Herding carp with sonic prods




It’s like a submarine movie, but instead of “The Hunt For Red October,” think “The Hunt For Silver Carp” — complete with sonic depth charges and electromagnetic disruptions spurring invasive fish right into naturalists’ trap on the Mississippi.

The Minnesota Department of Natural Resources (DNR), together with their opposite numbers in Wisconsin, as well as several federal agencies, on Monday began an experimental method for controlling invasive carp species. The boat sorties will target pernicious varieties of fish, such as silver carp and grass carp, while attempting to avoid beneficial native species. If it works, the DNR can repeat the method as an early-warning system in other water bodies to determine how far and to what extent the foreign carp species have invaded. To spook the carp into going where scientists want them to, sound waves from underwater speakers will serve as a shot across the bow. 

Instead of the North Atlantic, their hunting grounds for an underwater foe are in Pool 8 of the Mississippi River. Their battle plan is the Modified Unified Method (MUM), developed by the United States Geological Survey (USGS) based on commercial fishing methods observed in China. It has only been attempted a handful of times before in the U.S., and never in Minnesota. 

It was pioneered by Missouri-based fish biologist Duane Chapman. In addition to being a research scientist at the USGS’s Columbia Environmental Research Center, Duane Chapman has garnered national fame as an invasive-carp talking head, an expert consulted by national outlets when they do stories on the carp problem sweeping America’s waterways. During a trip to China, Chapman learned that Chinese fish harvesters could get as much as 85 percent of targeted fish by dividing the water into segments and then herding the fish by banging on the side of their boats. “And so I said, ‘I want a piece of that action,’” he recalled. 

According to a profile of Chapman in his hometown paper, the Columbia Missourian, he has had a connection to fish and their environment for almost his entire life. His father died when he was just four years old, so he was taught to fish by his paternal grandfather and uncles. Decades later, his pastime is now threatened by the spread of invasive species of carp, which crowd out and suppress native species beloved by anglers and essential to the ecosystem. Carp tend to particularly affect crappies, since they eat the same zooplankton as crappies do. 

Silver carp can also pose a direct threat to boaters’ personal safety: they have a habit of jumping up out of the water when frightened. Chapman himself has been hit in the face by an errant carp. 

To achieve the goal of herding the carp into a confined space so they can be plucked out of the water, MUM combines conventional fish harvesting elements with a brand new techno twist. DNR crews will drop the equivalent of about 25 miles of netting, which segments the water into different cells. Then, they will shock the water with electricity, giving the fish what Chapman describes as a “tickle” that startles them and causes them to flee. The boats clear each cell one by one so the invasive fish have a smaller and smaller space to run to. A sonar sweep by the command boat tells the naturalists when each cell is clear, so they can move on to the next one. 

The systematic cell technique is the essence of the Chinese Unified Method, and the USGS improved on it by adding sonar and electrolysis to help create MUM. But MUM takes the Chinese method one step further. 

Using electrofishing equipment risks electrotaxis, a condition wherein the natural electromagnetic field the fish use as a compass is disrupted, so that they are compelled to move toward the disruption. The Pied Piper effect is nice for commercial fishermen, but scientists using MUM want the fish to move away from the boat, not toward it. 

However, biologists have studied invasive carp to the degree that they can predict how carp will react to a more refined type of stimuli, in a way that makes them different from other kinds of fish. In response to bursts of sound waves, native fish will hide — going deeper to escape — whereas invasive carp will run, by fleeing horizontally. That gives naturalists the advantage of being able to scare some fish in one direction, other fish in a different direction, an advantage which MUM capitalizes on. 

To help corral the invasive carp, the mission in Pool 8 will use underwater speakers playing a recording of a hammer hitting metal, which has been sped up to a higher pitch. The resulting sound is akin to firecrackers going off. If someone was forced to listen with a hydrophone, they might be driven batty, but to the normal human ear above water, the sound is barely audible. 

Chapman said the electric field is a sort of failsafe in case the fish are curious about the sound or want to fight it, instead of fleeing. 

“We use settings and probes on the electrofishers that are not ideal for normal electrofishing work, because we want our electrofishing field to be broad and felt as far away as possible but to paralyze or roll over as few fish as possible,” he said. 

The electric/sonic pings drive the carp from the hounds to the hunters. Once trapped in a tidy small area, a commercial fishing seine will scoop them up for federal scientists to dissect and study. 

The most fish Chapman and his team have ever pulled up during past MUM runs is about 240,000 pounds, he said. That’s 120 tons of floppy, ugly, invasive carp. 

However, Chapman doesn’t anticipate getting that much during the Pool 8 mission. The MUM expedition south of Winona is the first time the technique will be used in a preemptive way, as a sort of reconnaissance. DNR naturalists have already bagged more than 50 silver and grass carp in Pool 8 over the past 12 months, but without going after the schools with the intensity of a MUM operation, they can’t be sure how many more are lurking below the surface of the water. 

The study goes far beyond simply counting the number of carp caught. After the carp are dissected, scientists can use the fish chunks to determine the carp’s age, which in turn indicates how long they’ve been present in a particular water body. Forensic analysis of the dead fish’s DNA can also reveal their origins and helps tell the story of how they got into this area of the Mississippi River. 

Even if they’re not in command of a sonar-equipped armada of scientists, nature-conscious anglers can still help the DNR track invasive carp. If one is caught, call 651-587-2781 or email invasivecarp.dnr@state.mn.us. Take a picture of the carp and submit it to the DNR, so it can be studied. 



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