrofishing technology, large boats and nets, employees from the Wisconsin and Minnesota Departments of Natural Resources are trying to find out how many carp have made their way up the river to La Crosse, and how many of them may be spawning new carp into the environment. 

The operation so far has netted about 30 carp over the first few days of the operation, enough to establish that the fish have been able to finagle their way this far north, putting other fish and organisms in danger. 

The effort is focused on taking on bighead, grass and silver carp, non-native species that have caused damage to ecosystems in Illinois, Missouri and Ohio river systems, and are spreading north via the Mississippi. The portion of the river in which this effort is taking place is called Pool 8. 

The removal of the invasive fish comes in response to the discovery of 39 silver carp and 12 grass carp in the Pool 8 region in 2020, as well as an additional eight carp caught in the area this year. 

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Not exactly known for their good looks, carp are a family of fish native to Europe and Asia that can grow to be 100 pounds, according to the National Park Service. The fish were introduced in the U.S. in the 1970s to control algae, weeds and parasitic growth in aquatic farms, and weeds in canal systems and lakes within the Mississippi River watershed.

Workers from the Wisconsin DNR, Minnesota DNR, U.S. Fish and Wildlife Service and U.S. Geological Survey remove netted fish as part in an intensive invasive carp removal effort Thursday, April 8, 2021, on the Mississippi River in La Crosse. They found only four common carp in the net along with a variety of other species. The goal is to remove invasive carp in Pool 8, south of Lock and Dam No. 7, and to curb the potential for invasive carp reproduction and prevent their establishment in the Upper Mississippi River Basin.

The fish eventually escaped into the Mississippi River basin and began breeding, slowly making their way up the river. Carp are known for being able to lay thousands of eggs at a time, quickly increasing their numbers. 

The invasive fish are a threat to the aquatic ecosystems because they out-compete other fish for food and space. Carp are also thought to lower water quality, killing off organisms such as native freshwater mussels.  

Work began on the Mississippi River on April 5, and in addition to natural resources staff, the U.S. Geological Survey and the U.S. Fish and Wildlife Service are aiding the effort. The work is likely to last up to 10 days, with pauses on Saturdays and Sundays. Commercial fishing is temporarily suspended, as well as recreational uses of the area. 

So far, the operation has netted about 30 carp, with an additional 35 fish escaping capture. On Thursday, a day members of the media were allowed to observe the operations, no targeted carp were netted and an estimated 10 invasive carp jumped over nets during the day.

How the fish are captured

The fish are being captured using a technique called the Modified Unified Method, which combines netting and herding maneuvers to drive and concentrate fish from a large area into a small zone for removal.

According to the USGS, the method uses block nets to create compartments or “cells” from which the fish can be driven. Then electrofishing boats that emit electric currents to drive the fish and boats outfitted with underwater speakers are used to herd carp from cell to cell. 

When one cell is cleared, another net is used to close the cell and prevent the fish from returning. The process is repeated one cell at a time, gradually creating a smaller area available to the carp and concentrating the fish into a harvest removal area, where a large commercial net will be used to capture the congregated carp.  

Fish native to the area do not respond the same to the sound stimulus, preferring to hide rather than run and therefore are not captured. 

This is the first time the method has been used in Wisconsin or Minnesota, and the effort in the Mississippi River near La Crosse is the first time it’s been used as an early detection and rapid response technique. 

Workers from the Wisconsin DNR, Minnesota DNR, U.S. Fish and Wildlife Service and U.S. Geological Survey take part in an intensive invasive carp removal effort Thursday, April 8, 2021, on the Mississippi River in La Crosse, Wis.

Technique learned in China

The technique was learned overseas and brought back to the states, said Randy Hines, the wildlife biologist and outreach coordinator for the U.S. Geological Survey. 

“We went to China back in 2015 and learned how they were using the unified method to harvest carp in their country,” he said. “In the US, adopting that procedure has been looking at using innovative new technology like electrofishing gear, as well as speaker boats with sound speakers underwater and then using sidescan sonar.” 

The goal is to remove the invasive fish and curb the potential for reproduction and prevent the species from taking hold in the Upper Mississippi River Basin. Officials also hope to gain more knowledge about the location of different types of invasive carp in the pool. 

Once removed from the water, the carp are taken to a USGS office where they check for the fish’s health, age and breeding ability, Hines said. 

Five carp previously tagged with acoustic transmitters will complement this effort by increasing the effectiveness of targeting fish in the area and providing insights into their movements and other behaviors. 

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While carp have been located in small numbers all along the northern portion of the Mississippi River, they haven’t yet been spotted in Lake Michigan, though DNA of Asian carp have been detected near the lake in an Illinois lake.

Previous efforts to stop the spread of invasive carp in river systems have failed.  

The fish have been able to easily spread, according to the National Park Service, because they’re capable of jumping over barriers such as low dams. Flooding also increases carp travel by connecting bodies of water not normally connected. 

Steps people can take to prevent the spread of carp in the Mississippi River and surrounding areas, according to the National Park Service, include trailering boats around locks, not harvesting bait or transporting any water from infested areas, disposing of unwanted bait in the trash, never releasing fish from one body of water into another, and draining and rinsing your boat when you’re done boating. 

Invasive carp captures must be reported immediately. If you believe you have captured an invasive carp, put it on ice and send a picture of the carp to Jordan Weeks, Wisconsin DNR Mississippi River fisheries biologist, at Jordan.Weeks@wisconsin.gov or 608-386-0970.

Mark Hoffman of the Journal Sentinel contributed.

Laura Schulte can be reached at leschulte@jrn.com and on Twitter at @SchulteLaura