You may know that fish enter a state of dormancy over autumn and winter, that they enter hibernation below certain temperatures, but is this true, what does it mean and what should you do about it?
Do pond fish go into hibernation?
Yes, temperate and coldwater fish species like goldfish enter a dormant state over winter, much like hibernation, although this is not the same as our mammal hibernation.
In cold temperatures, fish enter a state we call torpor.
It is a different way of conserving energy, one that still allows the fish to be awake and move around, but preserves precious fat reserves and nutrients during tough times.
What is Torpor?
Entering Torpor, or a state of torpidity, is a state where the body closes down specific functions and organs which consume lots of energy.
It is an extreme survival measure in response to very harsh environments, massive drops or rises in temperature or scarcity of food and water.
Many animals can enter torpor, including fish, mice, a lot of marsupials, many small birds like hummingbirds and many invertebrate species.
Reptiles and amphibians can also do this, however it is slightly different, and is called Estavation, where the frog or snake will reduce its bodily functions, but still be ready to pounce at food or run from predators at any time, very similar to torpor.
During torpor, a fish will remain very still, but they will still occasionally move around and are still sort of half awake most of the time.
During torpor, it is important not to interfere with the fish in any way if possible. Aside from just to check on them, do not feed them and perform little pond maintenance.
Any stress can bring them out of torpor temporarily and this can shock their bodies, leading to disease and death.
Why do fish hibernate?
Many fish species enter torpor as they cannot escape the cold weather, as they are trapped in a lake, pond or river, the freezing temperatures are inescapable, so their bodies have evolved to cope with it.
During winter, the temperatures are cold, preventing the fish from maintaining a healthy body temp.
There is no food, since most of the insects and algae that fish feed on die off and water freezes over, preventing effective oxygen diffusion.
All these things added together make life over winter very difficult for a fish, so their technique is to conserve what energy they have and wait it out until better times arrive.
For goldfish and carp, winter opposes another challenge, as these fish lack an acidic stomach. Instead, they must rely entirely on their gut bacteria to extract nutrients from their food.
Below 10C / 50F, the bacteria within a goldfish’s stomach stops working and enters dormancy, this means that in these temperatures, goldfish cannot digest food!
This is why it’s so important for them to enter torpor, as it is a natural process which prevents them from starving.
When do pond fish go into hibernation?
Most fish species will start to enter torpor when the temperature drops below 10C / 50F for coldwater fish like goldfish and koi; however, this temperature is a lot lower, at 4C / 39F.
Depending on your geographical location, this will be at a different time of year, or may not occur at all.
For most of the northern hemisphere, fish will begin to enter torpor around November and will continue right through until February – they may still continue to enter in and out of torpor right up until mid April.
Keep an eye on the temperature and feed them wheat germ when it gets above 5C / 41F.
How do I know if my fish are hibernating?
You will know when your fish have entered torpor as they will become lethargic, lazy and will hide and stay around the bottom of your pond, barely moving.
They will not come to the surface to eat and will probably be cautious and afraid of your approach. They will move slowly and carefully but will sit motionless in the water throughout most of the day.
If you see your fish like this and it is bitterly cold outside, check the temperature and if it’s below 5C / 41F then your fish are in a torpid state.
Around winter and spring, we always recommend using a thermometer on your pond.
Why should I use a thermometer in my pond?
Using a thermometer on a pond is imperative – we can’t stress enough how important it is, especially during the winter months.
You must understand how hot or how cold your pond actually is, and have an accurate reading on it.
Sticking your finger in the water and judging based on your 37C body temperature isn’t enough!
The reason you need to be so aware of the temperature especially during the colder months is because it tells you when you need to change the diet of your fish, when you may want to perform extra maintenance and when you should completely stop feeding your goldfish.
For reference, once temperatures reach below 10C, you need to switch your fish’s diet onto wheat germ, and feed much less.
Once it reaches below 5C, do not feed your goldfish at all, until temperatures rise again. Any food given now will sit in the fish’s stomach all winter, it will rot and cause infection inside the fish’s gut, which can lead to bloat, dropsy and death.
Using the thermometer as well as the fish’s behaviour during spring can also tell you when to start feeding them again.
Fish hibernation in mud
You may have heard that fish can hibernate in mud too, yes, but only a few select species of fish can do this. Some of these are the lungfishes – lungfish are actually our closest living fish relatives.
They are named lungfishes because… they have lungs!
This allows them to take in extra oxygen from the air when the dissolved oxygen levels in the lakes and ponds they live in get too low.
Lungfish can enter a state similar to brumation in reptiles and amphibians. They do this because in their natural habitats, the lakes they come from often evaporate in the dry season, and so the fish need to preserve themselves.
They do this by burrowing into moist soil and forming a cocoon around themselves made out of their slime coating. This cocoon protects them from the drought.
The fish then falls asleep and turns off many of its non-essential body processes, leaving just enough to keep it alive.
When the rains return, the cocoon dissolves and the fish wakes up again!
Yes, fish in torpor will be seen swimming at the bottom of the pond, motionless, with only slight movements.
They do this because it is the safest place for them to be, especially since only half of their motor skills and brain functions are active.
Staying at the bottom is a good way to avoid being eaten by birds and other predators. It also protects them from the weather, since the bottom of the pond will be the warmest part and will be the last part of the pond to freeze.
When do pond fish come out of hibernation?
As mentioned, this will depend entirely on your local area – this is why we suggest using a thermometer to see what time of year is best to begin feeding your fish again.
Each year the meteorological seasons change, this means that one year spring may arrive early, or you may get a particularly long winter, so it is impossible to say exactly when your fish will come out of hibernation.
- They will return to normal once the temperature gets above 5C / 41F however, during this time they should be fed on wheat germ and nothing else.
- Once the pond gets above 10C / 50F you can then switch them back to their normal varied diets.
We say this because protein is difficult to digest at low temperatures and the fish need to be warmer to absorb it properly.
Feeding fish high protein foods at this stage is a large cause for dropsy!
When do koi fish come out of hibernation?
Koi will come out of hibernation at the same time goldfish will, once the water temperature gets above 5C, and just like goldies, they need to be fed wheat germ until the temperature gets above 10C.
For the majority of the northern hemisphere, this is around late February early March, but the temperature may not stay consistent until mid April!
Confusing health problems with torpor
As mentioned, torpor is characterised by fish staying around the bottom, expressing lethargy and lazy movement, becoming skittish, not eating and so on.
These symptoms can also be seen in fish that are stressed or ill, however, there are differences which can tell you if your fish is actually sick, or if it is undergoing a perfectly normal process.
Labour intensive swimming
This is one often overlooked symptom in sickly fish, it is characterised by a lethargic fish who spends lots of time resting, and then when they do swim, they put their whole body into it.
Their heads will sway side to side dramatically and they will use lots of effort to achieve very little movement.
A fish in torpor will not do this, but an ill or highly stressed fish will.
In torpor, the fish do not physically lie down at the bottom, they hover just above the substrate. An unwell fish will touch its chest against the floor and fully rest for long periods of time.
A fish will only enter torpor when the temperatures are low, if your fish is expressing torpid symptoms and the temperature is high, something may be wrong.
Floating at the surface
This is another overlooked symptom, but a fish in torpor will avoid the surface as much as it can.
If, in the dead of winter, you find your goldfish is swimming near the top for long periods of time with its head touching the surface, something may be wrong.
What to do?
If you pick up on any of these symptoms during winter with your fish, then the first thing to do is to quietly observe them.
You do not want to bring a fish out of torpor, as it can shock and kill them.
If you end up finding any health problems like a wound, parasite or fungus growing on the fish, then treat it accordingly.
But try to avoid stepping in if you can, unless you know it is a serious problem.
Disturbing fish in hibernation can cause extreme stress, loss of immunity against disease and infection, shock and a host of other health problems that could kill your fish.
You should not introduce new fish and by extension new disease and pathogens to your pond near winter time, as you are inviting disaster.
Luckily, the cold weather often reduces the risk of infectious pathogens affecting your fish, since the microbes cannot function properly in freezing temperatures, however there are strains of fungus and bacteria that can survive in winter and these you need to look out for.
During the winter time, you need to leave your fish to their own devices, but be there to check up on them.
Make sure the water is clear and the filters are running smoothly, and your fish should look after themselves during winter.