Fish generally do not die when lakes freeze, a survival attributed to the unique physical properties of water and the biological adaptations of aquatic life. Most fish populations in temperate and cold climates successfully navigate the winter season by relying on a stable, liquid environment beneath the ice. The conditions that lead to fish death are not the freezing of the surface itself, but rather the compounding environmental stresses that sometimes occur under a prolonged ice cover. Understanding the fate of fish in winter requires examining the physics of a frozen lake and the physiology of the fish.
The Physics of Freezing
The survival of aquatic life hinges on the unusual density properties of fresh water, which allow a lake to freeze from the top down. Unlike most substances, water reaches its maximum density at approximately \(4^{circ}text{C}\) (\(39.2^{circ}text{F}\)). As surface water cools in autumn, it becomes denser and sinks, forcing warmer, less dense water to rise. This process continues until the entire water column reaches \(4^{circ}text{C}\).
When the surface temperature drops below \(4^{circ}text{C}\), the water molecules expand, making the colder water less dense. This less dense water remains at the surface and eventually freezes at \(0^{circ}text{C}\), forming a floating sheet of ice. This ice layer acts as an insulator, sealing off the liquid water beneath and preventing the entire body from freezing solid. Due to this thermal layering, the deepest parts of the lake remain stable at about \(4^{circ}text{C}\) throughout the winter, providing a consistent refuge for fish.
How Fish Survive the Cold
Fish endure the cold because they are ectotherms; their internal body temperature mirrors the surrounding water. As the temperature drops, the fish’s metabolic rate decreases dramatically, entering a state of dormancy or torpor. This physiological slowdown is a powerful energy-conservation mechanism, reducing the fish’s need for food and oxygen.
In this low-energy state, the fish’s heart rate and activity level are significantly reduced, sometimes by as much as 50 percent compared to warmer months. Many species move to the deepest parts of the lake, settling into the stable \(4^{circ}text{C}\) thermal layer. Some Arctic fish possess specialized adaptations, such as antifreeze glycoproteins, which inhibit the growth of ice crystals to prevent the freezing of internal fluids. This combination allows fish to survive for months on minimal resources.
When Fish Do Die
Fish mortality occurs in winter, a phenomenon known as “winterkill,” caused by a lack of dissolved oxygen (anoxia) rather than freezing. The ice layer creates a physical barrier, sealing the lake off from the atmosphere and preventing the vital exchange of oxygen. Oxygen consumption continues unabated, driven by the decomposition of organic matter on the lake bottom by aerobic bacteria.
This oxygen depletion is often accelerated by heavy snow cover on the ice, which severely limits sunlight penetration into the water below. Without sunlight, aquatic plants and algae cannot perform photosynthesis, the process that produces the majority of the lake’s oxygen supply.
Shallow lakes and ponds are particularly vulnerable to winterkill because they have a smaller total volume of water to hold dissolved oxygen. When levels fall below a critical threshold (often 2 to 3 milligrams per liter for game fish), the fish begin to suffocate. Mass die-offs are typically only observed after the ice melts in the spring.