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Zebra mussels appear to be stabilizing in Green Lake

After an initial explosion in numbers, the invasive species appears to be reaching a stable level at which populations will likely fluctuate up and down from. The full impact of zebra mussels in Green Lake, and what to expect in the years ahead, remains the subjects for study.

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Zebra mussels cover a rock pulled from Green Lake in this West Central Tribune file photo.
Tom Cherveny / West Central Tribune file photo
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SPICER — Since first being found in Green Lake in 2014, zebra mussels have spread throughout the lake and as is usually the case when invasive species first reach a water body, exploded in numbers.

Their numbers appear to have peaked in 2017, when research by Gary Montz, an invertebrate scientist with the Minnesota Department of Natural Resources, counted densities as high as 32,500 zebra mussels per square meter of lake bottom.

This many years later, and after five consecutive years of research by Montz, there appears to be signs that the zebra mussels population in the lake is stabilizing.

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AIS inspectors Beverly Jones and Jabran Mustafa and watercraft owner Don Frost inspect his craft after leaving Green Lake in this Tribune file photo.
Tom Cherveny / West Central Tribune file photo

In most lakes, the total number of zebra mussels tend to fluctuate around a stable point, going up and down from year to year, according to Montz. That is what appears to be happening in Green Lake.

“I would not be surprised if the zebra mussels population just kind of maintained itself out there,” Montz told the Tribune. “Some years see more. Some years less.”

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Montz did not sample for zebra mussels in Green Lake last summer, but he collected data for five consecutive years, from 2016 through 2020. The years may represent the peak period for the species in the lake. Just maybe, he said, we may see them establish themselves at a lower level over the long-term. But make no mistake: They are an will remain in the lake in large numbers.

Montz has 53 different locations scattered about the lake where he monitors their numbers. He drops a Ponar device to the bottom and scoops up a sample to bring back to the laboratory. It’s a process he hopes to continue in the years ahead, although not on an annual basis. “We definitely want to keep monitoring that lake because we have a really good baseline of data on the zebra mussels,” he explained.

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A quiet, summer afternoon is enjoyed at the Saulsbury Beach on Green Lake. Early fears that zebra mussel shells would litter the lake's beaches have not proven true.
Tom Cherveny / West Central Tribune file photo

Less is known about how the zebra mussels are affecting native fish populations in the lake, but many believe their arrival is detrimental to walleye. Walleye numbers have been on a decline as water clarity, in part a result of the zebra mussels filtering zooplankton, has increased.

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A Ponar system grabs a sample of the bottom of Lake Carlos as part of the process to determine the density of zebra mussels. The same system has been used on Green Lake to monitor zebra mussel populations at 53 different locations scattered about the lake.
Photo courtesy of the Minnesota DNR.

But there are some signs of change, and positive ones at that. Dave Coahran, fisheries supervisor with the DNR in Spicer, said the fisheries crew was able to capture 10, young-of-the-year walleye per hour when sampling the lake last autumn. That’s the highest number in more than 10 years. In the previous years, the crew was lucky to nab one in an hour’s worth of work in the fall.

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A homeowner found this zebra mussel on his dock on the southeast side of Green Lake in the fall of 2014, when the aquatic invasive species was first discovered in the lake.
Carolyn Lange / West Central Tribune file photo

Not only was the catch of young-of-the-year walleye up, but these fish were robust. “Nice sized. Good growth to them. Looked healthy. Good length to them,” said the fisheries supervisor.

He has no way to know for sure, but he pointed out that water clarity in the lake seemed to be down last year. And possibly, he said, the zebra mussel numbers were down as well.

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Many watercraft owners have been diligent in efforts to inspect their boats for Aquatic Invasive Species to slow the spread of zebra mussels and other invasive species. Tribune file photo of boaters checking their craft after leaving Green Lake.
Tom Cherveny / West Central Tribune

Many lakeshore residents reported seeing fewer zebra mussels on their docks and boat lifts when they pulled them from the lake last fall, said Scott Carlson, president of the Green Lake Property Owners Association. While there are plenty of zebra mussels in the lake, anecdotal evidence suggests last year was a down year for them.

Oscar Oakes, of Oakes Lawn Service of Willmar and Water Works Dock & Lift, of Spicer, can attest to the declining numbers of zebra mussels on docks and boat lifts. Year by year, he and his crew have been finding fewer zebra mussels attached to the lifts they pull. “(We’re) not seeing them completely covered like they have been,” said Oakes.

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Young zebra mussels are settled on Chara in Green Lake.
Photo courtesy of Minnesota DNR.

He’s noticed a big increase in the numbers of native crayfish found with the lifts, and suspects they may be preying on zebra mussels that attach to them. When he brings a lift into the shop, as many as 15 to 20 crayfish will emerge. In previous years, it was rare to find so much as a single crayfish on them, he explained.

It’s given Oakes reason to hope that zebra mussels numbers are starting to decline, and that some change for the better is occurring.

Another change on Green Lake — which may or may not be related to zebra mussels — is the perch population. Well before the arrival of zebra mussels, the fisheries crew was recording a steady decline in the number of perch they netted in the lake.

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A sample of zebra mussels from the bottom of Lake Carlos pulled up with the Ponar system. The same process is used to sample 53 different locations in Green Lake to determine the density of zebra mussels.
Photo courtesy of Minnesota DNR

Now, the lake is being examined as part of a larger study to determine what is happening to perch populations across the state. Coahran said it is believed that the perch are still in the lake, but are not growing large enough to be captured in nets. They are reaching sexual maturity at a much smaller size, putting their energy into reproduction instead of growth, explained Coahran.

What may be triggering this is unknown. But this past year, for the first time in many years, the fisheries crew have been netting a few “larger” perch, in the five- to seven-inch range. It’s too early to know if this is a sign of change for the better, but Coahran hopes it is.

Right now, the size and maturity level for perch in Green Lake is the lowest found in the whole state, the fisheries supervisor said.

Montz said long-term research on zebra mussels in the lake will help us learn more about what is a very complex system. What is certain is that zebra mussels are here to stay. He points out that zebra mussels have established themselves throughout the entire lake. Even if there are die offs due to heat or oxygen stress, zebra mussels are prolific. In other lakes, their numbers quickly rebound after adverse conditions.

“From what we see, they are not going to all of a sudden disappear out of the lake on their own,” said the researcher. “They may drop to lower numbers, but they are going to be in Green Lake.”

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A heavy coat of zebra mussels covers boat lifts that were removed from Green Lake in Spicer in fall 2015, a year after the species were found in the lake. Carolyn Lange / West Central Tribune file photo
Carolyn Lange / West Central Tribune file photo

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Tom Cherveny is a regional and outdoors reporter for the West Central Tribune.
He has been a reporter with the West Central Tribune since 1993.

Cherveny can be reached via email at tcherveny@wctrib.com or by phone at 320-214-4335.
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