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Why Do Lakes Winterkill?

4/21/2014

CHEYENNE - Every spring as ice leaves Wyoming’s lakes and reservoirs, fisheries biologists can be found checking lakes to determine the health of the fisheries resource in the various waters. On some lakes one of the things biologists are checking for is winterkill.

When wildlife managers talk of winterkill it is usually in reference to losses that big game experience during severe winters. But, even though winter’s toll on fish may be less obvious than with big game, harsh winters especially following drought years, will also take an unmistakable toll on the fisheries resource.

In Wyoming, motorists may occasionally see animals that have been the victim of road kill and in more severe winters, carcasses sometimes are spotted next to fence lines along roadways. With fish it’s a lot less dramatic. When they succumb to the winter, they do it under water and out of sight of most of us. With big game, winter loss is usually associated with deep snows and poor forage. With fish it’s much the same, as deep snow can affect the amount of oxygen in a body of water and low oxygen can lead to fish loss.

Many may remember studying the process of photosynthesis in basic high school biology. This process requires sunlight. The aquatic vegetation within a given body of water produces dissolved oxygen through the respiration process of photosynthesis. If there is not enough dissolved oxygen in the water, the fish suffocate just like they would if they were taken out of the water. During harsh winters, thick ice coupled with a heavy snow cover prevents sunlight from reaching aquatic plants. This lack of sunlight can kill plant life and when this happens, the plants no longer release oxygen into the water. The problem is compounded as the dead vegetation actually uses up oxygen as it decomposes.

If the water is naturally shallow, the problem can be further compounded. Shallow lakes allow more vegetation to grow and when this vegetation dies, the decomposition uses a tremendous amount of oxygen. Wyoming fisheries biologists have had some success in lessening the loss on a few lakes known for winterkill by installing aeration systems. These systems can be fairly costly, but in some instances, have helped prevent loss. Aerators sometimes are able to keep oxygen levels high enough that massive die offs are prevented. For the angler, winterkill may mean that the larger fish a lake has been growing for the past few years are no longer there. In the spring, Game and Fish biologists regularly net lakes suspected of winter loss to see what’s left. Lakes that have winterkilled are often restocked to keep the fishery available for anglers.

One question the Game and Fish sometimes get is why the trout appear to be gone following a harsh winter and the non-game fish or rough fish like carp are still around. This is easily explained in the oxygen requirements for the different species. The various salmonids (trout) need around five parts per million (ppm) of dissolved oxygen in the water to survive. Contrast that with carp that only need one ppm to stay alive. So in lakes where the oxygen levels have dipped low enough to kill trout, carp may get along just fine.

(Contact: Al Langston (307) 777-4540)

-WGFD-

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