The belief that fish must freeze solid when the surface of a lake or pond turns to ice is a widespread misconception. The vast majority of fish in freshwater and marine environments do not freeze. Fish have evolved remarkable environmental and internal biological strategies that allow them to survive in water near or even slightly below the normal freezing point. These adaptations ensure their survival through the coldest months, often in a state of reduced activity and energy conservation.
How Water Density Protects Fish
The primary reason fish do not freeze relates to the unique physics of water itself. Unlike most liquids, water reaches its maximum density at approximately 4°C (39°F). This density difference is key to creating a life-sustaining refuge beneath the ice.
As the air temperature drops, surface water cools and becomes denser, sinking toward the bottom of the lake. This circulation continues until the entire water column reaches 4°C. Once the surface water cools below this maximum density point, it becomes lighter and remains at the top.
This lighter, near-freezing water forms an insulating layer of ice at 0°C (32°F) on the surface, a process known as winter stratification. The ice layer acts like a blanket, shielding the water below from the colder air temperature. This stratification ensures that a pocket of water near 4°C remains stable at the bottom, providing a safe and thermally stable environment for aquatic life.
Fish move to the deepest parts of their habitat, effectively staying in this warm water zone. This movement keeps them away from the ice-water interface, preventing temperatures low enough to cause freezing. In this deeper sanctuary, the fish can sustain their slow winter metabolism.
Internal Biological Adaptations to Cold
Fish possess sophisticated internal biological mechanisms to cope with near-freezing temperatures. One significant change is a dramatic metabolic slowdown, sometimes referred to as torpor or brumation. As cold-blooded animals, their body temperature mirrors the surrounding water, which reduces their heart rate, respiration, and overall activity level.
This reduced metabolism is a crucial energy-saving strategy, as food resources are often scarce under the ice. Since biological processes operate at a reduced rate, their need for dissolved oxygen is also lowered. This is important because ice cover prevents the water from exchanging oxygen with the atmosphere, and decaying matter on the lakebed can consume what little oxygen remains.
Some species, particularly marine fish living in the sub-zero waters of the Arctic and Antarctic, have evolved specialized defenses called antifreeze proteins (AFPs) or glycoproteins (AFGPs). These proteins circulate in the fish’s blood and bodily fluids, binding to the surface of incipient ice crystals to prevent them from growing. This process uses thermal hysteresis, which lowers the freezing point of the fish’s blood below that of the surrounding seawater. For example, Antarctic notothenioid fish survive in water as cold as -1.9°C by lowering their internal freezing point to about -2.7°C.
The Rare Exceptions to Freezing
Although most fish avoid freezing, rare circumstances challenge this survival strategy. In extremely shallow ponds or small intertidal zones, the entire water body can freeze solid to the bottom. In these cases, fish are almost always killed, as the formation of ice crystals within tissues causes irreparable cellular damage.
Certain species, such as the crucian carp, exhibit an extraordinary tolerance for low-oxygen and near-freezing conditions, surviving in a state of anabiosis. While these fish do not survive being frozen solid, they can endure conditions that would kill most other species. Their survival is dependent on preventing the actual formation of ice within their cells.
True freeze tolerance, where an animal survives the actual formation of ice crystals in its body, is typically observed in other cold-adapted aquatic animals, such as the wood frog. These animals produce high concentrations of cryoprotectants, like glucose, which control where ice forms, drawing water out of the cells and protecting their vital organs. Fish generally rely on freeze avoidance mechanisms like AFPs.