Fish can survive winter in bodies of water that freeze on the surface. They inhabit the liquid water that remains underneath the frozen layer, not the ice itself. This survival is made possible by the unique properties of water, coupled with various biological adaptations and behavioral strategies developed by fish.
Water’s Peculiar Freezing Process
Water has a peculiar property: it reaches its maximum density at about 4 degrees Celsius (39 degrees Fahrenheit). As water cools from this temperature towards its freezing point, it becomes less dense and expands. Consequently, ice is less dense than liquid water, which is why it floats on the surface of lakes and ponds.
This floating ice layer is crucial for aquatic life, acting as an insulating barrier. It prevents colder air from significantly lowering the water temperature below, maintaining it at or near 4 degrees Celsius. If ice were denser and sank, bodies of water would freeze from the bottom up, solidifying completely, making survival impossible for most aquatic organisms.
Fish Adaptations to Frigid Conditions
Fish, being cold-blooded creatures, adjust to their surrounding water temperature. When water temperatures drop in winter, a fish’s metabolic rate slows. This reduces their energy and oxygen needs. Fish enter a state called torpor, a deep sleep-like state where their activity levels decrease, and they stop actively feeding.
Some fish species, especially in polar or extremely cold waters, have an additional adaptation: antifreeze proteins (AFPs) or glycoproteins in their blood. These proteins bind to ice crystals, preventing internal ice formation within the fish’s cells and body fluids. This allows these fish to survive in water that is slightly below the normal freezing point of their blood.
Winter Refuges and Oxygen Depletion
During winter, many fish seek refuge in deeper parts of lakes and ponds. The water there often remains a stable 4 degrees Celsius, providing a warmer environment than the surface. Some species, like certain types of mud minnows and lungfish, can even burrow into the soft sediment at the bottom for insulation and protection.
Despite these adaptations, a significant threat to fish in frozen waters is oxygen depletion, known as “winterkill.” When ice and snow cover a body of water for extended periods, they block sunlight from reaching aquatic plants, stopping photosynthesis. The ice also prevents atmospheric oxygen from dissolving into the water. As organisms consume oxygen and organic matter decomposes, levels can drop to dangerously low concentrations, leading to fish mortality.