SPICER — Oscar Oakes and his crew at Oakes Lawn Service and Dock Removal in Spicer have been helping residents on Green Lake pull their docks for the season, and this year is no different than the last few years: Along with the docks have come scores of zebra mussels, evidence that the invasive species has established a strong population in the lake.

Oakes said their numbers seem to fluctuate year to year, depending on location. Last year, some of the highest numbers of zebra mussels were found on docks in the area of the Olde Mill Inn. This year, the numbers of zebra mussels on docks in that area appear to be down.

Oakes believes that as the zebra mussels consume much of the food in an area, their numbers decline. When the food builds back up in the area, so do their numbers. The fluctuations between the peaks and lows seem to be shrinking some, he added. He suspects their numbers will eventually stabilize.

Gary Montz is conducting research on the lake to determine if that will be the case.

Montz, an invertebrate scientist with the Minnesota Department of Natural Resources, has been conducting zebra mussel counts on Kandiyohi County’s most popular lake since 2016.

The invasive mussel was first detected in the lake in 2014. As is usual, the zebra mussel population exploded following their arrival.

Montz has identified 50 sites distributed throughout the lake where he collects samples and determines the density of adult zebra mussels each year. The sites include a mix of water depths and different habitats, from rocky areas to stands of aquatic plants.

The work showed zebra mussel densities of 3,943 per square meter in 2016, according to information provided by Dave Coahran, fisheries supervisor with the Minnesota Department of Natural Resources. He shared the data during an open house meeting last month on special regulations for walleye on the lake. The numbers soared to 7,263 per square meter in 2017, and dropped back down to 3,890 per square meter in 2018, according to the information.

Montz said he won’t know this year’s numbers until later this winter, or possibly early spring. This year’s samples are waiting in his lab along with hundreds of others from lakes around the state. Examining and counting the samples is work he performs during the winter.

Based on the data collected to date, Montz told the West Central Tribune that it is too early to know if the numbers on the lake are stabilizing, or if this is a lake where the numbers will naturally fluctuate from year to year. In some lakes, numbers peak after their introduction and then, after a couple of years, reach and stay at a stable level. But there are other lakes where the numbers bounce up and down from a set point year to year, he said.

Montz said work he performed earlier this year on Green Lake showed that zebra mussel veligers remain abundant in the lake, but that doesn’t necessarily mean adult numbers will be up. Larval reproduction doesn’t have a real strong correlation with the number of adults, he said.

He can say this much for sure: His 2019 sampling confirmed again that Green Lake has a healthy zebra mussel population and the mussels are well distributed throughout the lake.

The DNR is also conducting research to see how their presence is affecting fish populations in the lake.

An adult zebra mussel will filter a liter of water per day. In Green Lake they have caused a dramatic decline in phytoplankton, the microscopic plants they eat. In turn, this has led to a decrease in zooplankton, which feed on the phytoplankton.

During the open house meeting, Coahran pointed out that the DNR is looking at how this reduction in zooplankton affects walleye and perch. Both fish rely on zooplankton as a food source in the first months of their lives.

One of the questions for Green Lake is whether the walleye and perch are adapting to this change by switching at a younger age to eating invertebrates as a food source. By eating phytoplankton, zebra mussels will fertilize shallow areas of lakes with their feces and increase invertebrate numbers.

Coahran said the Spicer fisheries staff will be collecting tissue samples from small perch and walleye. The tissue will be analyzed at a laboratory where it can be determined whether zooplankton or invertebrates are being used as the building blocks for the tissue.