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Following the (rough) fish: Radio trackers implanted in 15 Lake Okabena carp this week

WORTHINGTON -- Fifteen carp captured in Lake Okabena Tuesday morning have been fitted with radio tracking devices that are hoped to shed light on where carp congregate and spawn.

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Biologists from Wenck Associates insert a radio frequency tag into an adult carp captured from Lake Okabena in May 2019. (Julie Buntjer / The Globe)

WORTHINGTON - Fifteen carp captured in Lake Okabena Tuesday morning have been fitted with radio tracking devices that are hoped to shed light on where carp congregate and spawn.

The project, spearheaded by the Okabena-Ocheda Watershed District and financed through the city’s Clean Water Partnership fund, will target removal of the rough fish, which in turn could lead to improved water quality and fewer algae blooms on the Worthington lake.

Aquatic biologist Tom Langer of Wenck Associates, the firm hired to conduct carp assessments in Lake Okabena in 2017 and 2018, led Tuesday’s carp capture and the surgical implantation of tracking devices. He was assisted by fellow Wenck employees Nick Omodt, also an aquatic biologist, and Jeff Strom, a water quality scientist.

Earlier in the day, the trio circled the lake with an electro-fishing boat that emitted a charge powerful enough to bring fish to the lake surface. Of the 15 carp netted for the surgery, the sizes ranged from just over 5 pounds to nearly 21 pounds. There were at least five female carp, each of which was identifiable by the presence of eggs during the surgical procedure.

Langer, Omodt and Strom lined up large black totes filled with the carp on a Centennial Park dock and created a makeshift surgical area Tuesday afternoon to implant the tracking devices. To sedate the fish, a therapeutic-grade clove bud oil was added to the water of one tote. The oil coats the carp gills and acts as an anesthetic. The carp were sedated one at a time, with the process taking several minutes.

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As each carp slowed in movement, it was removed from the bin, weighed, measured and implanted with a small, cylindrical tracking device attached to a foot-long thin gauge metal wire. Once the cylinder was inserted through an approximately 2-inch incision on the lower side of the fish, the wire was fed through a smaller hole to rest parallel to the fish, and the incision was stitched up.

Omodt returned each carp to the lake, holding it for about five minutes as it came out of the anesthesia and was able to swim away.

Langer, Omodt and Okabena-Ocheda Watershed District Administrator Dan Livdahl returned to the lake Wednesday morning to look for any carp that didn’t survive the surgery, and to see if they could locate the implanted carp through the radio tracking equipment.

“We only found seven of the 15 fish,” Livdahl reported. “We’re assuming that those we couldn’t get a signal from were in deeper water and were trying to stay warm.

“We wanted to locate them all, but the fact that we didn’t get a signal wasn’t necessarily bad news,” he added. “There was only one fish (emitting a signal) within 150 feet of the dock from where the surgery was done. If any were dead, they would have floated to the surface.”

Tracking begins The radio tags implanted in the carp should last for about three years, giving the watershed district key information about where the rough fish gather to overwinter and where they go to spawn.

Each radio tag has a unique frequency, which can be picked up from up to a mile away with the tracking device. Livdahl will be tracking movement of the radio-tagged fish once a week.

Livdahl asks that the public report any deceased carp with a radio tracking wire to the Okabena-Ocheda Watershed District, 372-8228, so that the device can be removed and implanted in another carp. Fishermen who catch a carp with the tracking device are asked to return it to the water immediately.

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With the information collected, Livdahl said the watershed district can then determine where to do wintertime seining to capture the greatest number of carp - as well as where to establish barriers to prevent carp from spawning in areas where predator fish aren’t as prevalent.

Removing carp from Lake Okabena will decrease total phosphorus loading in the lake, thereby leading to a reduction in algae blooms.

Langer said carp begin to have a significant impact on water quality once they reach a population of 89 pounds per acre. An October 2017 assessment conducted by Wenck on Lake Okabena determined carp populations were double the critical threshold, and Langer said that may have been underestimated because the water had already cooled and the carp may have been in deeper water.

Because of that uncertainty, a second assessment was conducted in July 2018 - this time in both Lake Okabena and Sunset Bay. Roughly 312 pounds per acre of carp were found in Lake Okabena, with approximately 330 pounds per acre found in Sunset Bay.

“Between 2018 and what we sampled (Tuesday), we estimate about 40,000 carp exist (in the lake),” Langer said, adding that the fish caught this week were bigger than in previous surveys.

While removal of the carp will help, Langer cautioned that total eradication of the rough fish in Lake Okabena is “nearly impossible without Rotenone treatment or a water drawdown, but that targets all fish.”

E.O. Olson Trust awards funds to project Langer spoke for nearly an hour about Lake Okabena’s carp assessments and the management plan developed from it in a presentation to the E.O. Olson Trust board Wednesday. The trust was asked last month to contribute financially to the carp reduction project in the lake.

Following Langer’s presentation, the board authorized $37,500 for the project. Specifically, the dollars will be used to identify locations where the carp are reproducing in the system, to install barriers at Sunset Bay and storm water ponds adjacent to the lake, and to fund carp removal through commercial seining in the winter of 2020-21.

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The city of Worthington previously approved $52,000, which includes funding commercial seining on Lake Okabena next winter, paying for the fish tagging equipment, doing the implants, obtaining the necessary DNR permits, telemetry training, developing a data entry system for tracking the carp and researching where fish barriers may be needed on the lake.

Julie Buntjer became editor of The Globe in July 2021, after working as a beat reporter at the Worthington newspaper since December 2003. She has a bachelor's degree in agriculture journalism from South Dakota State University.
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