WORTHINGTON, Minn. — Located in the heart of Worthington, Lake Okabena is home to a variety of fish, including an abundance of common carp.

The carp stir up sediment and phosphorus, reducing water clarity. Meanwhile, high levels of nutrients impacted Lake Okabena’s water quality, causing it to be added to Minnesota’s impaired waters list in 2010.

Designated impairments led to a diagnostic study to determine the lake’s Total Maximum Daily Load (TMDL), followed by plans to implement best management practices on predominantly agricultural lands upstream.

Incentives offered by the Okabena-Ocheda Watershed District for landowners to plant filter strips along upstream waterways has helped. Then, in December 2016, the district was awarded a $428,000 Clean Water grant from the Minnesota Board of Water and Soil Resources to construct a water storage and filtering project on a former golf course north of Worthington.

Those efforts resulted in improved water clarity in Lake Okabena. For the first summer in at least a decade, the water body wasn’t frequented by a plethora of algae blooms.

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To continue the momentum, watershed managers are now taking a targeted approach at Lake Okabena’s carp population with the help of Wenck, a Twin Cities-based engineering firm that specializes in hydrology, flooding and water quality.

“We have carp continually stirring up the lake bottom, and also preventing plants from growing on the lake bottom and using some of those nutrients,” explained Dan Livdahl, watershed district administrator. “In order to get our nutrients down, we need to keep the carp from stirring up the lake sediment.”

Wenck has worked with the watershed for the past eight years, starting with the TMDL study, followed by a trio of electro-fishing surveys to help estimate carp density in the lake.

“There’s a method in research — depending on how many carp you see — where you can get a rough estimate of the number of carp in the lake,” explained Jeff Strom, a water resource scientist with Wenck who focuses on water quality chemistry. “It came out high each time, consistently saying this was over the threshold that the literature suggests has an impact to water quality in lakes.”

It’s been determined that a healthy lake can withstand 89 pounds of carp per acre. In Lake Okabena, the electro-fishing methodology showed two to three times that level of carp per acre. Strom said the method, based on studies by the University of Minnesota, has been in use for more than 20 years.

After gathering the data, the watershed district in 2019 funded two carp tagging efforts. The first, in May 2019, involved surgically implanting 15 Lake Okabena carp with radio transmitters.

Livdahl was trained on the radio telemetry equipment and began tracking carp movement using each carp’s unique signal. He kept notes on where the fish were hanging out during bi-weekly surveys.

A second tagging event was in October 2019, with less invasive RFID tags implanted in approximately 180 carp.

What Wenck and the watershed board wanted to know was where the carp were congregating in the 776-acre lake. Knowing where they schooled would give a commercial fisherman details on where to conduct wintertime seining. Capturing carp with RFID tags during the seining could then provide a better idea on the carp population.

Unfortunately, the winter of 2019-2020 didn’t go as anticipated. The carp never schooled together and plans for a wintertime seining had to be scrapped. It happened on other lakes in southwest Minnesota, too, according to the commercial fisherman.

Strom said every lake is different, and another attempt on Lake Okabena is warranted. In other areas of the state, the U of M and Wenck have had success with winter seining.

In the Phalen Chain of Lakes in St. Paul, and the Riley Chain of Lakes south of Minneapolis, strong winter schoolings resulted in a significant number of carp seined.

“They were able to remove enough during these large-scale winter seining events to even get below the threshold,” Strom said.

Also in the metro area, Wenck worked with the Shingle Creek Watershed on carp removal efforts. There, lake debris wreaked havoc with fishermen’s nets during the seining, Strom said.

“We got a couple seine hauls and removed a large amount of carp,” he added.

Success elsewhere means the Okabena-Ocheda Watershed District will try again.

“If the carp population estimates are close to correct, we have tens of thousands of pounds of carp to remove from the lake,” Livdahl said. “The only way to do that efficiently from the lake is seining.”

This summer, board members authorized funds to surgically implant another 20 Lake Okabena carp with trackers. To be implanted this month, the devices emit a stronger signal than those used in 2019, which is hoped to help Livdahl with wintertime tracking efforts.

“We walked across the ice with the radio telemetry equipment (last winter) and also drove across with an ATV,” Livdahl said. “The signal was so weak through the ice that it was difficult to track them.”

Of the 15 carp tagged in the spring of 2019, he still finds a signal for 13 of them. However, some of the signals have been in the same location since last fall — giving the assumption those carp are dead.

“I think we have signals coming from eight live carp,” Livdahl said earlier this month.

Not only is the district using the information garnered from carp tracking to plan a seining event, they are also considering efforts to keep carp from successfully reproducing. Lake Okabena is connected to Sunset Bay through a culvert system, and last spring’s tracking revealed most of the trackable carp migrated to the bay to spawn.

“We are looking at ways to trap the carp or seine them in the bay,” Livdahl said, adding that trapping leads to a new set of challenges — rotting fish in a shallow bay surrounded by homes and a campground. Any decision to trap will need approval from the city and the neighborhood.

The efforts being put into Lake Okabena’s carp issue won’t end with a successful seining event or fish kill.

“When we started the process of following the carp around, we knew this was going to be a multi-year process,” Livdahl said. “It’s still going to be an annual maintenance thing after we get below the (acceptable) level, and we will have to remove a few hundred pounds at a time rather than thousands of pounds.”