The arrival of winter transforms aquatic environments, often covering lakes and ponds with a layer of ice. This raises questions about the fate of fish living beneath the frozen surface. While it might seem fish would freeze solid, their survival reveals natural adaptation. Many fish species employ strategies to endure the frigid months, maintaining life when their surroundings turn to ice.
How Ice Forms in Aquatic Environments
Water’s unique properties dictate how ice forms on lakes and other bodies of water. Unlike most substances, water reaches its maximum density at approximately 4°C (39°F). As surface water cools towards this temperature, it becomes denser and sinks, displacing warmer, less dense water which then rises to the surface to cool. This process continues until the entire water column reaches about 4°C.
Once the surface water temperature drops below 4°C, it begins to expand and become less dense as it approaches 0°C (32°F). This less dense, colder water remains at the surface, where it eventually freezes, forming an insulating layer of ice. This top-down freezing mechanism allows water below the ice to remain liquid, maintaining a temperature of around 4°C at the bottom. This creates a thermal refuge for aquatic life.
The ice layer, especially when covered by snow, also impacts light penetration into the water. Snow and ice can reduce the amount of sunlight reaching submerged aquatic plants. This reduction in light affects photosynthesis, which is the primary way plants produce oxygen in the water.
Fish Survival Strategies in Winter
Fish employ various strategies to survive winter’s cold, icy conditions. Many species exhibit behavioral adaptations by seeking deeper parts of the water body. The water at the bottom of lakes and ponds, typically at 4°C, offers a more stable and warmer environment than colder surface layers. Some fish may also burrow into soft sediments at the bottom, using the mud as insulation from the cold.
Physiological changes are also important for winter survival. Fish, being cold-blooded, experience a reduction in their metabolic rate as water temperatures drop. This state, often referred to as torpor or dormancy, is similar to hibernation. Their heart rate, breathing, and overall activity slow, allowing them to conserve energy and reduce oxygen demand when food sources are scarce.
Certain fish species can produce specialized proteins known as antifreeze proteins (AFPs) or antifreeze glycoproteins (AFGPs). These proteins bind to small ice crystals within their bodies, inhibiting their growth and preventing the formation of larger, damaging crystals. While more common in marine fish inhabiting sub-zero polar waters, some freshwater species, such as the Japanese smelt and burbot, also possess these adaptations to prevent internal freezing.
Dangers of Life Beneath the Ice
Despite adaptations, life beneath a frozen surface presents challenges for fish. One primary danger is the reduction of dissolved oxygen levels in the water. The ice cover acts as a barrier, preventing atmospheric oxygen from dissolving into the water. Additionally, decaying organic matter, such as dead plants and algae, continues to consume oxygen through decomposition.
If snow covers the ice, it can compound this problem by blocking sunlight, which prevents aquatic plants from photosynthesizing and producing oxygen. This oxygen depletion, often termed “winterkill,” can lead to fish dying, particularly in shallow or nutrient-rich ponds where oxygen reserves are limited. Fish that perish during winter are observed after the ice melts in spring.
Decomposition under the ice can also lead to the accumulation of toxic gases. Hydrogen sulfide, a gas with a rotten egg smell, is produced by anaerobic bacteria in low-oxygen environments from decaying organic matter in the sediment. This gas can stress fish and become lethal at higher levels. Food sources become limited in winter, forcing fish to rely on reduced metabolic rates and fat reserves.