During a brutally hot summer in 2006, Pete Jacobson noticed something heartening at Ten Mile Lake north of Brainerd: The cisco weren't dying.
Throughout northern Minnesota, record July temperatures were putting the squeeze on the sensitive fish, a small, oily species that is an important food source for walleyes, northerns and muskies. Oxygen-deprived in warmer water, dead cisco were washing ashore by the thousand.
But not in Ten Mile Lake.
"It's over 200 feet deep, very clear, just a beautiful lake," said Jacobson, a fisheries research supervisor with the Minnesota Department of Natural Resources.
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It's also one of 176 Minnesota lakes considered "refuge lakes," places that might remain cooler than other spots nearby even as the climate warms in coming decades.
Refuge lakes and similar locations on land are the heart of a climate adaptation strategy that focuses on unique places with microclimates that could still support species adapted to colder conditions.
This approach is the flip side to another, more controversial, strategy referred to as "assisted migration," in which forest managers trying to adapt to warming would bring in species that are better at surviving in warmer, drier conditions.
Scientists call these places "climate refugia." Think of them as "cold spots." They could offer a way to preserve habitat for fish, moose, trees and other species that residents and visitors have come to think of as normal in northern Minnesota.
Cisco, also known as tullibee and, in Lake Superior, as lake herring, started dying in the late 1970s — but the DNR really took notice in 2006. Learn more about cisco from the MN DNR.
"If you'd go across a lake where there was a kill underway, you'd see them on the surface," Jacobson said. "They literally come up out of the deep water and are gasping for air at the top, and you'll see them dead and dying and floating at the surface."
Cisco need cold, well-oxygenated water. In Minnesota, that's down deep in clear lakes, below the thermocline, a line separating warm water above from cold water below.
In cold lakes with good water quality, there's enough oxygen for cisco all summer long. But in others, deep water loses oxygen from the bottom, sometimes leaving just a thin band with enough to support the cisco.
And in really warm years, that band can shrink until the cisco have nowhere to swim.
But even during the record fish kills in 2006, Jacobson noticed that no cisco died in Ten Mile or other deep, clear lakes like it.
"The bad news is that climate warming is going to be tough on cold water fish like cisco," he said. "The good news is that there are lakes in Minnesota that still will be able to sustain cold water fish like cisco."
The key, he said, is to maintain good water quality by protecting the land near lakes. The 176 refuge lakes are all largely surrounded by forests, which control erosion and help soak up nutrients before they run into lakes.
Nutrients in the water, especially phosphorus from fertilizer, cause algae to grow. When algae decompose, more oxygen that cisco need is sucked up.
The DNR is trying to keep those watersheds forested through what are known as conservation easements, under which landowners are paid not to develop their property.
So far it has received more than $2 million from the state's Clean Water Fund to purchase easements and has tentative approval for another $4.5 million.
But the DNR estimates it will take $180 million to protect 300,000 acres of forest in the watersheds of all the refuge lakes - including $3 million just for Ten Mile Lake.
"We have probably a 10- to 20-year window to do this," Jacobson said. "It's a lot cheaper to protect than it is to restore an ecosystem.
"When I talk to lakes associations and members of the public, a certain percentage of them do not believe in climate change," Jacobson said. "And I tell them that's fine. I say I do, I've seen the effects, we're seeing the effects now.
"But even if you don't believe it's real. Think about the fix that we're doing here. We're protecting the water quality on some really nice lakes. So even if climate change didn't happen, the worst that would happen is that we protected some really nice lakes."
By focusing on these select refuge lakes, Jacobson acknowledges researchers are giving up on some others.
"There might be some small areas that would stay cold enough, so some of the boreal species might be able to persist there."
"That's one of the basic tenets of climate adaptation strategy, sometimes you do have to give up on some resources. Some of those 650 lakes will not sustain cisco."
Other fish will be affected as well. Some walleye lakes in southern Minnesota could become bass lakes; some trout lakes in the future might be better suited for walleye. Trying to keep some species where they exist today may be a waste of money, said Keith Wendt, chairman of the DNR's climate and renewable energy steering team.
"We can promise and fulfill good fishing," he said, "but we may not be able to promise the lake that you fished with your grandfather to catch walleye, you're still going to catch walleye there. And that's the reality of a changing climate."
Some species' ranges have already changed. Moose, for example, have largely disappeared from the northwest part of the state. In the northeast, their population has been cut in half in less than a decade, to only about 4,000 animals.
While the DNR is studying what precisely is killing the moose, the cause seems likely to be a complex interaction between warmer temperatures, parasites, predation by wolves and habitat change, said Wendt. "And that's the big challenge with climate change. It interacts with all these other threats, and that's what we need to manage."
The concept of preserving cold spots could work for moose as well.
That's the thinking behind research at Voyageurs National Park, where scientists are using GPS collars to track habitat moose seek out when it's really hot, said park biologist Steve Windells.
"We can either protect those habitats that we have, or try to put additional amounts of those habitats on the landscape," Windells said.
A similar effort is underway in far northeastern Minnesota to identify and preserve places where cold-loving boreal trees will last.
Late last summer Dave Chaffin tromped through the forest off the Gunflint Trail on the edge of the Boundary Waters. The forest ecology graduate was collecting 106 temperature sensors he'd placed around the wilderness area two years earlier to record the temperature every 20 minutes.
Trees like spruce, fir and birch are expected to lose most of their suitable habitat in a warmer future, and Chaffin is hoping to find the places least likely to change.
Chaffin worked in advertising in Minneapolis for 12 years before quitting to study the effects of climate change on the canoe area, a place he loves. He's now a PhD candidate in forest ecology at the University of Minnesota.
Climate models predict this part of Minnesota will be five to 12 degrees warmer by the end of the century. But those models use what he called very "coarse-grained" climate data.
Climate data used in many prediction models are based on one reading for each 12-by-12-kilometer square of land.
But within those 12 kilometer areas, there can be huge temperature variations. He stopped, for example, on the north side of a steep hillside under a thick tree canopy, likely much cooler than high on an exposed ridge.
University of Minnesota forest ecologist Lee Frelich, who is overseeing Chaffin's work, said they have found some pretty cold spots, especially on the eastern side of the Boundary Waters.
"So there might be some small areas there that would stay cold enough, so some of the boreal species might be able to persist there," Frelich said.
Land managers in the future could then potentially set aside those areas as refuges for boreal trees.
"It's a question of what Minnesota is all about," the DNR's Jacobson said. "We have this image of 'northness,' that I think is something that we identify with in Minnesota. I'd personally like to see that continue, even in a climate warmed Minnesota."