What do fish do all winter?

JACKSON HOLE, Wyo. — Winter is an especially hard time for most wildlife. It gets really cold, the wind blows a bit more, there’s often snow covering the ground, and ice forms on streams and lakes. Wildlife that we’re used to seeing in the summer has to adapt if they’re going to survive. Some animals like deer and coyotes grow extra heavy winter coats. Grizzly bears find sheltered areas like caves to crawl into and hibernate. Geese, ducks, and a host of other birds migrate to warmer climates.

But did you ever wonder what fish do? Fish in lakes don’t have many options aside from moving to deeper water. Fish in streams and rivers have a whole different set of challenges to deal with because of the wide range of habitat types in a river. Their survival depends almost entirely on how, when, and where ice forms.

Ice usually forms across an entire stream in late fall and gets covered with snow. This is an ideal situation for fish to survive the winter because they’re safe from predators and harsh weather conditions. Fish move to areas where the stream is flowing slow and deep—like pools or underneath stream banks. Because there’s snow on the ice blocking sunlight, fish do not move around much to try and eat during the winter. That’s ok, because there’s not much to feed on in winter and fish can live on their energy reserves for up to 5 months. Aquatic insects tend to hatch in the fall and are too small for fish to filter out of the water until the following summer.

While ice can be good for fish if it’s a solid cap on streams, other kinds of ice can be their worst enemy. Before streams freeze over in the fall, in the spring just after ice-out, or during any warm spell that causes the ice to melt, sudden cold weather can cause the stream to form slush ice.

For fish, slush ice is like a huge dust storm to you and me. As it floats downstream, the ice can clog the gills of fish that can’t get out of its way. Some fish move to shallow areas along the bank where slush ice may not flow. Others go to deep pools where the slush ice usually floats over top. But all too often, the fish that move to the shore get caught and eaten by mink that hunt for food along stream margins all winter. Small fish that move into deep pools can be eaten by bigger fish that live there.

Not many of us are likely think about fish, but the fact is winter is an especially tough time for them. We also tend to forget that the things people do with water that affect ice formation can have a big impact on fish. Game and Fish biologists work with water managers and dam operators around the state to keep the flow in rivers stable during the winter and help streams and rivers ice over. Doing that is good for fish as well as the people who like to catch them.