If you plan to overwinter your fish, you’ll need to provide support as they enter and maintain a state of torpor (hibernation) during the cold months. This is a state where their metabolism slows down, their activity levels drop, and they seem to settle into an endless nap. While they’re napping, they must not freeze solid. The pond must be deep enough that liquid water will remain at the bottom, even if the water at the surface has turned to ice. If you’re in a part of the country where ponds and lakes ice over only from time to time, you’re probably not going to need to worry about thick ice crowding out your koi. If you’re looking forward to season-long hockey games at the local fishing hole, though, you need a pond that will have sufficient liquid water below the normal ice depth, whatever that tends to be in your area. All pond owners, of course, should be aware that weather patterns are shifting and it’s best to be prepared for something wildly out of “normal” range.
In addition to liquid water, your fish must have sufficient oxygen while they’re in torpor. Your fish may not be moving, but they’re still respirating, and there must be sufficient oxygen to keep them alive throughout the cold months. Here’s another area where a sufficient volume of liquid water is important. You’ll also want to make sure that your starting water quality is optimal and in extra cold areas, consider using an aerator and/or pond heater to allow a small “chimney” of liquid where oxygen can enter and harmful gasses like CO2 can escape. Finally, your fish must be protected from predators like raccoons, herons and other fish-hungry animals who do not hibernate the cold months away. Your pond should be deep enough and provide enough spaces that all of your fish have somewhere to hide while they’re asleep and vulnerable. Generally, to account for each of these needs, a minimum depth of 4’-5’ is safest for overwintering your fish in a backyard pond. Koi can survive in ponds as shallow as 3’, but they are more vulnerable to seasonal variations in temperature at both extremes. Keep in mind that larger wildlife ponds and those filled by surface water runoff need to be considerably deeper. Your county’s soil & water conservation district office should be able to make some specific recommendations for you in that circumstance.
