This news release was archived on Friday, April 29, 2011
For fish in ponds and lakes winter can be a deadly time, a time for winterkill to strike. Winterkill is a natural event, but humans often dislike the results it brings. Fish need oxygen to survive, just like people. Since they live underwater, fish have adapted to their habitat by developing gills. Gills allow a fish to take oxygen from the water just as our lungs take oxygen from the air.
During warm temperatures with open water, plenty of oxygen can get into the water, and fish are fine. In winter, problems begin when snow covers the ice. The problem is complex, and involves both the plants and animals living in the pond.
As long as sunlight can pass through the ice, the pond's aquatic vegetation can stay alive. Plants living in the pond need sunlight to live and grow. In the process, they give off oxygen into the surrounding water. This oxygen is used by aquatic insects and fish.
If sunlight is blocked by snow accumulations on the ice, plants will die from lack of sunlight. When they die, they begin to decompose. This process removes oxygen from the water, the same oxygen fish need to live. If enough oxygen is removed, the fish will begin to die. As fish die, their bodies decompose and use up more oxygen, and the problem gets worse. When oxygen levels in the water get low enough all the fish die resulting in winterkill.
While the pond is covered with ice, people are often unaware that winterkill has taken place. Only when the ice melts in the spring is the problem discovered. Fish carcasses float up on the shore of the pond. It may seem harsh, but several animals benefit from winterkill. Birds like eagles, ospreys and gulls will feast on the dead fish, and animals such as raccoons and mink will join them. Nothing goes to waste. The decomposing bodies of the fish will even provide nutrients for plants.
Shallow ponds are affected most by winterkill. To reduce the frequency of winterkill, experience has shown a 12 foot deep pond is a minimum requirement for FWP to stock fish. When a winterkill does occur fish will have to be restocked in the pond. They will grow quickly, and in a few years, the pond will be back to the way it was before winterkill struck.
When we see that winterkill has struck our favorite pond people often are concerned. Remember though, that winterkill has always been a part of the ponds life cycle. The fish and aquatic insects have adapted to winterkill, and in time, they will recover.