Do Koi Hibernate? How Koi Survive Winter

Every fall, new pond owners ask the same worried question as the water cools and their fish go still: do koi hibernate, and will mine survive the winter outside? The short answer is that koi do not hibernate in the strict sense, they go dormant, and that a healthy koi in a properly set-up pond survives even a hard freeze with no trouble at all. The longer answer, and the useful one, is that the thing people worry about, the cold, is almost never what kills a winter pond. The real dangers are different, preventable, and worth understanding before the first freeze.

This is also where a lot of well-meaning advice goes wrong. You will read that you need to keep your koi warm over winter, or bring them all indoors, and for a normal outdoor pond both are mistakes. A koi is built for cold; what it is not built for is a pond sealed under ice with nowhere to breathe, or one so shallow the whole pond chills to the bottom. This guide explains what koi do in winter, what threatens them, and the three simple things that carry a stocked pond through to spring.

What koi do in winter: torpor, not hibernation

Koi are cold-blooded, so unlike a hibernating mammal they do not decide to sleep for the winter; their whole metabolism rises and falls with the water temperature. As the pond cools in fall they slow down in stages. Around 50F they become noticeably sluggish and lose interest in food. By about 40F they enter full torpor: they sink to the deepest, warmest water, hang almost motionless, breathe slowly, and stop eating entirely, often not taking food again until the water climbs back through the 50s in spring. This is normal and healthy. A koi resting on the bottom in January is not sick or dying, it is doing exactly what it evolved to do.

Because their system is idling, they need almost nothing from you in this state, and the fat they built up over summer carries them through. The single worst thing you can do to a dormant koi is wake its gut up with food it cannot digest, which is why feeding stops below 50F. For the full temperature-by-temperature feeding rule and why cold food is dangerous, see our guide on when to stop feeding koi for winter.

Cold does not kill koi. Suffocation and shallow water do

This is the heart of it, and it is the opposite of what most people fear. Koi can sit in water a hair above freezing for months and come out fine in spring, provided two conditions are met. When a winter pond loses fish, it is almost always one of these two failures, not the cold itself.

The first killer is suffocation under sealed ice. When ice caps the entire surface, it seals the pond: the carbon dioxide and ammonia the fish and the decaying bottom debris give off can no longer escape, and fresh oxygen cannot get in. Over a matter of days those trapped gases climb to toxic levels, and the fish suffocate under a surface that looks perfectly calm. This is why an open hole in the ice is the single most important thing in a winter pond, it is not for warmth, it is a vent. The second killer is a pond too shallow to hold a refuge. Fish overwinter by dropping into the deepest water, which, in a pond below the frost line, stays around a life-sustaining 39F even as the surface freezes. A shallow pond has no such zone; it chills all the way down, and can even freeze solid in a hard climate, leaving the fish nowhere to go.

The three things a koi needs to survive winter

Everything that keeps koi alive over winter comes down to three requirements. Get these right and you can otherwise leave the pond alone.

• Enough depth. The fish need a deep zone that stays liquid and holds that ~39F bottom layer. In mild climates a couple of feet can be enough with a reliable de-icer; in cold-winter regions aim for a deep section three feet or more so there is always unfrozen, stable water for the fish to drop into. Depth is the one thing you cannot add in November, so it is worth getting right when you build.
• An open hole in the ice. Keep at least one patch of the surface from freezing over all winter so gases can vent and oxygen can enter. A floating de-icer is the standard tool; an air pump running a diffuser set high in the water column does the same job and adds oxygen. On a hard-freeze pond, many keepers run both. What you never do is let the whole surface seal.
• No food below 50F. Once the water is consistently below 50F, stop feeding for the year. A dormant koi cannot digest food, and feeding it is one of the most common ways keepers lose fish over winter. The fish live off their summer reserves; they will not starve.

What not to do: heat, churn, or smash

Three well-meant mistakes cause more winter losses than the cold ever does. First, do not try to heat the pond. A de-icer is not a heater and is not meant to be one, it holds a small hole open, it does not warm the water, and trying to keep a whole outdoor pond warm is both futile and unnecessary for a fish built for cold. Second, do not run a pump that churns the deep water. The warm, dense layer at the bottom is the fish's refuge, and pumping it up to a cold waterfall mixes that heat away and chills the very water the koi are resting in. In winter you either pull the main pump or run only gentle surface aeration, keeping the deep water still. For the full rundown, see our guide on whether to run your pond pump in winter.

Third, never smash the ice to open a hole. The shock wave from a hammer or an axe travels through the water and can concuss or kill the dormant fish resting below. If the surface has frozen over, melt an opening gently, set a pot of hot water on the ice until it melts through, or run a de-icer, and never strike it. And resist the urge to bring healthy koi indoors for winter unless your pond cannot hold a safe refuge; a proper outdoor pond is where they belong, and the stress of capture and an indoor tank is usually the greater risk.

Which pond fish are winter-hardy?

Not every pond fish handles a freeze the same way, so know what you are keeping. Koi are thoroughly cold-hardy and overwinter outdoors across cold climates without trouble. So do the single-tailed goldfish, common goldfish, comets, shubunkins, and sarasas are all hardy and winter over fine in the same ponds. Small fish and this year's fry generally do as well as adults, sometimes better, since they use less oxygen. The clear exception is fancy goldfish, the round-bodied fancies like orandas, ryukins, and moors, whose deformed body shape makes them far less cold-tolerant; in a hard-winter climate those are the fish to consider bringing indoors, not the koi.

Koi winter survival questions, answered

Do koi die in the winter?

A healthy koi in a properly set-up pond does not; koi are built for cold and overwinter under ice routinely. When koi do die in winter it is almost never from the cold itself. It is from suffocation under a pond that sealed over with ice and could not vent its gases, or from a pond too shallow to hold an unfrozen refuge at the bottom, or from having been fed while dormant. Fix those and winter is a non-event.

How cold can koi survive?

Very cold. Koi tolerate water down to just above freezing and can sit in near-freezing water for months in torpor with no harm, as long as the pond keeps exchanging gas through an open hole in the ice. The temperature of the water is rarely the problem; it is a sealed surface or a pond with no deep refuge that does the damage.

Do I need to heat my koi pond in winter?

No. Koi do not need or want a heated pond, and trying to keep an outdoor pond warm is futile. What you need is not heat but gas exchange: a de-icer or an air pump to hold a hole open in the ice so carbon dioxide can vent and oxygen can enter. A de-icer is often mistaken for a heater, but it only keeps a small patch ice-free, it does not warm the water, and that is all a cold-hardy koi requires.

How deep does a pond need to be for koi to survive winter?

It depends on climate. In mild-winter regions a couple of feet can suffice alongside a reliable de-icer. In cold-winter climates aim for a deep zone of at least three feet, and often more, so there is always unfrozen water near the bottom holding around 39F for the fish to drop into. The key is a stable liquid refuge below the ice; depth is what provides it, and it is far easier to build in than to add later.