When the winter season approaches and the temperatures drop, animals, birds, plants, and fish start their seasonal preparations.
The trees drop their leaves, birds start their southern migration, and some animals, like bears, stock up for a long winter’s rest.
Fish vary in their response to colder temperatures. Some, like salmon, follow the example of birds and head off searching for water climes. Others simply adapt to the environment they’re already in.
What Do Fish Do In The Winter?
When the cold winter months arrive, many pond owners start to wonder if they have any fish left or if they’ve all perished in the freezing temperatures.
In most instances, the news is good – the fish aren’t dead, they’re just cold.
Fish are cold-blooded animals, meaning their body temperatures fluctuate with the temperature of the water. So when the winter season comes and the water temperature drops, fish also start to feel the chill.
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The internal temperature of the fish drops as the temperature of the water decreases. At the same time, the fish’s hearts slow, and their metabolism becomes increasingly sluggish. As a result of these physiological changes, the fish become sluggish and less inclined to move around.
In this sense, fish hibernate, but in reality, no eco therm, or cold-blooded animal, can enter a state of genuine hibernation. To do so, they would have to maintain their own internal body temperature, as the bear does, which is impossible when it reflects the ambient temperature.
Furthermore, fish can’t maintain the metabolic rate needed to keep them alive for a sustained period.
Hibernation, or dormancy, is a “long-term multiday torpor for the survival of cold conditions.” Fish are unable to enter hibernation because “they appear to be incapable of further suppressing their metabolic rate independently of temperature.”
Instead of hibernating, fish enter a state of torpor.
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What Is Torpor?
Torpor is a physiological response in which the metabolism slows and the animal expends as little energy as possible.
Although similar to a state of hibernation, torpor is more of a resting condition. Fish don’t need to hibernate because they’re cold-blooded, so they don’t need the energy to manage their body temperature.
As a result, they have relatively modest metabolic and energy needs and can make it through winter on far less caloric intake.
When resting, the fish’s heart rate drops along with its body temperature and metabolism. As a result, even the need for oxygen decreases.
Even in this sleepy state, fish continue to feed and perform all their usual bodily functions, but for much briefer periods than they would normally.
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A resting fish may remain dormant for several hours before becoming active and hunting for a few hours, after which it will return to its state of fish hibernation. It will do this repeatedly for several months between autumn and spring.
How Do Fish Survive Winter?
When the water freezes, most fish head down to the deepest pools because the warm water sinks into very cold water. There they simply school together, taking their rest communally. Not all fish use this tactic, however.
We already mentioned that some fish migrate, like salmon and eel, rather than sleep, seeking out warmer waters where the food is more abundant.
Others go to even greater lengths to stay warm. For example, Koi fish and gobies will burrow into the soft sediments at the bottom of a body of water and wait out the winter there.
As the fish aren’t in a deep sleep, they still move around, looking for the warmer water which is often in the deepest pools or anywhere “that can radiate heat from the sun,” such as dock floats and rock banks.
Fish will also seek out any freshwater sources, such as wells or springs, where the water might be slightly warmer.
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Ice fishing enthusiasts who brave the cold to seek out popular ice fishing species like the yellow perch and northern pike, highly-priced this information.
Species like these appear to be more resilient to the cold, remaining more active in the winter season than many pond fish species.
Yellow perch, for instance, “remain active and aggressive,” even in the most frigid of waters. Similarly, the northern pike is “most active in cooler water temperatures – around 65° F and below.”
How Do I Know if a Fish is Hibernating?
A hibernating fish will usually hang out at the bottom of the pond in an upright position but with its pectoral fins tucked underneath it.
You may see it swim slowly around the pond sometimes, but most of its time will be spent in the deepest water where the temperature is highest.
A hibernating fish will continue to eat as long as the temperature remains above 45℉, if it drops any lower, they will generally stop eating altogether.
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At temperatures as cold as this, fish slow down to such an extent that they no longer need to hunt. Fortunately, pond fish can get the nutrition they require from scraping nutritious algae from the rock growth underwater.
Which Type of Fish Hibernate?
Many types of pond fish hibernate especially popular species like Koi fish and gobies. During this period, all their internal functions and processes change, adapting to the temperature of the surrounding water.
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Fish will stay in this state throughout the coldest period, returning to their normal status only once the warmer seasons arrive.
Other types of fish hibernate in different ways.
The lungfish is a prehistoric species that performs something very similar to hibernation but in very different circumstances. Instead of using torpor to survive the cold, lungfish use a process known as estivation to get through the dry season.
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While hibernation and torpor usually occur during the winter, estivation is a form of summer dormancy that enables an animal to survive a drought or similarly dry period.
The lungfish has survived for some 400 million years thanks to its ability to extract oxygen from the air and water.
When the dry season comes, the lungfish buries itself in the mud, creating a cocoon of air around it in the form of mucous. It remains there until the waters return, living off the fat in its own tail.
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A study of saltwater fish in the Antarctic revealed that these species undergo “winter metabolic suppression irrespective of water temperature.”
Researchers observed the black rock cod, also known as the Antarctic yellowbelly rock cod, or Antarctic bullhead notothen, becoming less active in winter, reducing its metabolic rate at the same time as its “sensory and motor capabilities.”
In between their periods of torpor, the black rock cod displayed “periodic arousals” every 4 to 12 days, each of which lasted for several hours. During these active periods, the rock cod’s “metabolism increased to summer levels” before decreasing again as it re-entered torpor.
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The researchers concluded that the rock cod effectively put themselves on ice for the winter, regardless of the temperature of the salty ocean water, waiting for food resources to recover to their summer levels.
This study suggests that some fish at least “can enter a dormant state similar to hibernation that is not temperature driven and presumably provides energetic seasonal benefits.”
Most fish hibernate in one way or another, even if it’s only entering a temporary state of torpor.
Even popular ice fishing species enter torpor, so it can be challenging to fish in the winter. Fish are less active than usual and show little interest in fast-moving prey, preferring to conserve energy and wait for a bigger, slower meal to present itself.
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If you want to fish in the winter, you need a colorful lure and a lot of patience! You’ll probably have more luck if you wait until spring when the warmer water and increased food supply rouse the fish from their slumber.
Nicky is a British adventurer and animal lover who spends her time exploring the natural world and writing about her experiences. Whether on horseback, underwater, running, hiking or just standing with a fishing rod in hand, she embraces everything her adopted home of South Africa has to offer.