When winter descends and water bodies begin to freeze, a common question arises: what happens to the fish living beneath the ice? This article explains the intricate relationship between fish, water, and ice, shedding light on the survival mechanisms employed by fish during the colder months.
How Water Behaves When Freezing
Water exhibits a unique property: unlike most other substances, it reaches its maximum density at approximately 4°C (39.2°F). As water cools from this temperature towards 0°C (32°F), it expands and becomes less dense. This means that colder, less dense water rises to the surface and freezes first.
The ice that forms on the surface acts as an insulating layer. This layer prevents the water below from rapidly losing heat to the colder air above. As a result, the deeper water beneath the ice typically remains at a temperature of around 4°C, providing a stable and unfrozen environment for fish and other aquatic organisms. This insulating effect prevents entire bodies of water from freezing solid, which would be lethal to most fish.
Biological Adaptations for Cold Survival
Fish have evolved various physiological and behavioral adaptations to survive in cold, near-freezing water. Many fish species respond to dropping temperatures by lowering their metabolic rate, a state often referred to as torpor or brumation. This reduction in metabolic activity conserves energy, as their need for food and oxygen decreases during periods of scarcity. Fish may also move to deeper areas of the water body where temperatures are more stable and slightly warmer, typically around 4°C.
Some fish species, particularly those inhabiting polar regions, produce specialized substances called antifreeze proteins (AFPs) or glycoproteins. These proteins bind to nascent ice crystals in their bodily fluids, preventing them from growing and causing cellular damage. AFPs allow fish to maintain their body fluids in a liquid state even when the surrounding water temperature is below the typical freezing point of their blood, sometimes as low as -1.8°C (28.8°F) in saltwater. Supercooling is another adaptation, where some fish can keep their body fluids in a liquid state below their freezing point without ice formation, as long as they do not come into contact with an ice crystal. Additionally, certain fish, like koi and gobies, may burrow into mud or bottom sediments to find warmer, insulated areas, entering a dormant state until conditions improve.
Hazards of Ice Cover
Prolonged ice cover still poses significant challenges for fish survival. A primary danger is the depletion of dissolved oxygen (DO) in the water, a condition known as hypoxia or anoxia.
The ice layer blocks the exchange of oxygen from the atmosphere into the water. Any snow cover on the ice further reduces light penetration, limiting photosynthesis by aquatic plants and algae, which are primary oxygen producers.
Simultaneously, organic matter such as decaying plants and dead organisms continues to decompose under the ice, a process that consumes dissolved oxygen. This imbalance between oxygen consumption and replenishment can lead to low oxygen levels. The decomposition process can also result in the buildup of toxic gases, such as carbon dioxide, methane, and hydrogen sulfide, which cannot escape through the ice and can become lethal to fish at elevated concentrations. Shallow bodies of water are particularly susceptible to these issues due to their limited water volume and oxygen reserves, making them more prone to fish die-offs during extended periods of ice cover.