Updated: April 27, 12:01 p.m. | Posted: April 14, 6:45 a.m.
Under a gray sky threatening rain, Tim Adams steered his fishing boat across the Black River near La Crosse, just upstream from where it empties into the Mississippi.
He joined a small fleet looking for something that shouldn't be here: Invasive carp.
Adams wants the carp gone.
“I enjoy the river as much as anybody, like riding around in our pontoon boat in the summertime,” Adams said. “I don't want the things jumping in the boat."
Adams, of Wabasha, fishes and boats on these waters, both for fun and as a commercial fisherman. He's worried that invasive carp will crowd out native species.
"I'm afraid these things are going to really make an impact on it,” he said.
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Last week, Adams joined a team of biologists and technicians from state and federal agencies in an operation to capture and harvest invasive carp from this stretch of the Mississippi River known as Pool 8, on the border of Minnesota and Wisconsin.
It was part scientific survey, part defensive battle against a notorious aquatic invader that has been steadily progressing up the Mississippi.
During the weeklong effort, the team removed a total of 31 silver carp. Nearly that many jumped over their nets and escaped, said Jordan Weeks, supervisor with the Wisconsin Department of Natural Resources.
"That tells us there's at least a population of fish here,” Weeks said. “It's not a large population, but it's something that we’ve got to definitely keep our eyes on."
Invasive carp were brought to the southern United States in the 1970s to control algae in fish farms. They escaped, and have been making their way up the Mississippi ever since.
The term invasive or Asian carp generally refers to four species — silver, bighead, black and grass carp. They’re large, voracious eaters that tend to crowd out and outcompete native fish species. Silver carp are known to leap out of the water, sometimes into boats.
"Any time that you have a large increase in invasive species, it's concerning,” said Hallie Rasmussen, visitor services manager for the Upper Mississippi River National Wildlife and Fish Refuge, which extends 261 miles from Wabasha, Minn., to Rock Island, Ill.
"Because you know that they're spreading and potentially moving even further north from here, which would be obviously affecting the other native fish that are in this habitat."
In recent years, flooding on the Mississippi helped invasive carp migrate more easily upstream, as gates on locks and dams were opened and water flowed freely.
Fishing crews had caught invasive carp in this area, but mostly just one or two at a time. Then, last spring, came a startling discovery: About 50 silver and grass carp caught just south of here.
In response, fish specialists decided to try a herding and harvesting technique developed in China.
It's been used in other parts of the country, including on the Illinois River since 2016, but never this far up the Mississippi or in any Minnesota or Wisconsin waters, said Randy Hines, wildlife biologist and outreach coordinator with the U.S. Geological Survey, which participated in the capture.
“The idea is mass harvest, if they are here. That reduces any pressure on the native fish,” Hines said. “And also, we're deploying this technique to learn about whether we can use it for a quick response.”
The crews started by walling off a section of the channel with huge block nets two or three football fields long. The nets stretched from the surface of the water all the way to the river bottom, held in place by lead weights and floating buoys.
With additional nets, they cordoned off the channel one section at a time, creating compartments or cells. Then, they drove the fish out of each cell using a combination of tactics that invasive carp hate: using boats that put an electric current in the water, and others outfitted with underwater speakers.
Native game fish aren't as bothered by the sound, and prefer to hide, he said. The boats also used sonar to determine the location of the carp.
Once a cell was cleared, it was closed off to prevent the fish from returning. The process was repeated one cell at a time, concentrating the fish into a smaller area.
Then, Adams and his crew of fishermen encircled the area with a seine net and started hauling it in, using their boats and hydraulic winches and just pulling, hand over hand.
The process took hours. Sometimes the net snagged on a sunken tree limb, and the crew had to free it.
Eventually, the circle of net closed as it was slowly pulled toward shore. Crew members wearing rubber hip waders and gloves stood in the knee-deep water. They grabbed each flopping, writhing fish and identified it. If it wasn’t an invasive carp, they tossed it back into the water.
Finally, the net was empty. They didn't find what they were looking for. Weeks called it good news that there were no silver or bighead carp among the catch that day.
"The common carp was in there, but that's not what we're really worried about,” he said.
But Weeks said they know there are invasive carp in these waters. While their work has wrapped up for now, he said they'll be back in this area again, hoping to keep an eye on this unwanted fish and prevent it from becoming a permanent presence in the region.
Correction (April 27, 2021): An earlier version of this story misidentified Hallie Rasmussen in a caption. The story has been updated.