FERGUS FALLS, Minn. — Zebra mussels first invaded Minnesota lakes three decades ago, causing biologists and anglers to sound the alarm on the damage they could do to the state's fisheries. The thumbnail-sized creatures filter food particles out of the water, upsetting a lake's food chain and starkly increasing water clarity.

With walleye angling being a major recreational pursuit as well as the driver of a multi-billion dollar industry, the worry was that a collapse of the state's most popular fish would damage to an important piece of Minnesota.

While the pesky invasive species has spread relentlessly across the Land of 10,000 Lakes since first being discovered in Lake Superior in 1988 and currently infesting more than 400 lakes with the number growing each year, the jury is still out as to whether zebra mussels are a death knell for good walleye fishing.

In some lakes, most notably the famed Mille Lacs Lake in central Minnesota, the mussels have appeared to have a serious negative effect on walleyes because of increased water clarity. In other lakes, zebra mussels seem to have had little or no effect on walleye populations and size structure.

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But in almost all lakes infested with zebra mussels, walleye behavior has changed drastically because of changes to their preferable habitat.

"The effect of zebra mussels on walleyes is variable from lake to lake. No two lakes are the same," said Gretchen Hansen, an assistant professor in the Fisheries, Wildlife, and Conservation Biology Department at the University of Minnesota. "I know that's frustrating for fishermen because there's not one answer to the question, 'How have zebra mussels affected walleyes in my lake?' It's what I always say: It's complicated and it depends."

Hansen was a presenter at a virtual Midwest Fish and Wildlife Conference in early February. She's a top researcher in the relationship between zebra mussels and walleyes, and she talked about two of her papers at the conference.

The first, published in 2019, focused on Mille Lacs Lake and concluded zebra mussels (and improved septic systems) helped lead to increased water clarity and reduced suitable walleye habitat. Walleye prefer water with low clarity and cool temperatures.

The paper concluded a marked increase in water clarity "co-occurred with the onset of walleye declines" in the lake while "thermal-optical habitat area for walleye in Mille Lacs declined over time."

"Walleye habitat area in Mille Lacs declined over the past several decades, with important implications for fisheries management," the paper said.

The second paper, published in 2020, concluded that young walleyes grow 14% slower in lakes infested with zebra mussels (and 12% slower in lakes infested by spiny water fleas, another invasive species).

The research made comparisons fair by correcting for many variables, including temperature, and used data collected from hundreds of thousands of fish beginning in 1983 on major walleye lakes.

Hansen said slower growth in fish less than a year old could lead to higher mortality rates due to predation and less chance of surviving through the winter.

"The impact of invasive species isn't uniform in every lake. It might depend on the depth or size of the lake, what food sources are available, what competition is in the lake, many other things," Hansen said. "The word I'll go back to is variable."

The evidence can be seen at the local level. Minnesota Department of Natural Resources assistant area fisheries manager Howard Fullhart, based in Fergus Falls, said differences in walleye populations in infested lakes are evident in Otter Tail County.

Fullhart said netting survey results on Pelican Lake show a walleye population as large and healthy as it's ever been, while data on North Lida Lake show a declining walleye population since zebra mussels invaded. Other infested lakes in the area show the same differences.

"We don't really know what's going on. Is it because some of the lakes have had mussels long enough that the lake has re-stabilized? Is it because a lake like Big Pelican has a river running through it so more nutrients are going through there? Is it something else? We don't know," Fullhart said.

For anglers, zebra mussels have changed the game even on lakes where walleye populations have not been impacted. Chad Maloy of Fargo, a member of the F-M Walleyes Unlimited fishing club, said he's heard from many longtime anglers that fishing on zebra mussel-infested lakes isn't as good as it used to be. That includes Pelican, a popular walleye destination for decades.

"The lake has changed because of the zebra mussels. I don't think a lot of anglers have changed with it and that's where you hear the comments like 'fishing isn't good there anymore'," Maloy said. "Because the water is clearer, the weed edges are deeper because light penetrates further. You can't pull spinners in 12 feet of water along the weed edges like you used to. Now the fish are in the weeds and you have to find pockets and drop a slip bobber and a leech in there, or a pull a minnow through the weeds.

"It's not relaxing, easy fishing anymore. It's tough fishing that's a lot of work. But Pelican Lake the last several years is some of the best walleye fishing I've ever seen for numbers and size. It's just different and anglers have to be willing to adjust."

Hansen, the University of Minnesota researcher, said the next steps are to find out why walleyes react differently to zebra mussel infestations in different lakes, and what can be done to help.

"Where we see a decline in walleye, is there something fisheries managers can do to make those lakes more resilient or are there inherent characteristics that would prevent that? That's the next thing we need to look at," Hansen said.