How climate is changing the habitat of some of the favorite fish of Wisconsin anglers

For as long as anyone can remember, the highly prized walleye has been a staple of Wisconsin’s northern lake fisheries. For some anglers, nothing matches the walleye for the kinds of fishing thrills it provides. They’re elusive, cautious and always a rewarding catch. They’re also a mainstay of the state’s tourist industry. 

But in many lakes and other waterways, they’ve been on the decline. Is it climate change that’s causing state waterways to warm? Or are there other factors at work? That’s the focus of today’s second installment on the changing northern Wisconsin forests and lakes. Read Part One here. 

Dan Isermann is unit leader of the U. S. Geological Survey’s Wisconsin Cooperative Fishery Research Unit, based at the University of Wisconsin-Stevens Point. The research efforts he leads are often collaborations with state and other federal resource agencies.

Dan, are northern lakes and streams warming in the Upper Great Lakes?

That’s certainly the focus of our research. They are definitely warming to varying degrees, and it depends on various factors. We’re also focusing on invasive species in the Great Lakes, and on ecosystem health and the ability of fish species to respond.

How are walleye populations faring?

We’ve seen a decline in naturally reproducing walleyes in the past two decades. We’re no longer catching naturally produced walleye in the fall in some of these lakes. A lot of the lakes where we’ve seen declines didn’t have robust populations to begin with, not to say we haven’t seen declines in better systems. We’ve had a series of graduate students who have focused on trying to identify what is going on. 

Is this due to a lack of successful hatching? 

That is what we’ve seen in a lot of the lakes that we might classify as having declining natural reproduction. With rare exceptions in study lakes, those fish are not showing up in fall. Where we see them reproducing, we’re seeing them in the larval form, but if we’re getting larvae [that aren’t developing further], we’re looking at why. There are two possible causes. Typically, we see zooplankton in the diet of fish less than an inch long, but we were seeing larval perch, not zooplankton. We thought maybe it was how zooplankton was distributed. Perhaps there are water clarity and climate impacts. We’re still in the process of sampling.

Second, are they being preyed upon? We’ve seen that warm-water fish have expanded. We have some of the best habitat on earth for bluegills, crappies, bass and other fish in that family. But we didn’t see very many walleye larvae in small panfish.

In one experiment with the Center for Limnology [at UW-Madison], we removed 280,000 [warm-water] fish from McDermott Lake in Iron County to see if it would boost walleye populations. There never was a robust population, but the lake did have some. We’ve not seen any strong response from the walleye population. It may be a little early. It takes a while for fish populations to respond, but they’re not really showing us a response to that kind of management action.

Are other species showing declines?

We’ve been looking at interactions between walleyes and yellow perch. Environmental factors that regulate walleye recruitment are similar to what they are for yellow perch. In a lot of these inland lakes, we don’t know a lot about perch. We have a current student looking at whether perch follow the same pattern as walleye in lakes that sustain and those that do not. For the most part, perch sort of tracked the same patterns as walleye. We saw fewer perch at various stages in the summer. So it could be that environmental conditions are such that both species are reacting in the same way.

So, what does the future hold for the treasured walleye?

I think there’s a lot of different ways to look at it and different strategies we can adopt. It’s hard, because of the diversity of fisheries available. Some lakes didn’t support large walleye populations, but people occasionally caught some, and it was important to them.

I have a post-doctoral student working on fish stocking using the premise of let’s assume there are fewer lakes to support fisheries, so we were stocking to support fisheries. But if there’s an increased demand for stocking in the future, producing fish to stock can be a limiting resource. Our theory is demand will increase, but we’re not sure we can do a lot on the supply side. In northern Wisconsin, with hundreds of lakes, how do we make decisions to stop stocking in some places?

Will anglers adapt to the new world of fewer walleyes?

We’ve been doing some human-dimension work, where we surveyed anglers about where they would fish based on hypothetical situations. What we saw is there is a fair number of anglers interested in fishing opportunities on a broader scale. Certainly, there are many interested in walleyes, and they are an important part of our fishing population. But [warm-water species like] bluegills, bass and crappies support immensely popular fisheries in Wisconsin. Really, bluegills are the bread and butter of fishing in the Midwest.

We have research on how bluegills might respond to climate changes. We’ve just started it. We’re trying to identify factors related to bluegill size structure, looking for harvestable-sized fish. We’re doing this on a Midwest scale as well. What drives size? Can we simulate what might happen 30 years down the road? In northern lakes where shorter growing seasons limit size, maybe some of those [warming] lakes might grow larger bluegills. Of course, it’s not as simple as just a longer growing season.

And there definitely are bright spots for walleyes out there, what I would say are flagship populations. Their success is probably related to habitat that’s already there. “Walleye bright spots” is the title of research we’re working on with the Center for Limnology. What are the ingredients? What can we do to move the needle?