SPICER — Phone calls and comments from some residents on the shores of Green Lake had Dave Coahran hopeful.

Their reports of die-offs of zebra mussels earlier this summer had the fisheries supervisor with the Minnesota Department of Natural Resources in Spicer thinking the losses might be significant.

No such luck. An ongoing study of the infestation by the aquatic invasive species continues to show that zebra mussels are more than maintaining their own in the lake.

“There’s still a healthy number of them out there and (they are) still of decent size,” said Gary Montz, an invertebrate scientist with the Minnesota Department of Natural Resources. He has been conducting zebra mussel counts on Kandiyohi County’s most popular lake since 2016.

“We’ve still got a lot to learn about zebra mussels overall,” said Montz. “Not only in Green Lake, but also how they affect things around the state.” Green Lake can provide information pertinent to all of the state’s lakes, since data has been collected from early in the infestation, he explained.

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The first report of a zebra mussel infestation in the lake surfaced in the fall of 2014. Their numbers began growing the next year. Montz identified 53 different locations in the lake in 2016 and began counting their densities at those sites. He has returned every year to the GIS marked spots, which are scattered throughout the lake.

All are in water deeper than five feet. Shallow waters are subject to disturbances from the weather and other activities that can compromise the value of the data.

Montz was worried that restrictions caused by the COVID-19 pandemic might not allow him to continue the work this year, but his supervisor helped make it happen. The five, consecutive years of data will go a long way in helping to get a better understanding of the impact of zebra mussels in Green Lake, as well as other lakes in the state.

The die-offs reported this year in Green Lake did not surprise Montz. He has seen zebra mussel die-offs in other lakes when certain conditions exist. Usually, it occurs when warm temperatures and a lack of winds to mix the waters allow dissolved oxygen levels to drop in areas of a lake. Even when significant die-offs occur, the zebra mussels maintain a population and their numbers rebound as conditions improve.

One of the goals for the study on Green Lake is to determine what level of population the zebra mussels will maintain in the lake. When the study began in 2016, densities were relatively low. One site had a peak of over 20,000 in a square meter of bottom, he said.

The numbers exploded in 2017, which is not uncommon when a new infestation takes off. Montz said he had sites with peak densities of 32,500.

The numbers in 2018 dropped down to levels closer to that seen in 2016. Last year, they rose, but not to the levels seen in 2017.

Montz will analyze the data collected this year during the winter, so it’s too early to know how the numbers are doing. What he called a “very, very preliminary” observation from the densities at a couple of sites showed that zebra mussels remain abundant.

“It’s kind of wobbling along,” said Montz of the year-to-year, overall changes in the population.

The Spicer fisheries crew is helping researchers determine how zebra mussels are affecting the food chain and, especially, walleye numbers in the lake. They are collecting tissue samples of young fish for laboratory analysis.

Zebra mussels cause a decline in the amount of zooplankton in a lake. Zooplankton is an important food source for young fish, walleye in particular.

The tissue samples being collected by the local crew will help determine if fish in Green Lake are switching at a younger age to eating invertebrates as a food source in place of zooplankton.

Coahran said tissue samples collected from Green Lake have been sent to the University of Minnesota for analysis, but that results probably won’t be available anytime soon.

The local crew will continue to collect tissue samples, as well as other work to monitor the fish populations in the lake. The crew conducts annual gill net assessments and electrofishing.

It will take time, but Montz said all of the data being collected will help give us a much better picture and understanding of what’s happening to the lake. “The fisheries folks are really committed to following this along too,” he said. “It’s good to have lots of different pieces of the biology out there as well.”