From a report in the StarTribune in 2011:
Those big December snowfalls have crews on some Minnesota lakes heading onto the ice earlier than usual this winter in an effort to prevent mass fish kills. They're on a rescue mission to install aerators and create open water before oxygen levels plummet to the point that fish essentially suffocate under the ice. Some lakes are already showing faster-than-usual oxygen depletion...A new photo and additional details in 2014:
Winterkill is a natural process that happens when fish don't have enough dissolved oxygen in water, he said. Because of the ice cover, oxygen in winter comes mainly from aquatic plants, which receive enough sunlight through ice to grow. But in years with lots of snow, sunlight penetrates ice less and plants stop growing. Instead of producing oxygen in water, the plants consume it as they die and decompose...
Sometimes it kills all the fish in a lake, he said, and sometimes it only affects part of a lake or some species of fish. It is more of an issue in southern Minnesota, he said, where more lakes tend to be shallower...
Lower photo credit: Allan Nistler, StarTribune.Lauer said the first fish to die are game fish: walleyes, bass, panfish, perch and northern. Then rough fish such as carp, suckers and bullheads succumb.“I’d say bullheads go last,’’ said Lauer. “If we get [dead] bullheads … we know it’s been a significant kill.’’..“I’ve never seen [this lake] winterkill,’’ said Frankie Dusenka of Frankies Live Bait and Marine in Chisago City. “There’s still 36-plus inches of ice around here; it’s amazing.’’DNR officials drilled holes around the lake and found very low oxygen levels — 1 part per million or less. Normally, the level would be 8 to 12 parts per million...The DNR assesses lakes with winterkill and determines whether to restock them or let natural reproduction occur. Sometimes the fish kills can help a lake by removing rough fish or reducing the number of small game fish, allowing survivors to grow larger.
Reposted again, because it's happening again:
Layers of ice, slush and deep snow on lakes north of the Twin Cities could lead to a rash of fish kills this year on shallow lakes susceptible to losses of oxygen, state fisheries managers say.Especially if the winter drags on without a big thaw, experts for the Department of Natural Resources say this could be the third winter in a row for partial kills of panfish and largemouth bass in vegetated lakes less than 20 feet deep...... anglers in the region [have been] turning on aeration systems in their favorite shallow lakes. Fifty-eight machines are now running in Carstensen’s area compared to 79 a year ago at this time...In other cases where there’s been a heavy fish kill on a shallow lake, the DNR sometimes will stock walleye fry. Without predation by other species, the baby walleyes can grow relatively quickly.