Core samples from Mille Lacs Lake may explain walleye woes

Researchers hope to get to the bottom of the challenges that face Mille Lacs Lake walleyes by digging deep into the lake's past.

A group of researchers drilled seven core samples from the lake floor Thursday and Friday afternoon, hoping to look back at least 50 years at what was going on then and now, specifically since the invasive species, spiny waterflea, was discovered there in 2009.

The group of scientists from the Minnesota Aquatic Invasive Species Research Center and University of Minnesota Duluth plan to analyze that 50 years worth of sediment pulled from the lake, after extracting about 200 years worth from the icy depths. Researchers hope to learn more about the role that spiny waterfleas have had in disrupting the food web and contributing to the decrease in walleye numbers.

Researchers will analyze the data to identify potential ecosystem impacts that could be felt by game fish like walleye. This study could also help researchers learn more about the long-term threat to fish posed by spiny waterflea, and how Minnesotans will be affected, according to Christine Lee communications specialist with Minnesota Aquatic Invasive Species Research Center.

Donn Branstrator, associate professor with the University of Minnesota Duluth partnered with the Minnesota Aquatic Invasive Species Research Center for this project. He was out hopping from hole to hole this week gathering samples from three main sites throughout the basin of the lake.

In his initial assessment of the sampling, there was not much to be surprised about except for the high volume of people on the lake.

"There are a ton of fish houses out here," he noted shortly after pulling the last sample from the lake.

What harm are they?

It's possible spiny waterflea made it into Mille Lacs Lake earlier than 2009 and went unnoticed for a while, Lee said. They were first discovered in Minnesota in 1987 in Lake Superior and are now in about 40 waterbodies in Minnesota. They are native to Asia and Europe.

The hypothesis with spiny waterflea is that they eat so much native zooplankton, they wipe out the bottom of the food chain and disturb the whole web, Lee said. However, in lakes that also have zebra mussels, it can be hard to pinpoint which species is doing what.

"There is a lot that is still unknown regarding their ecological impacts; we do know that in addition to these ecological impacts, they also clog the eyelets of fishing rods and cause problems for recreationalists," Lee said in an email.

As a way to better understand what effect spiny waterflea and zebra mussels have on a fishery, the researchers are not only looking at Mille Lacs, which has both invasive species, they also plan to take samples from Lake Winnibigoshish and Leech Lake (which have zebra mussels, but no spiny waterflea) and Kabetogama Lake (which has spiny waterflea, but no zebra mussel.)

Branstrator said the Minnesota DNR keeps records of water clarity, changes in the water column and fish numbers. Typically the records show a decrease in biomass with the introduction of the invasive species as the non-native species is competing for resources.

Once researchers are able to analyze the sediment, which could take at least one year, they could then start comparing the new data to the historical data to see what conclusions could be drawn, Branstrator said.

While the spiny waterflea may seem frightening with it's long spine, barbs and large black eye, fish do consume it.

"Very small fish have difficulty eating it," Branstrator said of those less than 2-3 inches in length. "Once they eclipse 3 inches, fish can readily feed on them."

Even so, there have been records of large fish suffering from punctured stomachs after ingesting the tiny crustacean morsels. And even though the species can be eaten safely by some, the volume of spiny waterflea eating the main source of nourishment can cause more harm than good.

"It's a prolific carnivore," Branstrator said. "Even if fish can eat it, they are competing with it."

There is no eradicating this species either. It's about containment, Branstrator said.

Sampling Mille Lacs

Researchers drilled numerous holes throughout Mille Lacs looking for semi-solid soil for the best samples. Branstrator said the basin of the lake provided good samples in and around 30 feet of water. The samples are not taken near shore because the spiny waterflea prefers life in the darker depths, where other fish that prey on them have a harder time seeing them.

Those core samples are taken back to Duluth to be refrigerated and later carefully sliced apart like cheese, a half centimeter at a time, Branstrator said.

Scientists glean through the sediment looking for aquatic body parts and fossils that tell a tale of what was there and what has changed over time.

While many parts of the creatures under water dissolve away, skeletal remains including the spine of the spiny water flea remain for many years leaving a path to when the species may have actually first appeared.

This technique was used on Island Lake Reservoir near Duluth to determine the spiny waterflea was actually there about 10 years before it was originally discovered, Branstrator said.

The study of these lakes is set to run through June of 2019.