Why Don't Fish Freeze in the Winter?

Have you ever looked at a frozen pond or lake and wondered about the fish? How do they survive the winter in frozen water?

Herring gull with a freshly-caught fish. (Photo courtesy of John Absher)

It may surprise you to know that the fish in our lakes and rivers are able to survive during the freezing temperatures of winter. That’s because they are cold-blooded. Their body temperature comes from the environment they are in. Humans, like all mammals, are warm-blooded. We create our own body heat, and it remains mostly steady, no matter what the temperature is around us. Cold-blooded animals do not need to generate as much energy, because they are able to get their body heat from their environment.

As the water gets colder in fall and winter, so does a fish’s body temperature. As a lake or river begins to freeze, the warmer water moves to the bottom. That’s where the fish go too, under the ice in the warmest water.

Fish don’t hibernate, like bears and groundhogs. But in the winter, they go into a resting state. During this time, they aren’t very active. They stay at the bottom of the lake or river, moving around very little. While resting, they don’t need as much food or oxygen. Their heart rate also slows down.

As winter starts to give way to spring, the water will start to warm up. The fish will do the same, gradually moving around more and more as their bodies warm up. And soon enough, they will be active, until winter starts to set in once again.