Fish can survive in a frozen pond, but their survival depends entirely on the pond’s physical characteristics and the duration of the ice cover. The ability of fish to live beneath solid ice relies on unique water physics and their biological adaptations. Deep ponds maintain an unfrozen layer, creating a refuge where fish lower their energy consumption to endure the cold winter months. However, this delicate balance can be easily disrupted, leading to “winterkill,” the primary reason fish fail to survive a harsh winter.
How Ponds Avoid Freezing Solid
A pond avoids freezing solid due to the peculiar properties of water. Unlike most liquids, freshwater reaches its maximum density not at its freezing point of \(0^{\circ}\text{C}\) (\(32^{\circ}\text{F}\)), but slightly above it at \(4^{\circ}\text{C}\) (\(39^{\circ}\text{F}\)). As surface water cools in autumn, it sinks when it hits this temperature, displacing warmer water toward the surface in a process known as thermal stratification. When water cools past \(4^{\circ}\text{C}\), it becomes less dense and floats, eventually freezing to form an insulating layer of ice that protects the water below from colder air temperatures. Pond depth is a crucial factor; deeper ponds provide a larger volume of water that stays consistently at \(4^{\circ}\text{C}\) at the bottom, offering a stable thermal refuge, while shallow ponds are much more likely to freeze completely, which is lethal to the fish population.
Fish Adaptation to Cold Water
Fish are cold-blooded (ectothermic), meaning their body temperature mirrors the surrounding water. As the water cools, their internal biological processes slow down significantly, causing them to enter a state of reduced activity or torpor. This metabolic slowdown is the primary winter survival strategy. By decreasing heart rate, respiration, and movement, they drastically reduce their energy needs and oxygen demand. Fish instinctively seek the warmest available zone—the \(4^{\circ}\text{C}\) water layer at the pond’s bottom—to maintain this reduced metabolism and conserve energy.
The Danger of Winterkill
The primary threat to fish under a frozen pond is not the cold, but asphyxiation, leading to “winterkill.” Winterkill is caused by a severe depletion of dissolved oxygen beneath the ice. Although dormant fish require less oxygen, the supply is no longer replenished from the atmosphere once the ice cover forms. Two main factors contribute to this depletion.
Decomposition
Bacteria on the pond bottom continuously break down organic material, such as fallen leaves and dead algae. This aerobic decomposition process consumes the limited dissolved oxygen in the water.
Blockage of Photosynthesis
Aquatic plants and algae normally produce oxygen through photosynthesis, which is the main source once the pond is sealed by ice. If snow persists on the ice, it blocks sunlight. Without light, plants cannot photosynthesize and instead consume oxygen, accelerating depletion below the threshold needed for fish survival.