“Chilled Out” Koi: How Koi Survive Frozen Ponds in Winter

How Koi Survive Frozen Ponds in Winter

If you live in a northern climate then chances are good that you see snow and ice as does your koi pond.  Its natural to worry about your koi during the winter periods, especially if they have been around for years and years.  Fortunately, koi are adapted to dealing with cold temperatures and an iced-over pond.  They have a few tricks up their “sleeves” to deal with the inevitability of ice cold water. 

Regulating Body Temperature

Thermoregulation of animals or how an animal regulates its body temperature can be a confusing subject.  For example, within the subject of thermoregulation there is:

– Ectothermic

– Endothermic

– Mesothermic

– Heterothermic

– Homeothermic

– Poikilothermic

– “Cold-blooded” 

There’s a lot of variations in the world of temperature regulation in animals but to make it more straightforward for koi owners you should know that your fish are ectothermic,which means that their internal body temperature is governed strictly by the ambient temperature (or in this case, the water temperature).  So that must mean that all fish are ectotherms, right? Not necessarily.  Unfortunately, biology is not always consistent. 

For example, the bluefin tuna and some sharks create internal heat from muscle activity yet are still largely affected by water temperature which puts them in a category known as mesothermy.  Furthermore, the term “cold-blooded” is actually not all that accurate.  A “cold-blooded” lizard in the hot desert sun can achieve an internal temperature greater than that of humans.  So in everyday conversation its just 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 temperatures get colder your fish start to become less active.  As a result, they require less food and at somewhere between 50 and 40 degrees F they stop eating all together.  Ectotherms are able to pull this off because they don’t have to maintain a certain internal temperature and, in fact, they can get away with using as little as 10% of the energy of what a mammal would need.  As temperatures fall, the rate of internal biological activity decreases which includes things as basic as how fast a muscle can twitch.  This concept in biology is known as the Q-10 coefficient. 

Planning for Winter

There’s not much activity going on with your fish, on the inside or outside.  They don’t need food and not as much oxygen however its still a good idea to keep some of your pond unfrozen with a de-icer for gas exchange (and some pond owners will run aeration all year long).  One of the things your koi will do is try to hang out in the warmest part of the pond and that will subsequently be the deepest part.  In general though, its a good idea, when designing/building a koi pond, that you make it at least 3 feet deep to avoid the possibility of total pond freeze.  Another thing you may want to keep in mind is that adding salt to your pond before winter will lower the freezing point of water and artificially cause your water to reach a super low temperature which can potentially harm your koi.

Koi in Dormancy

So what exactly are they doing under the ice? Sleeping? Playing cards?  As with thermoregulation there are a lot of different ways to go inactive during winter (or periods of less-than-ideal conditions).  There is:

hibernation in mammals

brumation in reptiles

diapause in insects and

aestivation in invertebrates

but ultimately your koi under ice are in a state of dormancy.  Essentially, they are simply “chilling out” in a state of super decreased activity and metabolism while waiting for spring.  Perhaps you don’t get to enjoy your koi as they mill around under the ice but just think of the money you are saving not buying koi food!

11 thoughts on ““Chilled Out” Koi: How Koi Survive Frozen Ponds in Winter”

  1. Thank you for the useful article. This winter my koi pond developed a leak and drained down to 4 inches of water while still covered by 4-5 inches of ice resting on the plant pots. [My worst nightmare.] It took me 4 days of 20-degree weather — replacing the water and using a water jet to cut through the ice before I could get all the koi out. They are now in my garage in a small temporary pond with constant aeration. I haven’t lost any (so far) and water is now about 44 degrees, but they were definitely stressed and still seem too active for the temperature. Do you have any additional advice for calming them or should I just figure on feeding them when the water is at about 50 degrees?

    • I’m sure you’re fish have calmed down by now but I know some pond owners will cover the container with the koi with some sort of black covering to calm them (much like bird owners do). I would start off slow with the food -their metabolisms may be slow to get up to speed.

  2. Liked your writing about Koi. I enjoyed it very much. Think ive always been ok in winter with my fish. I have two ponds.largest 12ft by 7ft. Small pond 8ft by 6ft. Winter time i cover both ponds about three quaders with warerproof materal. Seems to work well. I also leave the pumps and filters running in both ponds. Thank you

  3. Hi I am new to breeding koi carp I am soon to be a fish breeder I am nervous ofcourse about this new move or idea as such as the fish themselves are quite deer if this event goes wrong then I will lose money and have to start over again especially if I have a large breeding site active. If you could drop me any hints as to get my newly found project started this would be of great use to my knowledge I may even jot down this usefull information. Thank you ever so much all the best Paul Hadfield

  4. Would it wise to leave some Hyacinth plants in the pond over the winter in Connecticut.

    What problems would there be to the Koi and comets.

    • I don’t believe you will experience any issues to your koi doing this but i’m not so sure the hyacinths will come back the next year. Fortunately they are fairly inexpensive and grow pretty quick so you should be back in business in the spring.

    • James, I am currently working on a book that will outline all the key aspects and steps to take when getting into the business and raising and selling koi -I can let you know when it gets released.

  5. I live in the Pacific NorthWest (WA State). Is it ok to leave waterlillies in my pond all winter? They are well established, but the leaves do die. If so, should I make sure and take out the dead leaves? I have 6 large Koi. My pond is 4K gals.

    • Yes, you can leave them in. They will go dormant during the colder months. Definitely try to get as much dead stuff out of your pond as you can.


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