Thousands of invasive carp swim the channel of Ramsey County's Rice Creek and its connected lakes, rooting along the bottom and setting off a cascade of damaging effects that harm native fish, birds and plants.
Now University of Minnesota ecologists, who have spent years studying the life cycle of this unwanted fish in the Rice Creek system, are using that research coupled with new technology including "an electric fence for fish" to remove thousands of carp each spring.
"We will not meet our water-quality goals for these lakes without managing carp," said Matt Kocian, lake and stream specialist for the Rice Creek Watershed District, which is managing the carp removal program.
The goal is to remove the carp, which will improve water clarity, lower algae blooms and support native species.
"There is a huge biomass of carp in that system," explained University of Minnesota assistant research professor Przemek Bajer, who is part of the U's Minnesota Aquatic Invasive Species Research Center. "To restore that system you need to remove 80 percent of those carp."
Last year, was their first full season of operation on Rice Creek in New Brighton. Bolstered by those promising results, they're back again this year for the spring spawn.
"It went well. We captured two-thirds of the spawning run. That's a little over 10,000 carp," said Bajer, who started the company Carp Solutions to develop technology and handle the removal.
Last summer, they removed an additional 6,000 carp with baited traps. The Rice Creek watershed includes Long Lake in New Brighton and the Lino Lakes chain of lakes in Anoka County.
"I think we put a big dent in the population last year," Kocian said. He said the watershed has invested about $200,000 in the equipment and installation. A state Clean Water, Land and Legacy Grant helped to kick-start the program. It will take several years to see results, they said.
Scientists have spent decades trying to remove invasive carp from American lakes and streams, often relying on seine fishing, which involves dragging a net across the bottom of a lake usually below the ice in the winter when the carp school.
"This method, it's difficult because a lot of the lakes have snags on the bottom. The nets get caught, allowing fish to escape," Bajer said.
Carp Solutions' new method traps and removes the invasive fish in the creek during spring spawning season.
The common carp found in Rice Creek and lakes and streams across the country came from Eastern Europe in the late 1800s.
"They were introduced purposefully by the U.S. Fisheries Commission. The settlers from Europe, they were requesting them," Bajer said.
Common carp were brought from Germany, bred in ponds in Washington, D.C., and then transported across the country in specially designed railcars. By the early 1900s, it started to become clear that introducing carp to American waters was a mistake.
"Within 20 years, people realized what had happened. They dig around in the bottom looking for food," said Bajer, citing a 1912 research paper on the plight of a Pike Lake in Wisconsin. "The vegetation was disappearing. The waters were turning muddy, and the ducks were not using the lake anymore."
Bajer said he began studying carp in the Twin Cities in about 2006. In 2013, scientists attached radio transmitters to adult carp in Long Lake and started tracking their movement. In the spring more than 90% of Long Lake's carp population left and swam up Rice Creek to spawn in the Lino Lakes chain of lakes.
It was a eureka moment.
"If you have a big migration, that is very tempting to design something to remove them," he said.
Scientists installed a low-voltage electric guidance system in Rice Creek made by a Polish company, which funnels the fish into underwater traps. They had a conveyer belt specially designed to remove the live carp from the water.
They monitor the traps daily during the spring spawning season and regularly run the belt to remove the fish. Native fish caught including bass and bluegills are returned to the creek.
During the crush of migration, 95 to 99% of the fish caught in the trap are carp, Kocian said. An individual carp can weigh between 5 and 15 pounds. Smaller panfish can easily swim through the trap.
The carp are immediately euthanized with a natural fish anesthetic, Bajer said.
The carp are buried on farmland in northern Minnesota. That's because Carp Solutions is not licensed as a commercial fishing operation.
They hope their work at Rice Creek can serve as a model for carp removal in other water systems.
"Labor is the major cost in carp management," Bajer said. "We want to create something that is as automated and autonomous as we can."
