Did you know that koi hibernate during the winter? Yep, just like many other animals, koi are very sedentary during the fall and winter months.
Now, to be clear, koi do not technically "hibernate" during the winter. And their activity is not like other hibernating animals. There is no burrowing, no hoarding of food, no stealing of pic-a-nic baskets. But they go into a hibernation-type state just the same.
When the water turns cold, you will notice a dramatic decrease in the activity level of your koi. They will "sit" on the bottom of your pond, floating upright with their pectoral fins tucked up underneath the body. Their heart rate and metabolism slows during the cold period.
Check out this video clip of koi in cold fall water. (Sorry in advance for the bad cell phone quality). Notice that while some of the koi are still moving slowly about the pond floor, others are already sedentary, floating in place with their fins tucked.
Koi should not be fed (and in most cases will not eat) when the water gets below 50 degrees. That's right, they don't need to eat at all. Koi will go through your entire winter season without eating pellet food added to the pond.
Although koi can live through the winter in a frozen-over mud pond, it is a good idea to keep at least a part of your pond's surface unfrozen. The climate that you live in will determine what you need to do to prevent freezing. In some warmer climates, keeping the water circulating constantly may be enough. Colder climates might require a heater.
If you have any questions, look for a qualified koi and pond dealer in your area. Someone local will be best equipped to answer your questions about koi and the cold winter season.