The weeks before winter are often a time for much preparation. As temperatures cool, most plant life and a fair portion of animals begin to stock up on necessary nutrients, ensuring they can make it through harsh conditions without much food. The diminishing daylight hours trigger physiological responses that may induce a period of hibernation or dormancy. Cold-blooded reptiles, furry mammals, waterfowl, and yes, even fish, may begin to slow down and retreat to their makeshift forms of shelter.
In ornamental or wildlife ponds, fish may begin to gravitate toward the bottom layers of a pond in response to cooling surface temperatures. In regions where temperatures are cool enough to cause a portion of the pond to freeze, an adequate depth is crucial for fish survival. Cold-water ornamentals, such as koi and goldfish, may survive beneath a layer of ice if there is enough oxygen and space for them to remain warm and comfortable.
As goldfish are valuable ornamentals, much emphasis must be given to proper pond care and winter preparation. Though it may be acceptable to leave them in an outdoor pond through the season, not all kinds can exhibit the same measure of hardiness towards the cold. Moreover, even if their metabolic rates drop severely, they may continue to require some form of airflow between the surface and the atmosphere.
Temperatures Tolerated by Goldfish
Today’s modern-day goldfish breeds hailed from ancestors who lived out their lives in the temperate zones of China. Our domesticated varieties of Carassius auratus are thus equipped with the means to tolerate seasonal changes, including those that are associated with frosty winters.
Goldfish can tolerate an impressive range of water temperatures. While they aren’t guaranteed to survive through drastic temperature changes, especially if these large changes cause them to go into shock, they can slowly acclimatize to both warm and cool conditions. Generally, they thrive best at mild to moderate temperatures ranging from 60 to 73˚F (16 – 23˚C). When provided with enough time to adjust to increasing cold, they can persist in temperatures down to 34˚F (1˚C). Below this limit, their internal organs may cease to function.
Cold-blooded, goldfish are unable to adjust their internal temperatures. Much of their bodily functions are directly affected by the temperature of their surrounding waters. As a result, where they situate themselves in the water column is influenced by temperature. In ponds with a frozen surface, goldfish will naturally remain close to the bottom, where the temperature is higher and much more stable. Here, they may enter a resting period as their metabolism slows; at around 48˚F (9˚C), a goldfish’s body digests food at a much slower rate than normal.
Do Goldfish Hibernate in Winter?
The aquatic equivalent for hibernation in fish is called “torpor”. This period of inactivity can last from just a few hours to several weeks. Goldfish may lapse in and out of torpor when the need to move arises. For example, if a resting fish senses the presence of a predator, it can swim toward a patch of plants, where it may feel safer. It may also attempt to forage for small treats, like insect larvae or tadpoles. This is why, beneath a layer of ice, ornamental fish may still be observed to move.
A state of torpor allows a fish to conserve its energy stores at a time when food is expected to be scarce. Moreover, as its metabolic enzymes are less likely to function in cold temperatures, torpor ensures that it can last through the season without feeding regularly. Once temperatures begin to warm, re-activating important enzymes, you can expect your goldfish to regain its appetite.
Feeding Goldfish in Winter
A winter pause for feeding is recommended as goldfish in torpor are unlikely to eat. Uneaten feeds may simply sink to the pond bottom and use up oxygen as they decompose. Once the water temperature drops to 39˚F (4˚C) and below, stop feeding your goldfish completely. Resume feeding at a maximum rate of just 2x per week once this rises to around 40 – 50˚F (4 – 10˚C). Normal feeding rates may be resumed at higher temperatures.
Cold Tolerance of Common vs. Fancy Goldfish Breeds
Common goldfish breeds, which are usually distinguished by their simple features and more streamlined shape, tend to be more cold-hardy than fancy goldfish breeds. Some types of fancy goldfish, such as bubble eyes and orandas, have external organs with distinctly fine membranes. These are much more susceptible to temperature changes, which may significantly impede their function. Moreover, as these breeds are easily injured, they are best kept in a tank.
Temperature variations that are as small as +/-2˚C (3.6˚F) are enough to stress fancy goldfish breeds. As the conditions of outdoor ponds are directly affected by unpredictable elements, they are not suitable habitats for fancy goldfishes, which seldom thrive in temperatures below 59˚F (15˚C). Hardy goldfish breeds should be fine if they are left outdoors through mild to moderate winters.
Harsh winters with prolonged frosts and subzero temperatures are more of a concern. If torrential winter rains or snowstorms occasionally plague your area, it would be best to overwinter all of your ornamental fish in a protected and preferably heated tank. Keep in mind that excessively cool temperatures in relatively shallow ponds can cause ice crystals to form in a goldfish’s body.
How to Ensure Goldfish Can Survive in a Pond in Winter
In outdoor ponds, fish survival through winter is never truly guaranteed. A wealth of factors affects how easily a fish may cope with cold temperatures. One of the most important factors is pond depth. As the pond’s surface freezes, the thickness of the ice layer can take up a large portion of small and shallow ponds. Keep in mind that, as the fish are pushed to the bottom, they’ll continue to require a suitable volume of liquid water. Though goldfish can be left in ponds that are at least 2 feet (61 cm) deep, an even greater depth is recommended – especially if larger ornamental fish are present.
Gas exchange is another crucial aspect to consider when preparing your pond for winter. If the entire surface freezes over, oxygen will not diffuse into the pond. While aquatic plants and microbes may continue to produce oxygen, which is naturally found in higher levels in cold water, its concentration will seldom be adequate to support several fish for more than a few weeks. Consider maintaining a hole in the ice or using a de-icer for at least one part of the pond to ensure that gas diffusion may continue to take place. This should also allow any harmful gases in the water to escape.
Some pond owners opt to maintain pond aeration levels using a pond pump or aerator, which naturally prevents a part of the pond from freezing over. If your pond freezes over intermittently or for just one short period in winter, an aerator would not be necessary. In contrast, if ice covers the pond throughout winter, an aerator is recommended.
Final Considerations
Take note that light deprivation due to a layer of snow on ice impacts how efficiently algae and pond plants can assimilate nutrients in winter. Abnormal spikes in nutrient levels, coupled with low oxygen conditions, can stress your goldfish. Another aspect to consider is your goldfish’s survival upon exposure to winter predators. Slow-moving fish are easy prey for hungry wild animals.
A layer of ice does afford fish some protection, but it may not be thick enough to shield your fish from a bird’s beak or a feral cat’s claws. Some hiding places or cold-tolerant emergent and submerged plants may help keep your goldfish safe – but note that these may consume trace amounts of oxygen too.
At the end of the day, it’s best to remember that one can never truly control what happens outdoors. Ponds are at the mercy of sudden weather changes. While you can take measures to prepare for seasonal changes, it simply isn’t possible to predict natural disasters and how these may impact your pond. To an extent, you might want to trust in your goldfish’s biology and natural instincts for survival. Simply aim to give your goldfish enough space, light, and oxygen to make it through the coldest months.
