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Photo of a frozen lake

Frozen lake

For many of us, winter might be our favorite time of year. The air is cold, and ponds form ice, where we can slide and skate with our families and friends. Some people even enjoy fishing through the ice!

For fish in ponds and streams though, winter can be a deadly time, a time for winterkill to strike. Winterkill is a natural event, but humans often do not like the results it brings.

Fish need oxygen to survive, just like people do. Humans have lungs which supply oxygen to our bodies. 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.

When the weather is warm, plenty of oxygen can get into the water, and fish are fine. In the winter, ice covers lakes and ponds, and problems begin for the fish. Shallow ponds seem to be affected most by winterkill. These problems are complex, and involve both the plants and animals living in the pond.

As long as sunlight can pass through the ice, the pond's aquatic plants stay alive. However, if snow falls on top of the ice, it may block sunlight from entering the pond. 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 from the pond, plants will die. After they have died, they begin to decompose. This process uses oxygen from the water, the same oxygen fish need to live. The ice and snow block any new oxygen from entering the pond.

If enough oxygen is used, the fish will begin to die. As fish die, their bodies decompose and use up more oxygen, and the problem gets even worse. Soon no oxygen is left in the water, and all the fish have died. Winterkill has struck.

Since the pond is covered with ice, people are often not aware that winterkill has taken place. Only when the ice melts in the spring is the problem discovered. Hundreds or maybe even thousands of fish float up on the shore of the pond. It may seem very bad, but many 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.

To help solve the problem, the pond must be made deeper. New fish will have to be put 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.

Rivers suffer from different problems than ponds. Because the water is always moving, ice may not form on the surface at all. If ice does form, it is often not as thick as the ice on rivers or ponds. Also, the moving water brings lots of oxygen to fish.

Rivers have special problems of their own however. In rivers, ice may form on the rocks which are underwater, even though the surface of the river may not freeze. This is called anchor ice. Anchor ice can kill aquatic bugs such as mayflies, stoneflies and caddis. These insects are an important food source for many of the fish in the rivers. With their food supply gone, it means hard times for fish. Fish eggs waiting to hatch in the gravel also may be frozen and killed.

In many rivers, ice jams and high water flows cause a problem called scouring. Scouring takes place when the rocks and gravel in the bed of the stream are scraped and rolled by large rocks, high water flows, and chunks of ice. This scouring can also kill aquatic insects and destroy underwater plants. The bottom of the stream is greatly changed, and the habitat (the place the animals live) may not be suitable for them to live there anymore.

One of the greatest problems with scouring in rivers happens to young fish. Most fish in rivers lay their eggs in the gravel and small rocks on the bottom. When high water, rocks and ice scour the bottom, the eggs may be crushed and destroyed. Sometimes even the young fish themselves are killed, or they may be washed away in the strong flows.

When we see that winterkill has struck our favorite pond, or that a stream near our home has been scoured by rocks, humans often are sad. Remember though, that winterkill has always been a part of the pond and river system. The fish and aquatic insects have adapted to winterkill, and in time, they will recover. And remember the benefits of winterkill-all those dead fish provide a tasty meal for fish eating birds and animals.