In the chilly winter water live many chilly fish. Scarcely breathing, slowly swimming, they pause, waiting for the spring. Theirs is a dark and slow-moving novel with pages and pages to read. During the winter, water temperatures rarely rise above 35° under the ice. Under this frozen shell, the world of cold-blooded animals slows to a crawl. Turtles, frogs and many aquatic insects go into hibernation, but fish remain active, though much slower than usual. Most head for deeper water, tucking themselves in under rocks, logs and snags, while others like carp will actually burrow partly into the mud. Conditions make food more difficult to find – there’s no gnat for dinner under the ice – but due to the cold temperatures, most fish slow their metabolisms in the winter and eat very little anyway. Breathing, however, is still a necessity.
Fish take in water through their mouths and, as it passes over their gills, they are able to absorb dissolved oxygen in the water. The water becomes oxygenated as it moves through waves, riffles and waterfalls or as aquatic plants release oxygen during photosynthesis. Minnesota’s frigid winters cloak most rivers, streams and lakes with thick layers of ice and snow that both prevent the water from mixing with the air and also reduce the amount of sunlight available for underwater plants. The plants enter a hibernation-like state, producing very little oxygen, and as the winter drags on, there is less and less available for the fish to breathe. Very active fish that require more oxygen, like brook trout, move near areas of open water where springs or quickly moving currents keep the water from freezing. Slower moving fish, like smallmouth bass, settle in to wait for the thaw. Each day is a struggle for these chilly, barely-breathing fish, and sometimes spring doesn’t come in time.
During very snowy winters, especially when the snow comes early, aquatic plants are more likely to die from lack of sunlight, and when that happens, the plants decompose and use up dissolved oxygen in the water. Without adequate food or oxygen, many of the fish die as well, a phenomenon known as winterkill. When spring’s warmth finally arrives, it reveals a gruesome sight – hundreds of dead fish, strewn along the shore.
Nature isn’t kind or fair and surviving the winter is difficult for all animals, not just fish. Sometimes, human changes to the landscape tip the scales just enough to make survival impossible. Most streams in the metro area have been altered from their natural states, straightened, smoothed and widened. These shallow, slow moving streams are more likely to freeze in the winter and lack riffles to keep the water oxygenated. Along lakeshores where people have removed fallen trees and aquatic plants to more easily access the water, there are fewer places for fish to find shelter during the winter and fewer plants to create oxygen as well. Nature stacks the deck against them, and then we add a few more cards.
Still only December, there are many chapters left in the winter fish’s tale. Snow has fallen, ice has formed and the fish have begun to wait. Their story is still a mystery. No one yet knows their fate.