How do fish survive winter?

In this post, we will answer the question “How do fish survive the winter?” and what mechanisms fish use to stay alive even in frozen lakes.

How do fish survive winter?

Fish that live in ponds can survive the winter when the water freezes because they can adapt to drastic climate change. When a lake freezes, it is never completely solid. Even when temperatures are very low.

How does a lake freeze?

When the surface water temperature reaches 4 degrees Celsius, its density starts to decrease and water with a temperature lower than this temperature begins to float, once it is less dense than the bottom waters.  At that point, the surface water continues to cool.

As it cools, a thin layer of ice is formed on the surface of the lake (river or sea), this layer tends to increase, that is, to become thicker, with the freezing of new layers of water that are below it. As ice has a lower density than water, it floats, making this layer serve as a thermal insulator between the air that has a temperature below 0ºC and water that has a temperature of 4ºC.

For all the water to freeze, low temperatures must be maintained for long periods, at least for many months.

How is it possible for life to exist in frozen lakes?

It may seem impossible, but under that thick layer of ice that supports skaters, life goes on. Only in slow motion. Unlike what happens in summer and spring, some species of fish naturally overwinter and slow down their metabolism: they eat less and barely move.

How do fish live in the cold?

With the decrease in temperatures during the coldest months, the average water temperatures also decrease, causing a reduction in the level of fish activity. 

Exothermic animals generally have an optimal temperature range for their organisms to function, called “thermal comfort”. When the average water temperature drops and goes out of the comfort range, these organisms show a decrease in their body activities.

During the cold winter months, lakes and rivers in various parts of the world end up freezing, forming a kind of natural mirror. However, fish and other aquatic animals manage to survive even facing these adverse conditions. This draws even more attention to the Arctic Ocean, which tends to have these conditions practically all year round.

Change in Metabolism

The metabolic rate of a fish swimming under a thick layer of ice in a lake drops dramatically compared to the summer months. As the water gets colder and colder, the body temperature of fish also drops. They reduce their metabolic activities and are no longer as fast as they usually are in warmer water. 

The energy that the fish need to survive is much lower in the cold, which is a good thing since there is not the same abundance of food found in the summer. Some species found in ponds, such as catfish and carps store as much fat as possible in their bodies, eating heavily in the summer and fall so they have enough reserves during the winter. Some catfish and eels also bury into the mud at the bottom of the lake and wait for spring to return.

The circulatory system of fish

The circulatory system is closed and a heart with two cavities is observed: an atrium and a ventricle. In this important blood-pumping organ, only blood rich in carbon dioxide (venous) circulates. 

Circulation in fish is simple, since blood passes through the heart only once, in each complete circuit through the animal’s body. Blood enters the heart through the atrium, travels to the ventricle, and is pumped toward the gills, where it is oxygenated. This blood is then taken to the animal’s body.

Finding food in the cold

Some species of fish still actively feed on anything they can find. Perch, Lepomis macrochirus, trout, and pike are fish that have evolved and are now able to raise their body temperatures, even in cold water, to maintain their ability to eat. Fortunately, they do not need to ingest the same amount of food as they do in the summer. Larger fish eat smaller fish and can catch any baits remaining in the water, but smaller species depend on other food sources. 

Sun perch and Lepomis macrochirus feed on immature larvae of a wide variety of lake-dwelling aquatic insects that are found in the mud at the bottom. Small zooplankton is also consumed in winter as they remain active during this period. When the weather warms, plants at the top of the lake can become trapped in the ice and are pulled upwards as the ice rises and carries them with it. This causes the roots to come out of the ground and up, making aquatic insects visible and easy prey for hungry fish.

How to take care of my fish in the cold?

Many fish farmers have to adapt to climate changes in each season, many of them develop techniques to overcome these adversities and ensure a better quality of life for the fish.

Fish is a pecilothermic animal, which regulates body temperature with that of the environment. In winter, with the drop in temperature, the fish metabolism also drops, making them less active. Therefore, in the cold period, they eat less, swim less and breathe more rhythmically. In summer, fish become very active and require constant feeding. 

When building a lake in very cold places, it is recommended to have already designed a heating system for the water. There are several types of heaters and thermostats for tropical fish ponds. Another resource commonly used is the construction of lakes in deeper areas, to generate thermal comfort for fish. The stones also help to heat up, as they capture heat through the sun, especially on the banks.

Conclusion 

In this post, we answered the question “How do fish survive the winter?” and what mechanisms fish use to stay alive even in frozen lakes.

If you have any thoughts or doubts, feel free to drop us in a comment below!

Frequently Asked Questions (FAQs): How do fish survive winter?

How do fish survive in the cold?

Fish feel cold, but not like mammals. The body of the majority of fish is programmed to adapt to the variation in water temperature, heating and cooling occur gradually. Thus the fish can get used to the change and adapt their organisms.

Why do fish die in the cold?

As the fish have reduced metabolism due to the low temperature, the fish eat less, grow less, move less, and depending on the temperature they can die. They do not necessarily freeze.

What happens to fish when water freezes?

Fish that live in ponds can survive the winter when the water freezes because they can adapt to drastic climate change. This means that fish do not control their body temperature, which is very close to the temperature of the surrounding water.

Do Fish grow during winter?

In general, fish do not grow during winter. During cold winter temperatures the fish reduce their metabolic rate. Thus, the stop growing, as well as movement, and breathing rate.

Do fish hibernate in the mud?

Yes, some fish species can hibernate buried in the mud. An example of fish with this behaviour is the Little Mud Minnow or Umbra limi.

References 

Donaldson, M. R., Cooke, S. J., Patterson, D. A., & Macdonald, J. S. (2008). Cold shock and fish. Journal of Fish Biology, 73(7), 1491-1530.

Moksness, E., Kjorsvik, E., & Olsen, Y. (Eds.). (2008). Culture of cold-water marine fish. John Wiley & Sons.

Wohlschlag, D. E. (1960). Metabolism of an Antarctic fish and the phenomenon of cold adaptation. Ecology, 41(2), 287-292.

Cossins, A. R., & Macdonald, A. G. (1989). The adaptation of biological membranes to temperature and pressure: fish from the deep and cold. Journal of bioenergetics and biomembranes, 21(1), 115-135.

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