The Realities of Winterkill

January 31, 2009 by  

By Doug Leier

When it comes to violence and death, nature is not G rated by any stretch of the imagination.

If you’ve spent time outdoors, you understand that red fox will efficiently kill plenty of hen ducks keeping watch over a nest. Cormorants will eat many fish. There is no doctor to prevent disease, and battles against the elements can mean slow and painful deaths for the weak and the frail.

Visible events, such as a deer falling through the ice while attempting to cross a lightly frozen river, are etched in our minds.

These types of occurrences do not form a pretty picture, but they are part of nature’s fascinating cycle of predator and prey and animal against environment.

We’re familiar with some of nature’s unpleasantries because we’ve seen them personally, or viewed the pictures or video. But sometimes, what we can’t see is just as significant. While snow and cold create challenges for some animals on land, they can also make life difficult for fish underwater.

Winterkill carp discovered after ice out

Winterkill carp discovered after ice out

In some years the term “winterkill” is used frequently and this is one of those years. Winterkill describes a situation when water no longer contains enough dissolved oxygen for fish to survive. Without oxygen, fish suffocate, just as they do when you take them out of water.

Aquatic vegetation produces dissolved oxygen through the respiration process of photosynthesis, just as land-based plants release oxygen into the air. Fish then process oxygen with their gills.

Of course, sunlight is needed to start the photosynthesis process, and during some winters, the combination of thick ice and snow on top of it prevents sunlight from reaching aquatic plants. At some point, lack of sunlight kills aquatic plants, so they no longer release oxygen into the water. To make matters worse, dead, decomposing plants actually use oxygen.

If the water’s oxygen content falls low enough, fish start to die. And when that happens, their decomposition uses additional oxygen.

Can anything be done?

Winterkill has been a limiting factor on some North Dakota waters for decades.

It’s a word that draws winces of pain from anglers who were banking on another year of successful fishing in a lake that maybe wasn’t big enough or deep enough, or didn’t have the right aquatic habitat to sustain fish through a winter that produces thick ice and heavy snow.

Lakes that suffer a die-off can be restocked and once again become productive fishing waters. But every so often, maybe once every 5-15 years, the right combination of circumstances come together to threaten that fishery.

In a few North Dakota lakes where winterkill is historically a problem, fisheries biologists have installed mechanical aerators, or machines that artificially produce oxygen in water. Smaller lakes, such as Pheasant, Green and Hoskins in south central North Dakota, have aeration systems in place. The systems can work, but only if they are in place prior to a time when oxygen becomes deficient.

The limiting factor is that the North Dakota Game and Fish Department manages some 400 lakes. At the start of each winter it’s impossible to tell which ones, if any, will succumb to low dissolved oxygen.

Another method for battling winterkill potential is to clear the ice of snow. While this sounds good, it’s much easier said than done. One look outside and I realize it’s hard enough to my keep my driveway clear of snow, let alone enough acres of ice to make a difference for even one lake where winterkill is looming. It’s easy to imagine that hours after clearing part of a lake, winds would quickly blow snow into the open space.

Playing the hand we’re dealt.

Winterkill is a factor of nature and predicting it is not an exact science.

Looking out over Lake Darling

Looking out over Lake Darling

Biologists monitor dissolved oxygen content in suspect lakes during most winters. This winter, at least a couple dozen lakes so far will likely suffer at least partial, if not complete winterkill. In this part of nature’s cycle, unlike human disease, a remedy is not forthcoming. Once winterkill is suspected, all biologists can do is go in with nets once the ice goes out and see what’s left.

So, while driving by a picturesque winter scene of snow and ice reflecting the bright sun, consider the fish below. On most lakes, they’re just fine and starting to gear up for the spring spawn and production of a new generation. On a few, however, the beautiful winter scene above the ice may belie the R-rated natural event taking place below.


One Comment on "The Realities of Winterkill"

  1. Roy J. Inhulsen on Tue, 18th Jan 2011 9:39 am 

    Now I understand the whole dead fish smell in the spring. I was raised in MI near Higgins Lake and Houghton Lake in Roscommon County. Higgins Lake is extremely deep and has several feeders and springs. Houghton Lake is real shallow, has very few feeders and no feedeers to speak of. In the spring Houghton lake would reak of dead fish; Higgins Lake had barely a hint. Thanks for the Knowledge.

Tell us what you're thinking...
and oh, if you want a pic to show with your comment, go get a gravatar!

Human Verification: In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.