Scientists are still trying to figure out how a changing climate affects walleye and other species of fish. Most don't expect the walleye to be a winner.
As global climate change continues delivering warmer temperatures and heavier rains to Minnesota, lakes and their inhabitants will feel it.
"One of the places you expect climate change to make a big difference is in changing the mix of species that do best in a lake," said John Magnuson, director emeritus of the Center for Limnology — the study of inland waters — at the University of Wisconsin-Madison.
Fish are cold-blooded creatures, so you'd naturally expect them to respond to changes in temperature, which affect processes as basic as reproduction and growth. But the drop in walleye numbers has been too big to blame on just an average rise of a degree or two, according to Paul Venturelli, an assistant professor in fisheries at the University of Minnesota.
"Warmer temperatures can stress walleye. But we think the major effect is indirect," said Venturelli, a member of the panel that helped the Minnesota DNR explore why the walleye population in Lake Mille Lacs has declined.
To understand how climate change is affecting walleye, he said, you need to look at the whole food chain.
The walleye's favorite meal happens to be a fish that seeks refuge in the coldest area of a lake during the summer: cisco, also known as tullibee.
"I liken them to sticks of butter swimming around," Venturelli said, "a pretty fatty fish with high energetic content."
Changes in temperature can alter the number of those fatty tullibee swimming around, he said.
As tullibee seek the deeper, colder areas of a lake in the summer, they're dependent on a limited supply of oxygen, which naturally declines over time. That's always been the case in Minnesota's lakes, said DNR fisheries research scientist Pete Jacobson. The difference now is a longer growing season, which lengthens the time that a lake is stratified into warm and cold sections.
"When we have earlier springs and later falls, that water is locked away longer, the oxygen concentrations decline even further than they used to, and we think that's affecting cold-water fish like tullibees," Jacobson said.
An analysis of declining tullibee populations led DNR scientists to consider factors like nutrient pollution from runoff, he said. But they found Minnesota's warming climate was the most likely culprit.
"It's one of those indirect ecological effects that you might not think of," he said. "It's not simply warmer surface waters that are doing it. A lot of these ecological systems are fairly complex, and sometimes the effects are rather unexpected. When you actually parse them out, they can be pretty strong."
The DNR is trying to preserve 176 designated refuge lakes where tullibee should be able to survive even with continued warming. Mille Lacs isn't one of them — it's a shallower lake, and even though it has tullibee now, it could have a hard time supporting the population in the future.
Another walleye food source scientists have their eyes on is the yellow perch, a fish that has also been declining in Minnesota lakes. Biologists haven't yet pinpointed the cause, but climate change is a possibility. A study published in July out of Lake Erie showed that warming temperatures might hurt yellow perch reproduction.
Study author Troy Farmer, a postdoctoral researcher at Auburn University, said that he had expected climate change might favor yellow perch because it's not considered a cold-water species, like tullibee.
"We actually saw just the opposite — that short, warmer winters were followed by fewer juveniles the following year," he said.
Farmer said more research is needed to find out whether yellow perch in smaller inland lakes could also be affected. "I think the potential is certainly there," he said.
But the connections among walleye, tullibee and yellow perch only scratch the surface of the complex lake food web. Scientists who analyze those connections use sophisticated computer programs to do it.
Venturelli said a study by other researchers of a small lake in Wisconsin looked at 182 species. The researchers concluded there were nearly 1,000 different connections among those species.
"Removing a species, changing the chemistry or the thermal properties of that lake and try to figure out what happens to the top predator, an example being walleye, is really difficult," Venturelli said. "There's an expectation, a demand for understanding, and for fisheries biologists, it can be very difficult for us to deliver most of the time."
Even if scientists know a lot more than they once did about what's going on in Mille Lacs and other Minnesota lakes, the complexity makes solutions hard to come by. Scientists say there's no quick fix. Even if there were one, like stocking plenty of walleye in lakes with low numbers, it wouldn't be practical for the DNR, said the University of Wisconsin's Magnuson.
"They're not going to have infinite resources to raise walleye and put them in all the lakes," said Magnuson, who has been warning about how climate change could affect walleye and other fish for more than a decade.
An added challenge is that while scientists see a consistent climate trend toward warmer temperatures in Minnesota, there's still a lot of year-to-year variability.
"The DNR will have to respond to that high variability and manage resources differently in periods that are slightly cooler versus when they're slightly warmer," he said. "It's going to be a challenge."
Scientists don't know exactly how big a role climate change is playing in the Mille Lacs walleye situation. But it's one of the many factors that will continue being part of the equation — for every fish species in every lake in the state.
Heinz Stefan, an emeritus civil engineering professor at the University of Minnesota, said computer models show that each lake responds to climate change in unique ways. Meanwhile, lake users plan annual fishing trips expecting things to remain the same. If something changes, they expect to know exactly why, he said.
"We're spoiled. We have expectations, and many of them have been fulfilled," he said.
Take the cars we drive, he said. They almost always perform the way their owners' manuals describe.
"Because they're man-made. But natural systems are not man-made, and there are a lot of systems that nobody put in there," Stefan said. "That's the big difference, and I think the public is struggling with that."
5 ways climate change threatens fish
Minnesota is warmer and wetter under global climate change, and scientists expect that trend to continue. But the effects on fish are not always obvious.
1) Food sources
A one- or two-degree average temperature change likely won't have much of a direct effect on most fish species, but it might have an impact on their food sources — everything from aquatic plants to insects to fish that are prey for game species like walleye.
Scientists have already found cisco or tullibee are sensitive to climate change in Minnesota, and they happen to be a favorite meal for walleye.
2) Hooking mortality
A study on Lake Mille Lacs found that for bigger walleye, being caught and released when it's hot outside can be fatal.
As climate change brings warmer temperatures to Minnesota, hooking mortality could become a bigger problem. In British Columbia this summer, wildlife managers banned all fishing in some rivers — including catch-and-release — during a heat wave, saying catch-and-release mortality rates could reach 40 percent.
As cold-blooded creatures, water temperatures drive a variety of biological functions in fish, including reproduction.
Scientists are still learning about how temperature changes under climate change will affect reproduction. But one study on Lake Erie showed yellow perch did not reproduce as well during shorter, milder winters. The results were confirmed in lab tests. Yellow perch are a food source for walleye.
4) Heat stress
High temperatures can have a direct effect on some species of fish, causing stress that interrupts their normal functioning. For example, a heat wave in 2012 in Iowa was found to be the culprit in large fish kills of shovelnose sturgeon in rivers.
In lakes, some fish will seek refuge in the deeper part of a lake where temperatures are colder than surface temperatures in the summer. But as temperatures rise, shallow lakes might have a harder time supporting cold-water fish species.
5) Oxygen levels
Lakes are stratified into warm and cold sections during the summer months. The cold bottoms of lakes are also where algae goes to decompose. As it decomposes, it sucks up oxygen, so the oxygen that's available in the coldest part of a lake can diminish over time.
Under climate change, Minnesota has a longer growing season, which lengthens the amount of time a lake is stratified. For fish like tullibee that seek colder temperatures, a lack of oxygen where they swim can kill them.
In addition to a longer growing season, climate change has led to heavier rains in Minnesota, which can bring more nutrients into a lake. Nutrients fuel algae growth, which can contribute to depleted oxygen levels.