Lakes-Canadians are winter-ready. We’ve put shovels by every doorway in the home in October. Wildlife migrates to warmer climates, sheds summer fur, and hibernates. Fish? At 0 °C, water freezes, but fish survive. But how?
However, only the water on the lake’s surface freezes, so there’s still plenty of liquid water farther down. As a result, ice fishing is a popular wintertime activity. Nonetheless, despite the pliability of the water, the lake fish still suffer several difficulties throughout the cold season. The lack of available oxygen is the main difficulty. In the summer, fish have plenty of oxygen to breathe thanks to the steady circulation of surface waters and the high rates of plant photosynthesis.
But in winter, when the surface is quiet and trapped by ice, plants consume oxygen rather than produce it since they can’t see photosynthesize. A condition called hypoxia can occur in shallow, tiny lakes, depriving fish of enough oxygen to survive. When a fish suffocates and dies, it decomposes, which uses more oxygen, and leads to more death. Known as winterkill, this can rapidly lead to the demise of the lake’s whole ecosystem.
In order to survive in lakes, fish have evolved a number of strategies for handling low oxygen. Before the ice forms, some fish, like the Iowa Darter, will migrate out of the lake and into the flowing waters. While some fish migrate out of the lake entirely, others, like the Brook Stickleback and Fathead Minnow, remain within the lake but head for the inlet, where the high concentration of dissolved oxygen is maintained by the fresh water from the streams that feed it.
Many of the lake’s benthic (bottom-dwelling) fish make their way to the lake’s shores when conditions are favorable. As the oxygenated shallow water extends all the way to the lake bed, they can relax there without worrying about drowning. To avoid hypoxia, mud minnows are one of a kind because they can take up oxygen from gaseous air bubbles trapped under the ice. Fish that don’t swim to the shoreline eventually run out of oxygen since lakes deplete it from the bottom up; therefore, they should swim to the surface and hang out near the ice, from which oxygen can slowly seep into the water. Pike takes this a step further, taking it upon themselves to create places of abundant oxygen. As they do so, they lay at an incline in the water, with their snouts virtually touching the ice and their tails pointing downward.
Once below the ice, they keep a weak stream of warm water moving by moving their pectoral fins and operculum (the hard, protective covering for their gills). Due to this, a dome of oxygen-rich water is formed as the ice melts. Even though pike is typically solitary fish, it is not uncommon to find them resting in these domes in pairs or small groups. Getting to the top, however, is not without its own difficulties. Water near the ice can be quite cold, often falling below 1 degree Celsius as the ice melts into the surface water. In consequence, fish are at risk of dying of hypothermia.
Fish typically choose one of two tactics to deal with this situation. Entering a semi-dormant condition is one way to save power.
The largemouth bass behaves in this manner. They slow down in terms of activity, breathing, and other fundamental bodily processes. Since they can’t digest much food, they must eat fat to survive. Therefore, fish might starve to death if winters are too long or too harsh. Increasing their heart and respiration rates is the second tactic. Yellow Perch behave in this manner. This requires continual feeding throughout the winter, which may be difficult given the lack of food.
Life struggles through the winter, but it always seems to persevere. Once the weather warms up in the spring, fish that have been hibernating can get back to their active lifestyles. Fish that migrated away from the lake are coaxed back by its abundant food and shelter. As fish resume hunting and reproducing, they recoup from winter losses and produce the next generation of hardy Canadians.