Ever wonder on a cold, snowy winter day what fish are doing under the ice?
The short answer is: about the same as the rest of the year. They breathe, eat and try to avoid being eaten. There are, however, some differences in winter.
First, it’s important to remember that freshwater fish like reptiles and amphibians are coldblooded. (Scientists prefer the more precise word, poikilotherms, but we’ll forgive them.)
That means their body temperatures are controlled through external means, in this case, water temperature. They do not maintain a constant body temperature like us mammals.
Close to 80 percent of human energy goes towards heating our bodies to a stable temperature. For fish, this is not an advantage. For one reason, their source of food, often insect based, is limited in the winter. In Montana, the winter temperature of water that doesn’t freeze usually is in the 30s or low 40s. To raise an organism’s body temperature to 98.6 in that cold environment would require a tremendous amount of food, which just isn’t there for fish.
Rather than devote large amounts of energy simply to heat up their bodies, fish have adapted to the cold rather than adopted a “central heating system” like a mammal.
And what a magnificent adaptation. A warm-blooded animal requires five to 10 times as much food as a cold-blooded animal of the same size and build; so, cold-blooded animals are better at surviving in barren environments, though not all fish species respond the same.
Cold water species, think burbot, are no longer confined to cold water pockets like in the summer and may range more widely. Other species, such as bluegills, may do the opposite – that is hang out and not expend much energy. They can accomplish this by not eating for weeks or months. Generally, however, everything underwater slows down, and life continues just swimmingly, thank you, as long as there’s oxygen. Ah yes, oxygen.
In an average winter, ice and snow form a lid on a lake or reservoir. If that lid is too thick, it prevents light penetration thus shutting down oxygen production from photosynthesis by aquatic plants. In addition to producing no new oxygen, the existing aquatic plants die.
Microbes at the bottom of the lake decompose the organic matter, the dead aquatic vegetation. If the lake is too shallow and the winter too long, those microbes will use up all the oxygen breaking it down. The result can be too little oxygen to support a lake’s fish, and they die.
In deeper lakes or mild winters, most fish survive. Plus, fish have adapted to slow down in the winter, as being too active would burn through their oxygen reserve.
Then, as water warms under the spring sun, ice-out occurs, insect life increases, and a fish’s metabolism rises just in time to take advantage of the increasing food.
Until then, fish mostly hang out in the same spot and wait for spring and open water.
For now, for all of us, warm water and summer sun are just a dream.
