Chilled Out Koi: How Koi Survive Frozen Ponds In Winter

If you live in a northern climate, you will most likely see snow and ice just like your koi pond. It’s natural to worry about your koi during winter periods, especially if they’ve been around for years and years. Fortunately, koi are adapted to deal with cold temperatures and a frozen pond. They have a few tricks up their “sleeves” to deal with the inevitability of freezing water.

Corporal temperature regulation

Animal thermoregulation or how an animal regulates its body temperature can be a confusing topic. For example, within the topic of thermoregulation is:

– ectothermic

– endothermic

– Mesothermy

– Heterothermal

– Homeothermy

– Poikilothermic

– “Cold blood”

There are many variations in the world of temperature regulation in animals, but to make it easier for koi owners, you should know that your fish are ectothermic, which means that their internal body temperature is governed strictly by room temperature (or in this case, water temperature). So that must mean all fish are ectotherms, right? Not necessarily. Unfortunately, biology is not always consistent. For example, bluefin tuna and some sharks generate internal heat from muscular activity, but are still largely affected by water temperature, putting them in a category known as mesothermy. Also, the term “cold blooded” is actually not that accurate. A “cold-blooded” lizard in the hot desert sun can reach an internal temperature higher than that of humans. So, in everyday conversation, it’s easier to refer to mammals and birds as endotherms and just about everything else as ectotherms.

Biological activity and temperature

You have probably noticed throughout the seasonal changes that as the water temperature cools, the fish start to become less active. As a result, they require less food and somewhere between 50 and 40 degrees F they stop eating altogether. Ectotherms can accomplish this because they don’t have to maintain a certain internal temperature and, in fact, can get away with using as little as 10% of the energy a mammal would need. As temperatures drop, the rate of internal biological activity decreases, including things as basic as how fast a muscle can contract. This concept in biology is known as the Q-10 coefficient.

winter planning

There is not much activity with your fish, neither inside nor outside. They don’t need food or as much oxygen, however it’s still a good idea to keep part of your pond unfrozen with a de-icer for gas exchange (and some pond owners will maintain aeration year-round). One of the things your koi will do is try to hang out in the warmest part of the pond, which will later be the deepest part. However, in general, it is a good idea when designing/building a koi pond to be at least 3 feet deep to avoid the possibility of the pond completely freezing over. Another thing you may want to keep in mind is that adding salt to your pond before winter will lower the freezing point of the water and artificially bring your water too low, which could harm your koi.

koi in dormancy

So what exactly are they doing under the ice? asleep? Playing cards? As with thermoregulation, there are many different ways to remain inactive during the winter (or periods of less than ideal conditions). There are:

hibernation in mammals

brumation in reptiles

diapause in insects and

aestivation in invertebrates

but ultimately, your koi under the ice are in a dormant state. Essentially, they’re just “chilling out” into a state of super-decreased activity and metabolism while they wait for spring. You may not be able to enjoy your koi as they move under the ice, but think of the money you’re saving by not buying koi food!