BOULDER JUNCTION — A team of Wisconsin university researchers is at­tempting to unravel the mystery of declining walleye numbers in North Woods lakes.

Among their projects is a first whole-lake experiment on McDermott Lake in Iron County to see if removing bass and sunfish would allow natural walleye reproduction to occur again.

Dr. Jake Vander Zanden, director for UW-Madison’s Center of Limnology, gave an update on the project during the annual open house last Friday at the Trout Lake Station research facility in Boulder Junction.

“The issue of declining walleye populations has been something that’s really hard to unravel,” he said. “It could be several different factors happening at the same time.”

The five-year study is being funded by a $1 million grant from the U.S. Geological Survey. Titled “Creating a Safe-Operating Space (SOS) for Walleye,” the project will continue through 2021.

The project involves removing bass, bluegill and other panfish from the 82-acre McDermott Lake located near Springstead. At one time, the lake had a self-sustaining walleye population. Today, just a handful — fewer than 40 — of adult walleyes are present.

What it does have in abundance are largemouth bass and sunfish, many of them stunted.

McDermott Lake is not alone with reduced walleye numbers. Across the North Woods, and also in northern Minnesota, “hundreds and hundreds” of cool-water lakes have seen their walleye numbers plummet in recent years, the professor said. The impact has been significant.

The tasty walleye is popular with hook-and-line anglers and also tribal spearers, who continue their treaty-granted spring harvest into the modern era. Recreational fishing pours millions of dollars into the economies of North Woods communities.

Testing the hypothesis

“The cause of these declines is not well understood, but is likely due to a combination of factors, including rising water temperatures, increased water clarity, loss of habitat, harvest, and species interactions,” says the project’s website at walleye-sos.weeb­

Led by graduate student Holly Embke, researchers are analyzing data, much of it compiled by the Wisconsin Department of Natural Resources (DNR) over several decades, as well as simulation modeling, and a whole-lake experiment to better understand the role of those factors in causing walleye declines. And that’s where McDermott Lake comes into play.

Removing young bass and sunfish that prey on walleye fry could trigger a resurgence in walleye numbers, according to a hypothesis being tested by scientists and students from UW-Madison and UW-Stevens Point. The DNR is also lending a helping hand, including issuing a permit for the study.

The research project began in 2017 when technicians surveyed McDermott and nearby Sandy Beach Lake to establish baseline information. Sandy Beach, similar in many ways to McDermott, continues as a reference lake and is untouched.

Last year, the goal was to remove 80% of the bass and sunfish, most notably bluegill and pumpkinseed. They used electrofishing, fyke nets and clover traps to accomplish that. They also collected information on other parts of the lake, such as the food for fish (macroinvertebrates, zooplankton, crayfish), plants, water chemistry, and temperature.

What they found were mostly small fish, due to overpopulation of those species. At the end of 2018 they had removed upwards of 85,000 fish (2,000 pounds) from McDermott Lake. This year through mid-July, they have removed 51,000 fish, according to Embke.

Removal of the target species will continue through this year and the following years. (Perch, muskies and other species are not being removed.)

What they did not find, over two years of sampling, was any evidence of young-of-the-year walleye, clear evidence that natural “recruitment” is nonexistent.

“In 2017 and 2018, we did not collect any young-of-year or juvenile walleye,” said Embke. “We will conduct our fall young-of-year/juvenile walleye surveys again this September so (we) do not know the results yet for 2019.”

Other factors explored

Vander Zanden explained that they are looking at other causes for the walleye decline: overharvest, whether by hook-and-line or spearers, or both; climate change; various species competing for a limited food source; shoreline development; and habitat loss.

Based on this study’s and previous studies’ examinations of stomach contents of bass, it doesn’t appear that bass are a significant predator of walleye young.

“We looked very hard and did not find very strong evidence for predation,” he said, qualifying it by saying it could be happening to some extent. Or the researchers didn’t catch them in the act,” said Vander Zanden.

“You might not catch that period of time when they are chowing down on them and knocking down the population because it’s a short period of time, or maybe they are not a major food item for the bluegill or crappie,” he said.

“In 2017 and 2018, we looked at many sunfish stomach contents and haven’t found any young walleye, but will continue looking this year as well as the next two study years,” Embke added. If so, removing them from the lake system should lead to more walleye recruitment and better growth rates if the hypothesis holds true.

“Although our main question still remains the same — whether removing bass and sunfish from the lake can relieve some predation and competition pressure for walleye and thus increase their population — we are looking at the effects of this removal on other aspects of the lake,” she said.

“For example, we’re also interested in how this removal will affect other fish species in the lake and therefore are looking at the entire food web. Additionally, given that we are removing lots of bass and sunfish but will certainly miss a few, we are interested in how this removal will affect those remaining bass and sunfish. For example, do those left-over bass and sunfish grow faster with fewer competitors?”

What the future holds

The website also notes that based on climate change modeling, it appears the future will bring fewer suitable walleye lakes, “while potential warm-water competitors (e.g. largemouth bass) will have increased habitat.”

Going back to overharvest, Vander Zanden said daily bag limits and spearing quotas may have to be adjusted if that’s determined to be a significant factor. They are keenly interested in the results of the Minocqua Chain of Lakes where there’s a five-year ban on walleye harvest, either by hook-and-line or spearing.

“Rules (on bag limits, etc.) in play 30 or more years ago may have worked at that time, (but) the world has changed,” he concluded.