Minnesota's famed walleyes giving way to bass and bullheads

Maybe it's prophetic that Minnesota is changing its new conservation license plates to show a jumping largemouth bass.

As 2 million anglers prepare for Minnesota's walleye season, which opens Saturday, some fisheries experts warn that the state's most popular fish may be harder to find in future years. That could mean the next generation of Minnesota anglers will be more excited about the Memorial Day weekend bass opener than the traditional mid-May walleye opener.

Scientists say that within a few decades some Minnesota lakes probably will be too warm for walleye to thrive. And even northern Minnesota lakes might get too warm in summer to hold key walleye food such as cisco.

Walleye are the backbone of the resort, fishing and bait industry and represent a century of family fishing traditions.

"We're going to have fewer lakes with walleyes in them, and fewer walleyes in other lakes," said Don Pereira, fisheries research and policy manager for the Minnesota Department of Natural Resources. "We need to start getting the public thinking about accepting other game species, like bass."

Average temperatures in Minnesota - already up a degree in the past century - are expected to increase another 3 degrees by 2050 and keep rising.

"Even if we act fast to do something, that's where we'll be. And we're talking by mid-century, not 100 years from now," Pereira told a conference organized by the DNR this past winter. "After that, it all depends on what humanity does with carbon emissions."

That's the projection of hundreds of scientists who study the issue. The warming appears to be caused by a doubling of carbon in the atmosphere over the past 150 years that is trapping more heat near the earth. A global panel of scientists says the carbon problem is caused by human activities, namely burning fossil fuels.

Minnesota scientists say the state's summers will be more like Kansas by century's end, and winters more like Illinois. The fish in Kansas? Few walleyes, and lots of bass and bullheads.

Scientists already have warned that a warming climate could mean trouble for fish and wildlife at the southern edge of their natural range, such as moose and lake trout. As more lakes hold more and bigger bass and sunfish, which thrive in warmer water, experts say, lake trout will retreat to just a few lakes in far northern Minnesota within decades, and even there take refuge in deeper water.

Minnesota's North Shore trout streams also could be imperiled, and already face fish die-offs during hot summers. Unlike trout streams in other areas that are fed by cold-water springs, North Shore streams are dependent mostly on surface water runoff that warms quickly.

It's unclear exactly how warmer air will affect the state's lakes. But a recent University of Minnesota Duluth study found Lake Superior's water temperatures are warming twice as fast as air temperatures.

And it's not just a Minnesota issue. A study of 209 lakes across the U.S., published in 2004 by University of Minnesota scientist Heniz Stefan, Environmental Protection Agency scientist John Eaton of Duluth and others, found that suitable habitat for cool-water fish like walleye will decline 30 percent because of higher temperatures.

It's the state's cold winters, and relatively cold waters, that keep walleyes from growing faster and bigger here. The world record 22-pound,11-ounce walleye came from Arkansas, after all. Minnesota walleye haven't hit 18 pounds.

Pereira said some northern Minnesota lakes will warm up into the walleye's optimal temperature range as the state's climate warms. Higher temperatures will mean many more days for walleyes to grow, and they should get bigger more quickly.

But when scientists look closer, the picture gets murkier.

Walleye do best in water that is between 59 and77 degrees Fahrenheit. Anything warmer than that, even for short periods, and walleyes become stressed. At 82 degrees they stop eating. At 88 degrees, they die. The biggest, most prolific breeding fish die first.

Water temperatures in Lake Pepin, a famous walleye lake along the Mississippi River near Lake City in Goodhue County, are projected to rise into the walleye's danger zone. Pereira said that not enough of Lake Pepin will remain below 77 degrees during hot summer months to support enough walleyes to keep the population thriving.

"If the projections are on target, Lake Pepin couldn't hold many walleyes," Pereira said, noting that warm summer temperatures already have caused Pepin's big walleyes to grow slower, likely due to stress. And during springs after recent hot summers, those stressed walleyes have produced fewer young.

In some lakes, walleyes can move to deeper water - if there's enough oxygen and quality food down deep. But scientists say that during the summer, deeper water will have less oxygen, causing "dead zones" near the bottom.

Most of Minnesota's top 10 walleye lakes - the biggest, most popular, most productive walleye lakes in the state - don't have a deep, cold-water refuge for fish to move into when water warms near the surface. Walleyes haven't needed that refuge in the past, but may in a warmer future.

"Lakes that mix - deeper-water lakes like Rainy, Cass and Vermilion, and the Boundary Waters - could see better walleye production in the future," Pereira said. "But the variable is what will happen with tullibee/cisco. (Walleyes) won't grow faster if they don't have the food."

Already, some of Minnesota's most famous and productive walleye lakes are seeing a marked increase in the number and severity of cisco die-offs, Pereira said.

Walleye can live eating just perch and minnows. But will they thrive, reproduce and grow?

"We're just starting to try to answer that question now. We don't know how important the cisco/tullibee relationship is with walleye," Pereira said, noting research soon will begin on Lake Mille Lacs, which has seen increased die-offs of ciscoes in recent years.

"Temperature is only part of the equation for fish, one of the factors they need to live," said Eaton, now retired from the EPA. "The temperature increase we'll see may not directly affect a walleye in a specific lake, but it might affect something in the (food chain) that disrupts tullibee that disrupts walleye. We will be seeing a lot of indirect impacts we aren't even thinking about yet."

John Magnuson, emeritus professor of limnology at the University of Wisconsin-Madison, said evidence already is mounting that fish in some lakes no longer grow all summer. Magnuson spearheaded research that has showed, using 150 years of ice-out data, that upper Midwest lakes are covered by ice nearly three weeks less than a century ago.

"Instead of seeing the peak growth period in summer when it should be, we're seeing a period where some fish stop growing. The charts look like a double-humped camel, with more growth in the spring and fall and no growth in the summer because it's too hot," Magnuson said. "We may see much longer growing seasons as we get warmer. But the middle of that growing season could well become lethal if the fish can't find a refuge."

Pereira outlined these dire consequences to highlight the importance of water quality in Minnesota. We likely can't stop the planet from warming fast enough to save walleyes in Lake Pepin and likely some other Minnesota lakes, he said, but we can keep our lakes cleaner so there's less stress on fish.

That means not just point-source pollution such as factories, but also runoff from cities, logging sites, farms and construction projects. That becomes more critical as scientists predict more rainstorms, causing more runoff and carrying more nutrients and contaminants into waterways.

"What's important is that we keep the lakes clean so that can continue to support walleye. Any decline in water quality is going to make the situation worse," Pereira said. "Climate change is going to take our current habitat issues and magnify them."

It's also time to start planning new fisheries programs, Pereira said. That means identifying which lakes that may be a waste of time and money to stock or encourage walleyes in, and instead focusing on species that will thrive in warmer water.

Pereira said Minnesota anglers in 50 years will need to know how to catch more largemouth bass and sunfish. But he's not suggesting that Minnesota change its official state fish. Not yet.

"We're still going to have some walleye lakes, and likely some good walleye lakes up north," Pereira said. "But lakes that are marginal now will no longer be able to support walleyes. And we're going to have to make some decisions on whether it makes sense to keep trying to force them to produce walleyes or shift to a more suitable, warmer water species."