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Fish can’t exactly migrate like birds do in the winter. So, what do they do, particularly fish that live in ponds where the water freezes over in winter?
While fish living in the sea or in rivers have the option of migrating to warmer waters (and moving water is less likely to freeze in the winter), fish living in small lakes or ponds do not have that option. Instead, they have to hunker down in the cold waters that they live in.
Luckily, fish have a few evolutionary mechanisms that they’ve developed to help them survive winters in ponds.
Do Fish Hibernate?
As the weather gets colder, fish will slow down their physical movements and even necessary bodily functions.
Fish are cold-blooded animals, which means as the water outside drops in temperature, their body temperature naturally drops as well. The animals will also lower their metabolism levels during cold weather, so they have to eat less and expend less energy.
Technically, fish cannot hibernate because only warm-blooded animals can do so. However, many fish enter a similar state called torpor during the winter. Torpor is a lighter, short-term version of hibernation when an animal will decrease its bodily functions, but only for a few days or weeks at a time instead of the whole winter.
Some pond fish go dormant for the entire winter, moving around occasionally and feeding a little. Others will only sleep for a few weeks at a time.
The decreased heart rate, metabolism, and breathing during torpor help fish conserve energy and survive the winter.
Where Do Pond Fish Go When It Gets Cold?
Most fish go to the bottom of the pond in the winter because warm water sinks while cooler water rises to the surface.
They will go as deep as they can, often congregating in deeper pools. Some species will even burrow into the sediment at the bottom of a pond to stay warm.
In shallow bodies of water, fish may have trouble surviving the winter because there is not enough deep water for them to retreat to. The first few feet below the surface are usually very cold.
Even if the pond freezes over, fish will be fine at the bottom of the pond.
The layer of ice and snow on top of the pond acts as an insulating layer and increases the temperature of the water.
However, if there is too much ice and snow covering the whole surface of the water, then that could be dangerous for the fish because harmful gases get trapped in the pond.
What Do Fish Eat in the Winter?
In the winter, fish decrease their metabolisms, so they only need to eat a fraction of what they would eat in the summer. However, they still need to eat something as they do not hibernate fully.
Even when a pond is frozen over, there is still food left for fish to eat.
In the winter, fish will subsist on a diet of aquatic plants and algae. Their caloric needs are much lower when dormant in the winter, so fish only need a small amount of algae to survive.
Other fish that are naturally adapted to cold water, such as trout, take advantage of the dormancy smaller fish go into and use winter as their prime hunting time. They will eat other fish hiding from cold water in the deep.
However, these predatory fish are not common in small ponds.
How Do Fish in Decorative Ponds Survive in the Winter?
If you have an artificial pond in your backyard, you may be wondering if your decorative fish will survive the winter.
Certain tropical species cannot survive the cold and you should take them to an indoor tank for the winter. However, hardier species such as koi and goldfish survive just fine. They use the same tactics as their wild cousins, lowering their metabolisms and seeking out deep water, to survive.
To help your fish survive the winter, break a hole in the ice.
Artificial ponds don’t have as much natural ventilation as wild ponds do. Carbon dioxide could get trapped beneath the ice, particularly if you have many aquatic plants in the pond. Drilling a hole allows for natural gas exchange and ensures that your fish will get enough oxygen to survive the winter.
Other than ensuring oxygen levels in the pond, there is not much else you need to do to help hardier captive fish survive the winter.
You don’t need to deice the pond—it’s actually recommended that you leave most of the pond covered in snow and ice to act as an insulator.
Feed the fish much less than you would normally as they need to eat much less and leftover food can break down into carbon dioxide.
Can Fish Die in the Winter?
Unfortunately, not all fish survive through the winter as it does require hardiness to make it through cold weather.
One of the main causes of fish death is gas imbalance. If there is not enough oxygen in the water, fish will suffocate as the ice prevents new oxygen from entering.
If there are too many fish in the pond or too many plants emitting carbon dioxide, that leads to gas imbalance.
Fish that get trapped in shallow ponds will also have a harder time surviving the winter as there is less warm water in the deep end to help them stay safe.
If there are too many underwater currents, that could also kill off fish in torpor as surviving currents could sap vital energy reserves.
Pond fish have adapted so that they are safe when they have to hunker down for the winter.
By slowing down their metabolism and bodily functions, eating the bare minimum, and heading for warmer waters closer to the bottom, pond fish survive the winter.
Even decorative pond fish such as koi use these techniques to survive the winter, as long as there is enough oxygen and food to survive.
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