The ice abides: In cold blood

What’s it like to live under the ice? We’ll never know the sensations, because after all, we’re not fish – and we can never relate to their experience because our metabolism is radically different from theirs.

We are warm-blooded; fish (along with frogs, crayfish, turtles and other water creatures) are cold-blooded. Our bodies regulate our temperature; much of the energy we consume as food goes to feed the inner furnace that keeps us at 98.6 (or so) degrees F. Cold-blooded creatures’ temperatures rise and fall with the temperature of their environment, which means right now the fish in your lake are at somewhere around 40 degrees F, just like the water.

What does that mean in a practical sense? It means the fish are quite sluggish. Their muscle movements rely on complex chemical reactions that proceed rapidly when warm and slowly when cold.

Imagine what it would be like to live in cold water as the warm-blooded mammals we are. Staying warm would be impossible. Even at 70 degrees, water pulls heat out of our bodies dramatically faster than does air at a similar temperature. At 40 degrees water temperature, our bodies simply could not keep up; we would be uncomfortable to say the least and would die soon from hypothermia. Think about the mammals that live in water (like whales) or spend a lot of time in it (seals, sea otters, walruses). Their adaptations tend to include heavy fur, a thick layer of insulating blubber, or both.

Cold-blooded creatures are perfectly fine in cold water. They don’t have to heat themselves, which means they don’t use a lot of energy. They don’t have to eat a great deal in winter because the cold tamps down their metabolism. Ice anglers can see evidence of this in the fish they catch and clean – the stomachs are often full of food. Prey that might digest in a day in summer may take a week in winter.

You may have wondered, if you ice fish, why you catch certain species in winter more so than others. It’s because fish react differently to the cold. Walleye and northern pike eat plenty as ice forms, weeds die back, and prey fish become more exposed. Bass and muskies, on the other hand, don’t move around much and eat just enough to sustain basic functions.

Where do fish go in winter? Some stay relatively near the surface where the water contains more oxygen. Some hunker down in the depths. Bullheads bury themselves in the bottom, not actually hibernating but moving very little.Many fish hang around the same kinds of places they haunted in warmer times – weed beds, brush piles, manmade cribs and other cover. Bluegills, for example, crowd around cover for protection. The flips side is that their presence in turn attracts predators, like northern pike.

Right now, the air temperature over my lake (Birch, near Harshaw, Wis.) hovers in the single digits, and a wind whips over the snow. Talk about wind chill all you want, but being in the water below the ice would feel much colder. This warm-blooded creature is glad to have a heated cabin and a bowl of soup to come to after a walk.