There were no catch limits on this fishing trip at Silver Lake in the north metro, and the fishermen used a backhoe to lift and load their haul — 6,000 pounds of invasive carp.

When they were done pulling more than 1,100 fish out of the water Wednesday, the back of a pickup truck was full of squirming carp packed in snow.

The professional fishermen who hauled the invasive carp out from under the ice of the St. Anthony lake were doing so to improve water quality.

The result: The lake is now nearly rid of the invasive fish, whose bottom-stirring presence lowers water quality. Wednesday’s yield was bigger than a one-day carp catch last year of about 3,800 pounds.

But the methodical removals have generated some debate, because fewer carp mean more weeds. More than a decade ago, Silver Lake, which straddles the border of Anoka and Ramsey counties, was designated “impaired” by the Minnesota Pollution Control Agency because of elevated phosphorus levels, which fuel algae blooms.

Carp eat aquatic vegetation and stir up sediment on the lake bottom, releasing phosphorus in the water.

The fish have found their way into waters like Silver Lake in minnow buckets and by swimming up ditches and other waterways.

Silver Lake’s shoreline neighbors — the Three Rivers Park District, the Rice Creek Watershed District, Ramsey County, the state Department of Natural Resources and the cities of St. Anthony, New Brighton and Columbia Heights — agreed to remove as much of the lake’s carp population as possible. It’s thought that population peaked at 1,300 fish in 2013.

“What we are doing is looking at this carp removal as a best-management practice,” said Tony Havranek, a senior environmental scientist with the Minneapolis-based WSB & Associates, who is overseeing the project. “It’s not a silver bullet, but it’s one tool in a suite of tools that we are going to apply to a water body to improve water quality and habitat.”

Another such tool, he said, is managing stormwater flow.

Imperiling the ecosystem

Generally, scientists like to see 89 pounds of carp per acre of water or less.

“That’s the ecological tipping point,” said Havranek, who has overseen carp removal at half a dozen lakes in Minnesota, Wisconsin and Illinois. “You go over that, and you see dramatic decrease in water quality and a large reduction in aquatic vegetation. It can have cascading effects on other parts to the ecosystem, including waterfowl, fisheries and human recreational use.”

When its carp population was at its peak, Silver Lake had an estimated 144 pounds per acre. Residents recall seeing the big fish swarm along the shoreline in the spring.

The removal project was started more than two years ago, when scientists implanted transmitters into seven carp and released them back into the lake. Carp school in midwinter, so fishermen were able to use the location of those seven fish to locate and net nearly the lake’s entire population of carp.

The fishermen drilled holes in the ice, then used a small remote-control submarine to deploy their net.

The harvest can be tricky. In January 2014, fishermen had 10,000 pounds of carp — nearly the entire population — in their net until it snagged on a submerged picnic table, allowing half the fish to escape. Fishermen removed an additional 800 pounds of carp over the warm-weather months. That brought the lake’s carp level down to 91 pounds per acre.

“We did some improvements — better water quality,” Havranek said.

But some residents were concerned that if the carp removal continued, the lake would become more vegetated, interfering with recreation, he said. After conferring with residents about best environmental practices, scientists hired by the cities and parks district proceeded with the removal this year.

‘It is a good thing’

On Wednesday, a sunken canoe caught at and tore the net, but a sizable amount of the lake’s carp were still removed. Havranek estimated that there are still up to about 300 carp remaining in the lake.

Other fish snagged, including catfish and bass, were thrown back into the lake.

The carp will be sold and processed, and could up in animal feed or in overseas markets.

Shoreline homeowner Tim Johnson came out to watch the action. He’s on the area homeowners’ association’s water quality board. He grew up on Silver Lake, and his parents and brother also own homes on its shores.

“There are some saying the weeds interfere with their swimming,” he said. But “the idea [behind the carp removal] is it will be good for the water quality. I am convinced by the experts it is a good thing.”