'Winterkill' In Minnesota: Why You Shouldn't Be Worried

Cold, long winters with lots of snowfall (sound familiar?) can lead to fish die-offs. It's been happening throughout the Minnesota this year, in lakes near Brainerd, Hinckley and in the Twin Cities.

“While seeing lots of dead fish can be disconcerting, we remind people that winterkill is normal and happens every year to some extent,” Neil Vanderbosch, DNR fisheries program consultant, said in a statement.

Once a lake is capped with ice, the amount of dissolved oxygen present in a lake depends on how much oxygen is produced by aquatic plants. Winterkill occurs when snow and ice limit the amount of sunlight reaching aquatic plants.

Without adequate sunlight, a lake's plants produce less oxygen. If the vegetation dies from lack of sunlight, the plants start to decompose, a process that further depletes oxygen dissolved in the water, according to a news release.

• Trout species require high dissolved oxygen levels and may begin dying off when a lake’s dissolved oxygen falls below 5 parts per million.
• Bluegill and largemouth bass are also sensitive to low oxygen levels. Walleye, yellow perch, northern pike, carp and crappie can tolerate dissolved oxygen levels as low as 2 ppm.

Winterkill rarely results in the death of all fish in a lake, but lakes with frequent winterkill events tend to be dominated by bullheads.

Winterkill can have some benefits. In lakes with overabundant panfish, occasional winterkill can increase growth rates of the fish that survive. Winterkill can also greatly reduce carp abundance, which leads to increased water quality and more successful stocking efforts.

People who see numerous dead fish after the ice melts should report their observations to the State Duty Officer at (800) 422-0798.

Will Minnesota's cold temperatures kill pests?

You're probably looking for some positive to come out of the state's cold and snowy spring. On that front, the DNR has good news and bad news.

The good news:

Some insect eggs, especially those of invasive species, will die quickly in the cold.

The bad news:

Many insects and ticks can avoid the cold altogether, however, by burrowing into the ground or bark. Temperatures must fall to at least 30 degrees below zero in order to kill a significant number of the emerald ash borer.

"Overall, we may see lower numbers of a few insect species in 2018, like gypsy moths," writes Jess Hartshorn, a DNR forest health specialist. "This minor setback does not mean much in the long-term, however. This winter will not be enough to kill all emerald ash borer, gypsy moth, or any other pest of concern."

Top image via Minnesota Department Of Natural Resources