Do Pond Fish Hibernate? (Explained)

Fish are cold blooded organisms, and as such they rely on the surrounding environment to help regulate their temperature. Specifically, fish are ectothermic poikilotherm vertebrates, meaning that their body temperature will fluctuate with the surrounding water temperature – so, if the water cools down, your fish will, too!

Humans, and most birds and mammals as well, are endothermic homeotherms, meaning that we produce our own heat (typically by consuming food for energy) and our body temperature does not vary much. In fact, deviate by more than just a degree or two and we become ill; more than that, and we can die.

Because of their environment, fish are well adapted to be able to withstand some pretty substantial temperature shifts before it causes any serious issues for them, as well as make use of a low-energy state to reduce food requirements over the winter.

Are Fish Hibernators Or Just Light Sleepers? (Hibernation Vs Torpor)

When an animal goes into a state of “deep sleep” or decreased activity, it’s commonly referred to as “hibernation.” However, true hibernation means that the animal is in such a state of inactivity that even things like loud noises or being touched will not wake them, similar to a coma. It’s also obligatory (meaning, the animal cannot simply decide to hibernate) and triggered by annual biological processes dependent on environmental conditions.

In true hibernators, their bodies have completely shut down all but the most essential of functions – the heart rate will slow down by as much as 98%, metabolism drops by about 99%, and in mammals body temperature will plummet to match that of the environment, focusing only on keeping the internal organs warm enough to function at the minimum capacity necessary. This can last anywhere from weeks to several months, but is generally seasonal and lasts from autumn to spring when conditions are the most harsh and food is the most scarce.

With all of this in mind, contrary to popular belief, bears do not actually hibernate based on these criteria – they’ll wake up periodically, move about a bit and perhaps even venture out for a drink, their body temperature doesn’t drop extensively, and heart-rate and metabolism slows but not to the extent of a true hibernator. Instead, bears enter a short-term state known as “torpor.”

What Makes Torpor Different? Why Is It Useful?

Torpor is a lighter, more temporary state than that of hibernation, typically lasting anywhere from a few hours to a couple of weeks. For example, in the northern United States, red squirrels and other non-hibernating mammals utilize torpor at night (or for several days if the weather is bad) during the winter to conserve as much energy as possible during the coldest period, re-emerging once the sun is up and has warmed things up a bit. Torpor can be described more so as a state of lethargy in which the organism doesn’t move around much or may appear to be asleep in some cases, but they’ll “wake up” periodically to warm up a bit (in the case of fish, by moving to water that may be slightly warmer), forage for some food (because the metabolism doesn’t slow to the extreme degree of hibernation), or simply due to some form of disturbance.

While hibernation is obligatory, torpor is flexible and can either occur instinctively or by choice of the animal, and ultimately the animal has some form of control over its commencement and duration. Torpor is useful when it’s not necessary to be inactive for extended periods of time, and enables the animal to become alert again fairly quickly if disturbed.

So, Do Pond Fish Actually Hibernate?

With all of this in mind, fish don’t technically hibernate (although, the word has now become a “catch-all” term within the hobby for winter inactivity) – they actually enter torpor. In fact, it’s biologically impossible for any known ectotherm to enter true hibernation, as they cannot regulate their own body temperature – it already matches that of the environment – nor can they regulate their own metabolisms to maintain internal temperature. True hibernators (endotherms) do both of these things during hibernation, and so by definition fish and other ectotherms do not and cannot truly hibernate. Because ectotherms don’t need to use up energy regulating their own temperature, their metabolic and energy needs are fairly low, meaning that they really don’t need to enter true hibernation – torpor is usually enough for them to survive the winter.

When the weather cools down, the metabolism of your fish will slow down as less energy is required to remain the same temperature as the surrounding water. This is why you don’t need to feed your fish much (or at all) during the winter, but because they’re in torpor rather than hibernation they still need to eat a little. For most fish, simply grazing on any existing plants or algae in the pond is sufficient. Specifically, during torpor the body releases hormones that suppress the appetite and conserve energy, while also decreasing protein synthesis and tissue growth (this is why juvenile fish will grow very little during the winter). Likewise, heart rate will slow down, oxygen consumption will decrease in response to lower dissolved oxygen levels in the water, and brain activity will be reduced.

When Do Fish Come Out of Torpor? (Things To Look Out For)

Goldfish, koi, and other fish will naturally come out of torpor once temperatures warm back up. This could be springtime, or if there is a mild period during the winter (don’t worry if this occurs, just give your fish a little extra food while they’re active and once temperatures drop they’ll re-enter torpor). It’s also natural for fish to come out of torpor periodically throughout the winter, as torpor is just a temporary state. Torpor that takes place over a span of months is actually a series of many periods of torpor. Fish may move about for a few hours, nibble on some algae, and then resume their apparent slumber.

Once water temperatures exceed 50°F for at least a few days, your fish should naturally regain normal alertness, activity levels, and feeding patterns again. It’s important that you don’t try to “wake up” the fish yourself, as doing so could cause shock, injury, or the usage of precious energy reserves that are still needed until the fish naturally revives itself. During this initial period of exiting torpor, your fish will display slight movement – not much at first, but this will gradually increase as temperatures rise. You’ll notice that they’re not huddled into a group for warmth and energy conservation, and that their fins aren’t held as closely to their body. Similarly, their appetite will return as their metabolism speeds back up – usually around 55 to 60°F for koi and a few degrees higher for goldfish. If you see that your fish are beginning to swim about and are no longer clumped together in a group, it’s likely that they’re searching for food and have fully come out of torpor.

What Temperatures Are Comfortable For Pond Fish?

As discussed in our previous article focused on pond water temperature, all fish have a certain temperature range in which they are comfortable and a slightly wider temperature range in which they are able to survive, even if they aren’t fully comfortable. Once outside of this range, fish often become lethargic, may develop some form of illness, are more prone to parasites and disease, enter torpor to conserve energy, or, in extreme cases, die (this is of particular concern if the temperature changes too quickly, which can result in death from shock).

For common goldfish, this temperature range is somewhere around 68 to 72°F, though they’re hardy and can withstand fairly significant temperature changes outside of this (so long as they don’t occur too quickly!). Koi have a greater ideal temperature range (59 to 77°F), but are less tolerant than goldfish of variations outside of that scale.

What Happens As Water Gets Colder In Ponds?

As winter approaches and temperatures begin to drop, you may notice your fish moving about more slowly, eating less as their metabolisms slow in response to decreased digestive enzyme functioning, and hanging around toward the bottom of your pond. The latter is due to winter stratification that results in the warmer water, which is now more dense (water is most dense at 4° C; anything warmer or colder will float to the top), shifting to the bottom of your pond while the cooler water moves to the top. Therefore, your fish are going to stay where water is the warmest and they can expend the least amount of energy to survive winter. An interesting video below shows the process of stratification which can occur in both deeper ponds and larger lakes: