Midges, which nourish trout in Minnesota streams through the winter, can't tolerate a milder climate.
From left, researchers Jane Mazack, Will French, Jenna McCullough and Dan Spence netted trout from Cold Spring Brook, near Zumbro Falls, as part of a study to learn about the relationship between midges and the fish that eat them.
ZUMBRO FALLS, MINN. - A flying insect that thrives in midwinter might seem like a creature from a frightening fictional Minnesota.
But Diamesa mendotae, a cold-hardy but delicate insect also known as a midge, is very real and may provide a measure of how the state's climate is warming, and what effect that might have.
Researchers from the University of Minnesota are working to understand more about the relationship between these unusual freeze-resistant insects and the fish that eat them in streams in the southeastern part of the state. The three-year project is primarily about brown trout, a popular target for anglers. "We're thinking that a changing climate and increasing air temperatures will affect water temperatures, and that could reduce [fish] populations," said entomology professor Len Ferrington, principal investigator on the project.
Midges are also at the heart of the study because of their unusual ability to thrive in the winter, when they serve as trout food. When most other insects are idling, with eggs and larvae hidden away from the cold, midges, armed with a sort of internal antifreeze, produce several generations of offspring. Stream anglers, skiers and others who might poke into the deeper recesses of southeastern Minnesota during winter see them flying in clouds above the water or speckling the streamside snow.
Jane Mazack, a graduate research assistant in the Water Resources Science program at the U, said Diamesa can remain active down to about 6 degrees below zero, which this winter would be the second-lowest temperature recorded in the Twin Cities.
The midges are also high in calories and nutrients -- "like pecan pie" for trout, Ferrington said. But their cold-hardiness is balanced by an intolerance for warmth. An increase of as little as 1.8 degrees in the average water temperature in a stream could wipe out an entire winter reproductive cycle for them, Mazack said. That could mean less food for trout during winter, or at least less nutritious food in the form of other bugs.
Most southeastern Minnesota trout streams are fed by groundwater, which emerges at the same temperature as the average temperature of the air above ground. That's about 48 degrees, which keeps the streams from freezing over and in turn allows insects to hatch, fly about, mate and lay eggs. But Minnesota's air temperatures have been creeping upward in recent decades, most notably in the winter, and Diamesa stop growing in water that's 50 degrees.
"It's really a narrow boundary," Mazack said.
Lori Krider, another graduate assistant in the Water Resources Science program, has found that the water temperature in groundwater-fed streams rises an average of .38 degrees for every 1 degree rise in air temperature. An average air temperature increase of 7.9 degrees by the end of the century, predicted by one Intergovernmental Panel on Climate Change model, could produce a stream temperature increase of about 3 degrees, which would be beyond midges' threshold. That could alter food sources and habitat for trout, the researchers said.
Hip deep in winter water
Researchers on the project, funded by a $300,000 grant from the Legislature, work in a rugged laboratory. Last week, three graduate students and a DNR fisheries specialist waded into Cold Spring Brook, just outside Zumbro Falls, to collect bug samples, then probed the stream with a long-handled, electrically charged device that shocked fish to the surface.
As a light snowfall settled in the folds of their bulky clothes, they collected several dozen fish -- almost all of them brown trout -- and carried them to a portable, heated tent. There, the team weighed and measured each now-anesthetized fish, clipped a fin to mark it, injected a small tracking device under its skin and scraped off some scales that can be used to determine a fish's age. Mazack then pumped a jet of water into each fish's mouth and massaged its stomach, forcing out the contents -- sometimes a dozen or more relatively intact insects -- into a sieve, then sealing them in small plastic envelopes. They then dumped the fish back into the stream.
The team will visit 12 streams several times during each of the project's three years. On return visits, as many as 40 percent of the fish they net might be ones they've caught before, giving them opportunities to track fish growth.
"It's been kind of eye-opening to see how much food is in some of these fishes' stomachs," said Will French, a conservation biology graduate research assistant. "They're feeding and growing in the winter -- quite a bit, actually."
French said his interest is in how well fish are faring in the winter in any one stream compared to others. Differences could lead to changes in how streams are managed; researchers in a separate part of the project are examining the banks, curves, bottoms and other structural elements of streams and how they might be changed as well.
Indeed, increasing warmth in the air, groundwater and streams could be slowed by more green buffers along streams, as well as shady plants and trees above them, Ferrington said. But for now, the potential problems of a warming climate are being examined in the cold.
"There's a lot of stuff going on here we really don't know anything about," French said. "We can't forget about winter. It is Minnesota."
Bill McAuliffe • 612-673-7646