Pond preparation is a must 

There is nothing more relaxing than sitting and watching a gentle waterfall caress the rocks as it cascades into a fully stocked outdoor fish pond filled with Koi. In warmer weather Koi are particular active and will provide you with hours of enjoyment as they are lively and remain highly visible. Thus it is a simple matter of observation that allows one to keep tabs on the Koi pond and the conditions of the environment and its inhabitants.

But even in Southern California, where ambient temperatures seem to perpetually stay well above freezing, those of you who have outdoor Koi ponds know winter can be a concern. When the cold weather descends on us, Koi tend to slow their activity.

Koi are poikilothermic or cold-blooded beings, and their metabolism is closely tied to the temperature of the water in which they live. This, in winter, when the water is coldest, Koi activity slows to a crawl, mirroring their metabolism. They mostly spend their time treading water near the bottom of the pond, where they find the warmest water in the Koi pond. The need for movement is minimal as it doesn’t take much for them to keep their joints limber and flexible.

Water reaches its maximum density at just more than 39°F, and in cold weather, the warmest water is at the bottom of the pond. That is where you will find your Koi in winter.

Keep an eye on them 

And unlike our other outdoor hobbies, like gardening, one can’t just ignore the pond, letting it freeze over (if you are in higher elevations or in less temperate climates) and pick it up again after the spring thaw. Do that and you will find yourself having to restock the pond every spring, a lesson most Koi novices learn just once.

If you do happen to live in a cold climate, it will be a while before you see your Koi again as they sit on the bottom of the pond in the warmest pocket of water they can find. This might lead one to think they are huddling together for warmth, but being cold-blooded creatures, but what is actually happening is, the Koi are entering a state of torpor.

Because torpor is measurably shorter in duration, it is not really considered hibernation, but many of the characteristic of torpor mirror what bears and other endotherms that hibernate experience:

reduced body temperature, slowed metabolism, slow reaction times, reduced breathing rate and primary body functions. Torpor allows the animal to save the energy that would otherwise be needed for higher levels of activity.

Keep things calm in the Koi pond

So because of that, it is wise to keep things as calm as possible around your Koi pond in colder seasons. If, for example, you live in a place where your pond might freeze over, and you need to open the ice, find a quiet, gentle method, such as using some boiling hot water to make a hole. Do not chop at it with a hammer or axe.

You can even use a cordless drill to get through the ice if it is too thick to be breached with the boiling water. But the key benefit to doing this is the open area in the ice will allow noxious gases, like ammonia, to escape from the pond.

As a general rule, you should guard against letting your pond’s water temperature fall below 34F. Temperatures below 34F will allow ice crystals to form on the gills of your Koi, which can kill them.

Of course, if you do your homework and have cleaned and prepared your pond for the winter, then you have little about which to worry. You can sit back and relax, enjoy the slower pace the fish will maintain in cold weather, and look ahead to spring, when the Koi will be back to their usual entertaining behaviors.