The Outdoors

By Scott Rall, Outdoors Columnist

It has a long time since my neck of south west Minnesota has dealt with area lakes winter kills. Winter kills is just a slang term for low oxygen content in the water of a lake. When the oxygen content gets too low the fish will start to die. There has never been a total 100% winter kill in any lake that I know of.

There aways seems to be at least some fish that survive the low oxygen, but as a percent of the total fish populations only a small percentage might remain. Partial winter kills, on the other hand, can happen quite often and may be localized to one small area in an overall larger lake.

Oxygen in water is measured by parts per million. There are fancy machines or electronic gadgets that do this measuring. Just drill a hole in the ice and drop down the sensor and in seconds you can read the parts per million.

I am sure it varies but in most of the lakes in my county and those surrounding counties around me the oxygen readings in a normal winter run about 12-16 parts per million.  Fish get in serious trouble when the parts per million gets down to about 2. I have seen readings in lakes where a draw down was intentional and almost all of the water in the lake was drained the prior fall when oxygen readings can be 0 parts per million.

Snow cover is really one of the biggest factors as to when and if a lake will winter kill to some extent. Last year we tested a lake that had 2 feet of ice and only 6 inches of water below it and the oxygen readings were totally able to support fish life. There was very little snow on the ice that year. The same lake this year has about 12-16 inches of snow cover, and when the sunlight is blocked oxygen content can fall very fast. Readings in the normal range fell to the fish in trouble range in as little as three days.

Plant life that grows in lake bottoms produce oxygen while they are alive.  When the sunlight is blocked, these plants begin to die and as they do so they actually consume oxygen and expel C02. When this flip flop happens, oxygen content moves negative very quickly.

If you read my column with any regularity, you know that I am a pike spear fisherman. I have seen lakes with clear water turn cloudy overnight.  This is as a result of an algae bloom. Little snow cover allows algae to be produced even during periods of very cold water. Growing plants make oxygen and dying plants consume it. Snow cover in above average depths cuts off the sunlight and winter kills become far more common.

The one big mystery to me is why some lakes of similar size and depth maintain much higher oxygen levels then other lakes only a few miles away with similar characteristics. Some never winter kill when lakes 5 miles away suffer significant fish losses. Most of the shallow lakes in south west Minnesota have man made aeration systems designed to either keep water open, thus allowing open water to absorb oxygen,  our bubble air from the lake bottom like an aquarium pump.

In Minnesota if the DNR intends to stock fish in a particular lake an aeration system is mandatory in most cases. Why would they want to stock a lake over and over if it just going to winter kill repeatedly anyway? These systems are not the panacea they might seem. They will certainly carry over fish that are in the close proximity but I have been told that you need not travel very far for these benefits to evaporate. An 800-acre lake with an aeration system might still winter-kill in 80% of those acres.

Many shallow lake systems benefit from winter-kills in that with all of the fish removed the vegetation can re-establish and make for much better overall fish and wildlife habitat. These lakes are usually called a boom or bust fishery. Fishing can be grand when conditions allow but more often than not an occasional winter kill makes the process start all over again and with some regularity. Winter-kills are either good or bad depending on what you are trying to achieve.

All I know for sure is snow depths can be hard on both fish and wildlife as well.  All you have to do is take a drive around and see all the pheasants working their tails off trying to find a few morsels to eat.

Spring will come and if we can get through the month of February, we will turn the corner for both the benefit of fish and other wildlife, what happens in the mean time is anyone’s guess.

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If you have any questions, reach out to me at scottarall@gmail.com.