As lakes grow warmer, the race is on to save Minnesota’s cold-water fish

ITASCA STATE PARK – From a boat anchored near the center of Elk Lake, deep among the towering white pines of Itasca State Park, researcher Will French lowered a sensor into the water. A pair of loons howled and kept their distance as French called out the readings of oxygen levels and water temperatures, meter-by-meter, from the water's surface to the bottom of the lake more than 90 feet below.

Just 30 feet beneath the surface, French said oxygen levels were already too low for most fish to survive.

That's ominous for this early in the summer, and an especially bad sign for cold-water fish because temperatures closer to the surface are too high, leaving a roughly 10-foot band of cool, oxygen-rich water these species depend on.

That band will narrow throughout the summer as the lake heats and algae uses up the available oxygen, pinching cold-water fish into a smaller and smaller space.

"They're getting squeezed," said French, sentinel lakes biologist for the Minnesota Department of Natural Resources (DNR).

By the end of August, the livable area for those fish in Elk Lake could be reduced to less than a meter in depth, some 2 feet or so, said Beth Holbrook, research scientist for the DNR. If that happens, Holbrook and French expect to see some fish kills and more strain on cold-water species that have been increasingly stressed in Minnesota for the past three decades.

So now the race is on. The DNR is using the data collected by French, Holbrook and their sentinel lake program to protect dozens of deep, cool Minnesota lakes that will stay cold enough and clean enough for key fish species to survive as the state's climate grows warmer.

Their target: a small, ghostly and sensitive fish called a cisco.

The fish, also known as tullibee, play a key role in Minnesota's aquatic food chain, as a main source of prey for some of the state's most prized game fish, including walleye, muskie and northern pike.

But cisco are also considered the canaries in the coal mine — the best indicators of a lake's health — because of their sensitivity to water clarity, temperatures and oxygen levels, said Peter Jacobson, DNR research supervisor.

"When we do have cisco we know we have better walleye and muskie populations that grow to a larger size," Jacobson said.

The collapse of the Midwest's cisco population is one of the many invisible but consequential ways that a warming climate is changing Minnesota. From songbirds along the Mississippi River to the green canopy of the northern forest, a landscape that Minnesotans have long taken for granted is increasingly under threat from rising temperatures and extreme weather events.

Across the Upper Midwest, cisco have been decimated by rising water temperatures. They've disappeared from more than a dozen Minnesota lakes and have lost more than half their total population over the last 30 years. They've nearly vanished from Indiana, historically the southern edge of their natural range, and have disappeared from nearly a third of their native lakes in Wisconsin.

While cisco were once found in about 650 Minnesota lakes, the DNR and University of Minnesota scientists believe that just 176 of those are deep enough and clear enough for cisco to survive as temperatures continue to rise. These "refuge lakes," if the DNR can protect them, will be the last stand for the species in Minnesota.

Dead zone

Cisco start dying off when a lake's water temperature reaches the mid-70s. They survive Midwestern summers by retreating to the coldest, deepest parts of a lake. But they also need oxygen-rich water, and deep water generally has less oxygen.

Compounding the challenge, oxygen in many lakes is being sucked out by increasing amounts of algae, caused by both warmer temperatures and increased runoff of fertilizers and other nutrients from farms and urban development. When the algae dies, it sinks to the lake bed and decomposes, burning through the oxygen at the bottom of the lake.

Which leaves the cisco sandwiched between the low-oxygen dead zone near the lake's bottom and the warm water at the surface.

Eventually, they simply run out of space, said John Lyons, a longtime researcher with the Wisconsin DNR who is now with the University of Wisconsin.

"A lot depends on how long they're trapped down there," Lyons said. "If you have a long summer, they're down there longer and there's more time for that finite amount of oxygen to be used up."

Statewide, annual average temperatures have increased by about 3 degrees since the 1950s, according to the Minnesota Pollution Control Agency. Lakes across the state are losing up to four days of ice every 10 years, according to the State Climatology Office.

While cisco are typically among the first lake fish to die off as temperatures increase, they're not the only species at risk. Freshwater fish are divided into three categories: cold, cool and warm — the general temperature ranges at which fish species swim their fastest, find the most food and have the most success spawning.

While warm temperatures can kill cold-water fish like cisco, they also put tougher cool-water species such as walleye at major disadvantages, Lyons said.

In southern Wisconsin and Minnesota, largemouth bass are starting to reproduce more than walleye, essentially taking over lakes.

"A degree or two can really shift things inside a lake," Lyons said. "It's the difference between who is doing well and who is struggling."

Triage for lakes

The saving grace for the cisco is that Minnesota has a lot of deep water. But protecting those lakes means ensuring that they will keep enough oxygen.

That, in turn, depends on reducing the supply of nutrients and runoff that are reaching some lakes. Hence the DNR's effort to identify and protect sanctuary lakes.

One challenge is that many of the refuge lakes on the DNR's list lie in the center of the state, where the population has been increasing for years — along with more lake cabins, housing subdivisions and retail development. State officials, conservation groups and lake associations are racing to buy up development rights within watersheds of refuge lakes to keep them wild.

The magic number seems to be 75%, Jacobson said. If the DNR and its partners can keep three-quarters of a deep lake's watershed forested and undeveloped, the water will likely stay clean enough for cisco to survive the summer, he said.

Minnesota lawmakers have set aside millions of dollars over the past several years to buy development rights near the center of the state — where the lakes are most at risk. The project is starting to show progress. Groups have just about locked down 75% in places like LaSalle Lake near Bemidji and Ten Mile Lake in Otter Tail County.

But even with intervention, hundreds of lakes are still likely to lose their cisco if temperatures keep climbing, potentially including highly popular lakes such as Mille Lacs.

Even those marginal cisco lakes with almost completely protected watersheds, such as Elk Lake in Itasca State Park, might lose their cisco as the fish are pinched into an ever-narrowing band between warm water and oxygen-deprived zones.

"The best we can hope for is that some of these really good lakes will be able to hold their own," Jacobson said.

John Magnuson, a retired professor of limnology at the University of Wisconsin, said the Midwest has begun to take a triage mentality for cold-water fish — don't try to save lakes that are as good as gone, but focus efforts on those that can survive.

"Think of this [as] doctors would after a bus crash, when they know they can't save everybody," Magnuson said.

Aside from the changes to the ecosystem, it's hard to quantify exactly what a lake and its people lose when cisco are gone, Magnuson said.

He remembers when Lake Mendota in Madison had enough cisco to support commercial fisheries. When those disappeared, cisco were still plentiful enough that people were drawn to the lake's rocky shores in the fall — just before the water started to freeze and the fish were spawning in sufficient numbers that they could be caught with hand nets.

"They'd scoop them out and take them home to smoke them," Magnuson said.

Now cisco are all but extinct in Lake Mendota.

"They're just gone," Magnuson said, "and except for one or two in our nets, you'll never see them here."