Fish are ectotherms, meaning their body temperature is regulated by their surrounding environment. When cold temperatures arrive, fish must adapt internally to survive the severe challenge of water temperatures dropping close to the freezing point. Survival requires a combination of behavioral shifts and physiological adjustments to conserve energy and avoid freezing. This adaptation allows fish to persist until warmer conditions return in the spring.
Seeking Thermal Refuge and Reducing Movement
As surface water cools, fish seek thermal refuge by finding the warmest available layer. This is possible because freshwater reaches its maximum density at approximately 39°F (4°C). In a lake or pond, this denser, warmer water sinks to the bottom, creating a stable thermal layer beneath the colder surface water and the insulating layer of ice. Fish congregate in these deeper areas, where the temperature remains consistently above freezing, escaping the harshest cold.
Fish also dramatically reduce their physical activity. Swimming speed and foraging efforts decrease significantly, which is an effective way to save energy when food is scarce. Some species may school together in the deepest pools or even burrow into soft sediments, like koi and gobies, to enter a semi-dormant “winter rest.” This reduced movement minimizes the energy expenditure required during a period of low resources.
Metabolic Slowdown and Energy Conservation
The internal survival mechanism relies heavily on reducing the rate at which fish consume energy. This slowdown is primarily a passive physicochemical effect, where cold temperature naturally slows down biological processes, known as the Arrhenius effect. This state of decreased activity is often referred to as torpor, involving a significant reduction in the overall metabolic rate.
The body’s primary functions respond to this cold-induced inertia by decreasing their operational speed. Both heart rate and respiration rate drop substantially. For many fish, this reduction in metabolic demand is a passive consequence of the cold environment, not an active suppression. Energy needs are met by utilizing fat reserves stored during warmer months, allowing the fish to fast throughout the winter.
Specialized Biochemical Adaptations
Some fish in extremely cold environments have evolved unique molecular tools to prevent their tissues from freezing. The most notable adaptation is the production of Antifreeze Glycoproteins (AFGPs), also called antifreeze proteins. These molecules are found in species like Arctic cod and Antarctic notothenioids, which live in waters consistently below the normal freezing point of their body fluids. AFGPs work by binding to the surface of tiny ice crystals that form inside the fish, preventing them from growing larger and causing lethal damage.
This mechanism creates thermal hysteresis, the gap between the temperature at which the fish’s body fluids melt and the temperature at which they freeze. The proteins allow the internal fluids to remain liquid even when the water temperature is slightly below freezing, a condition known as supercooling. AFGPs inhibit crystal growth, giving the fish a protective buffer against the formation of internal ice. The kidneys of some polar fish are adapted to conserve these AFGPs in the bloodstream, ensuring a constant supply of the anti-freezing agent.
The Threat of Winterkill (Oxygen Depletion)
Despite complex adaptations, the primary danger to fish in a frozen body of water is not freezing, but suffocation, an event known as “winterkill.” This occurs because a thick layer of ice and snow limits the light that penetrates the water. Without sufficient sunlight, aquatic plants and algae cannot perform photosynthesis, which is the main source of dissolved oxygen production during winter.
Bacteria on the lake bottom continue to decompose organic matter, a process that consumes dissolved oxygen. If the ice cover persists, oxygen levels can drop below the critical threshold of 2 to 3 milligrams per liter necessary for most fish species to survive. As oxygen depletion progresses, fish become stressed and eventually die. Shallow lakes with high amounts of organic material are particularly prone to this ecological disaster.