MAHOPAC, N.Y. - Early one morning last week, a large tanker truck with the words, “Live Fish” emblazoned on its side wound its way through town before parking next to the docks of Il Laghetto Restaurant on Lake Mahopac.

The truck was carrying a precious cargo that residents on the lake, along with all those who use the lake for recreation, have been waiting for some time: weed-eating grass carp.

Fifteen hundred of the hungry fish were offloaded into the lake via a long flexible tube that Supervisor Ken Schmitt and Councilwoman Suzy McDonough helped to direct.

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The fish, which were purchased from Keo Fish Suppliers in Keo, Ark., are charged with an important task: reducing the weed population in the lake, a problem that has been plaguing boaters and swimmers for several years now.

It’s been a long, arduous task getting the carp here. Approval was needed from the state Department of Environmental Conservation (DEC) and at first, the agency was reluctant to give it.

When asked how he felt when the fish were finally being stocked into the lake, Ed Barnett, chair of the Lake Mahopac Park District’s executive board, said, “I thought, finally. I was really happy to see it. We really need that.”

The last time Lake Mahopac was stocked with carp was in 1994 when 2,565 of the fish were brought in.

“We did a weed analysis back then and determined there were 175 acres of weeds (on a 580-plus-acre lake),” Barnett said. “The DEC allowed us 15 [carp] per acre [of weeds]. That’s been modified downward since then.”

The DEC approved 200 carp for last year, but that proved to be not nearly enough. Experts said it’s important not to overstock the carp because they could wipe out the weeds entirely, which would create an environmental disaster and harm other species of fish that reside in the lake. For example, Barnett said weed coverage is important for large-mouth bass, as it provides a place for them to lay eggs and protect them from predators.

To convince the DEC more carp were needed, the Lake Mahopac Park District conducted a biomass study of the lake, using a drone to map out weed coverage. The DEC went over the data and was convinced, subsequently approving another 1,500 of the triploid carp—a species that can’t reproduce.

“So, we put in 200 last year and now we’ve added 1,500,” Barnett said. “We shall see. We should be able to see some kind of results soon. We have fully cooperated with the DEC.”

The Park District has plans to conduct another biomass study in August, as well as some other tests.

“We do a water-quality study and the biomass study at the same time; we have been doing it since 1993,” Barnett said. “We have given [the DEC] the data because they don’t have the resources to get themselves.”

Barnett said the DEC informed him that a recent study it conducted revealed other fish populations in the lake are doing well.

“There is an outstanding number of small-mouth bass that the fishermen will be going after,” he said. “We also have perch and sunfish.”

Barnett said the recent explosion in weed growth around the lake is likely due to the old fish population from 1993 dying off.
“They ran their cycle,” he said. “They have a shelf life.”

However, some other lake residents have told Mahopac News they believe that runoff from lawn fertilizers containing phosphates have contributed to the problem.

Barnett said that when the weed problem came to the fore in the early ‘90s, the Lake District looked at several options before settling on the carp solution.

“We were looking at different ways of dealing with it,” he said. “We had a [weed] harvester, but it wasn’t practical. No one wanted it behind their homes when they were having parties on the weekend. And there were always mechanical breakdowns. We also said no to chemicals. There is a water district that draws [drinking] water off the lake, so we went biological—the carp.

“We were hoping to hit the sweet spot for the number of fish we needed to control the problem and still keep some weeds to maintain a fishery for breeding,” he added. “Some want the lake to look like a bathtub, but that’s not reasonable.”

The carp idea worked in ’93 and Barnett said there is no reason it won’t again.

“There were naysayers who said it wouldn’t work, but it certainly has,” he said. 

In 1993, the district paid $33,000 for the 2,500-plus fish, which have lasted over 20 years.

“That’s a pretty good deal for a 583.4-acre lake,” Barnett said.

This time, the district paid $11,000 for the 1,500 fish that were delivered last week.

“I am just thrilled we have [the carp],” Barnett said. “We have been pioneering this technology (the biomass surveys) and doing it most consistently on an annual basis.”