If you’re a fisherman in the Northwoods, you’re well aware of the decline in walleye populations over the last few years.
Why is this happening, though? And is there a way to reverse the trend?
That’s what Jim Bokern in Manitowish Waters wanted to know when he sent a question to WXPR’s Curious North series. Mackenzie Martin takes it from here.
Matt Chotlos is an undergraduate student at UW-Madison. For the last two summers, he’s been waking up at UW-Madison’s Trout Lake Station in Boulder Junction five days a week with a group of other researchers and driving to McDermott Lake in Iron County, where he traps bass and other sunfish–up to 2,000 a day–with the goal of removing every last one. By the end of this July, he had helped remove a total of 150,000 fish.
He has no interest in the fish. They’re donated to wildlife recovery centers as food for wildlife. The reason he’s doing this is to answer questions. The biggest of which is: What happened to all the walleye up here?
See, historically McDermott Lake has had a very healthy population of walleye, but today, there are only about 50 adult walleye in the entire lake. As the walleye population has gone down, the population of bass, particularly largemouth bass, has gone up. It’s a trend being seen in many northern Wisconsin lakes. Chotlos and a team of other researchers are removing bass and other sunfish to see if it helps the walleye population.
“We’re trying to change the community structure to see how it affects everything in the lake,” he says. “With a focus on walleye. The walleye are the star of this project.”
Walleye are the star because they’re a big deal in the Northwoods. A lot of people come up here in hopes of catching one, or at least eating one, which is why declining walleye populations are such a concern.
“Walleye are a pretty important game fish, they’re an iconic Northwoods fish,” says Chotlos. “There are a lot of people who are concerned that if these fish go away, then a lot of the money and things that are coming in through that industry will be gone as well. That’s one focus. There’s also just the concern from an environmental perspective of losing these species that have been around for so long.”
UW-Madison’s Center for Limnology is collaborating with UW-Stevens Point and the Wisconsin DNR on the project, who have years of intensive surveys of fish populations.
In a paper published last year in the Canadian Journal of Fisheries and Aquatic Sciences, researchers analyzed walleye populations in 473 northern Wisconsin lakes between 1990 and 2012 and found that walleye production had decreased by 27% over the 22-year period.
“The issue of declining walleye populations has been something that is really hard to unravel,” says Center for Limnology Director Jake Vander Zanden. “Because what we’re dealing with is hundreds and hundreds of lakes, each with very different characteristics. No two lakes are the same, and they all have different factors that are important in determining what that lake is like… So I won’t say here that I have the answer to why walleye populations are declining.”
What he can share is a couple of theories groups collaborating on this have, like that bass and other fish populations are influencing the walleye populations. Could it be predation? Vander Zanden says they looked pretty hard and didn’t find very strong evidence for that.
“Is it possible that some walleye are eaten by bass, sure, but it does not seem to be the major driving factor,” says Vander Zanden. “Another idea is that they’re competing for resources. Lakes have a limited amount of productivity and a lake can only hold so many species. A lake can’t have every possible species be abundant in it.”
Theoretically, if the issue for walleye is competition for resources, removing other fish from a lake should return them to historic numbers, right? Vander Zanden says another part of the problem is that walleye are having problems reproducing. Young walleye are routinely dying off in their first year of life.
“They’re hatching and then at some point early in the summer, they seem to just disappear,” he says. “It’s very mysterious. They’re there and then they’re just not there. And it’s not like you can observe what is happening to them. They just disappear. They may not have enough food or the environment for some reason might not be right for those really small walleye… It’s really a recruitment issue in many lakes.”
UW-Stevens Point has been looking into this extensively.
Another theory that’s being investigated is whether fish are being overharvested.
“The rules that were set into play 30 or more years ago may have worked really well for that time, but we’re 30 years later,” says Vander Zanden. “The world has changed. The environment has changed.”
Speaking of the environment, there’s a huge issue we haven’t yet touched on that’s impacting everything right now: climate change. Warmer temperatures are creating warmer lakes, and bass–particularly largemouth bass–are a warm water species, while walleye are a cooler water species.
“It’s not to say that if you warm up a lake, walleye can’t survive in that lake, but it could tip the balance in favor of bass thriving over walleye,” says Vander Zanden. “You just see that tipping over towards bass dominance versus walleye dominance.”
It’ll be a year or two until the project has enough data to say whether bass populations are impacting walleye populations. And by then, our lakes will have changed even more as a result of climate change. In the end, regardless of what comes out of this research, fishing in northern Wisconsin will look different in the future.
As to whether there’s anything we can do to reverse the trend, Vander Zanden says protecting natural lake shorelines and habitat is essential.
UW-Madison's Center for Limnology studies lakes and other inland waters.