Answer:
Some fish that live in open waters like rivers and oceans swim south for the winter. But many fish live in lakes and ponds that do not connect to other water. When it gets very cold, ice forms on top of their lakes and ponds, but it doesn't freeze down to the bottom of the lake (find out why). The fish live under the ice, which is why some people go ice fishing in the winter. But most fish slow down their heart rates and eat very little, much like bears in hibernation, so they're also harder to catch!

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Trout in Winter
http://www.gorp.com/gorp/publishers/foghorn/fish_trt.htm
This article is intended for trout fishers, but it gives a good explanation of what trout do during the different seasons. Find out why it's difficult, but possible to catch trout if you go ice fishing.