Now that winter is fast approaching, it’s high time to attend to outdoor features that may require extra care and maintenance. Both ornamental and wildlife ponds are particularly affected by cooling temperatures as they are complex ecosystems with both living and non-living components. In cool temperate zones, outdoor ponds may freeze over through winter. Their capacity to maintain a liquid state is influenced by depth, the presence of a heater, and more.
In the most severe winters of the Northern Hemisphere, ponds that are 2 feet (61 cm) or less may freeze over almost completely. In regions with mild winters, a depth of at least 18 inches (46 cm) is usually sufficient to ensure that a bottom layer of water remains fluid. If the pond is home to ornamental fish or any other animals of value, it’s usually best to stay on the safe side and maintain a depth of at least 3 feet (91 cm) or more.
In sufficiently deep ponds, fish will naturally gravitate to the bottommost layers – where the water is warmest – as soon as the first extended frosts occur. They may undergo many metabolic changes through winter, during which water temperatures can significantly affect their growth rate, overall energy levels, and their capacity to consume food.
Why Do Fish Move to the Bottom of a Frozen Pond?
When the surface of a pond is exposed to cool winter temperatures, it can quickly lose heat and become significantly more dense than the bottom layers of water. This causes a phenomenon called “pond stratification”, which results in warmer waters at depths below an increasingly cold or icy surface. The less dense, warm water becomes a zone where fish may survive.
The insulation potential of bottom layers is increased with pond depth. In ponds with a depth of 3 feet (91 cm) or more, many cold-tolerant ornamental fishes may survive. Thermal stratification and thick ice can actually create a more stable environment for aquatic life through winter. These reduce the impact with which extreme thermal fluctuations may disturb the bottom of a pond. Note, however, that prolonged ice cover reduces the availability of oxygen. In the absence of reliable aeration, fish kills may occur even in adequately deep ponds.
Moreover, fish may gravitate towards the pond bottom as a means of avoiding surface predators. As they enter a dormant period and consequently have less energy to escape from potential harm, they take advantage of the reduced visibility of deeper waters. Predators like birds and piscivorous mammals should find it challenging to access fish if they remain in the warmer depths, below ice.
Factors Affecting Fish Survival in Freezing Conditions
Several factors determine how a pond’s community of fish is affected by freezing conditions. Keep in mind that not all fish are able to survive in outdoor ponds through winter. Those that have any chances of survival are usually native to temperate areas. Those sourced from tropical to subtropical zones are ill-equipped to withstand drops in water temperature, so even the “warmer” bottom layer may be too cold for them.
1) Oxygen levels
As the surface of a pond freezes, the exchange of gases is restricted. Though some oxygen and carbon dioxide diffusion can occur through ice, a thick layer can cause dissolved oxygen levels to dramatically decline. In aquatic animals, stress-related responses to oxygen deprivation may include increased respiratory activity, the release of stress hormones, and organ failure. Maintaining adequate oxygen levels is crucial to the well-being of fish in coldwater habitats. Some pond owners promote gas exchange by using a de-icer and an aerator through winter.
2) Temperature
Of course, the temperature levels throughout a pond and along its surface determine how severely its waters may freeze. It also affects how long an icy surface is retained and how quickly it may melt to re-facilitate gas exchange. Furthermore, as fish are ectothermic and are therefore unable to regulate their own body temperature, cold water affects their immunity and metabolic capacity. Their enzymes and microbial biome may be shut down by temperatures nearing 1˚C (34˚F).
3) Ice thickness
A thicker surface of ice provides better insulation and protection for fish by minimizing exposure to the cold air above. In contrast, thin ice allows greater heat exchange with the atmosphere, leading to more rapid cooling of the water beneath. Fish often seek refuge in areas with thicker ice as it creates a relatively stable environment; this helps them conserve energy and maintain consistently low metabolic rates during winter.
4) Snow cover
While a layer of snow can provide additional insulation, protecting the water beneath from rapid cooling, excessive snowfall may have contrasting effects. A thick blanket of snow can impede light penetration into the water, limiting photosynthesis. Additionally, deep snow cover may create challenges for fish by further obstructing their access to oxygen and altering the distribution of temperature gradients within the pond.
5) Adaptations to cold temperatures
Biological adaptations and energy conservation strategies play crucial roles in determining how fish are affected by freezing conditions in a pond. Certain fish species exhibit remarkable adaptations, such as the ability to enter a state of dormancy during winter. This adaptive response allows them to conserve energy and endure colder temperatures with lowered oxygen demands.
Winter Physiology of Cold-Water Fish
As temperatures drop, cold-tolerant fishes exhibit a drastic slowdown in metabolic activity. This allows them to reduce their energy expenditure and adapt to increasingly cool conditions. Fish like carp, trout, perch, and largemouth bass enter a state of “semi-dormancy” or “torpor”, conserving their nutrient stores and energy levels all throughout winter. During this stage, they may stop consuming any and all food.
Some species of cold-water fish, especially those that are native to northern temperate regions, produce anti-freeze proteins in winter. These primarily prevent the formation of ice crystals within their tissues, safeguarding the integrity of their cells. In freezing waters, these proteins are crucial for preventing total organ shutdown and mass fish kills. Without them, most fish would be unable to endure the crippling effects of near-zero temperatures.
Ornamental fishes, like domesticated koi, showcase similar biological adaptations aiding in their survival in garden ponds during winter. Koi likewise slow down their physiological processes to match the conditions in their environment. Though they possess antifreeze proteins, they will naturally seek the warmest and most stable zones of the pond bottom. They regain their appetites once water temperatures begin to rise in spring.
Should I Feed Fish in Winter?
If fish are not actively consuming food due to their slowed metabolic rate in winter, feeding them may be unnecessary and can be counterproductive. It may lead to water quality issues due to the accumulation of uneaten feeds.
Nonetheless, there are some instances where pond owners may opt to continue feeding their fish with a specialized winter food – formulated to be easily digestible in the cold. These include instances involving mild winters (during which the pond never freezes over and most fish continue to remain active), the continuous aeration of a pond, and monitored feeding in high-density aquaculture ponds.
Which Ornamental Fish Should Not Be Left in a Freezing Pond?
The decision to move ornamental fishes into a protected location depends on the specific species, your local climate, and the availability of a heated, indoor tank. Generally, tropical and subtropical fish are more sensitive to cold temperatures and should not be left in icy ponds. These include:
- Tropical fish: species like tetras, guppies, mollies, and certain types of cichlids are adapted to warmer water temperatures. In colder climates, these fish may not survive winter conditions. Leaving them outdoors would be detrimental to their survival.
- Sensitive goldfish varieties: while common goldfish can withstand cooler temperatures, certain fancy goldfish varieties, such as the bubble-eye or celestial eye, may be more susceptible to the cold.
- Koi: while koi are hardy and can tolerate cold temperatures, extremely harsh winter conditions with prolonged freezing may warrant bringing them indoors. This is especially true for smaller, highly-prized, or more delicate koi varieties.
Pond owners should be aware of the specific temperature requirements of their ornamental fish species. If temperatures in your outdoor pond are expected to drop to levels that could be harmful to your fish, it’s advisable to set up a heated aquarium or indoor pond well in advance of winter. Additionally, providing proper acclimatization when moving fish between environments is crucial to ensure a smooth, stress-free transition.
