When ponds and lakes freeze in winter, you may wonder how fish survive. I know I did, so I wanted to find out more.
Fish are cold-blooded and can survive in cold bodies of water. Fish reduce their metabolism to save energy in winter and live in the unfrozen waters of ponds, lakes, streams, and rivers.
I have some great information below if you want to learn more about how fish survive during the winter.
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Cold-blooded
Unlike mammals, fish are cold-blooded and cannot regulate their body temperature. Its surroundings and heat influence the body temperature, but heat cannot be generated inside the body.
Fish live in a variety of habitats and temperatures. Fishes living in the Arctic seas have a low body temperature, almost close to the water’s freezing point.
Other tropical fish living on coral reefs have a higher body temperature, around 27 celsius.
How fish live with body temperature the same as their surroundings have a few essential consequences for fishes.
The temperature in many places can vary throughout the seasons of the year. Small freshwater fish may live in almost freezing water in winter but at a higher temperature during summer.
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Metabolism
Although they can live in different temperatures, this does affect their metabolism. During the colder winter months, small freshwater fish slow their metabolism down, not growing, hardly moving, not feeding, and breathing slower than usual. They hide in the vegetation towards the bank’s edge or some protection in the river bed.
However, in summer, the fish would be much more active. With plenty of food around, the fish will start to grow. The fish will become more engaged, with the internal organs functioning at their best.
Where Do Fish Live When Water Freezes?
Water during the winter is prone to freezing. As the water cools, it begins to contract. Once the water reaches four celsius, it starts to expand. Once it reaches the freezing point, it will stop growing. When the water contracts on top, its density contracts, and the ice will sink to the bottom.
Once the water drops below four celsius, it becomes less dense and stays at the surface. Ice forms at the surface, but the water beneath does not freeze. The temperature stays between the freezing point and four celsius for weeks. Although there is a layer of ice at the top of the water, the fish survive in the unfrozen water below.
This is the main reason that fish can survive in water during long periods of frost. By reducing their metabolism to a low level, fish can survive in unfrozen water.
In ponds or shallow streams, fish have a more challenging time surviving as there is not enough unfrozen water to live in. Water with a large amount of plant growth can also harm fish in winter.
If the water freezes with a layer of ice and snow, the light is reduced from the sun to the water. Because of this, plants produce more carbon dioxide than oxygen.
If too many fish are in the water, and there is not enough oxygen to survive due to the amount of carbon dioxide, many may die.
Although fishes in ponds can only escape to the deep water below, most wild fish migrate in winter to deeper, warmer waters.
When fish migrate, they can usually be found in the river’s deepest parts, where the water is slightly warmer. Fish will also move to the deepest parts of lakes in the winter.
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Do Fish Eggs Survive Freezing Waters?
Salmon spawn their eggs in winter, laying them in the gravel. The eggs develop in temperatures close to freezing by using shallow, running water. As the development is slow, between 6-20 weeks to hatch, the young are born in spring and can take advantage of food availability.
The amount of oxygen and carbon dioxide in the water can significantly affect the salmon spawn’s survival rate. Many adults die after traveling to the streams and laying eggs, as they have no energy to survive a trip back to the ocean. However, this is not due to winter conditions.
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How Do Fish Survive At Sea In Winter?
The sea’s immense size allows it to retain its warmth and is not as cold as rivers, ponds, and lakes. Sea fish are generally not as vulnerable to the cold winter months as freshwater fish, as the sea has a lower freezing point than freshwater due to the salt.
However, during severe weather, fish that usually live in shallow rivers and coastal waters move into deeper water to escape the cold.
Many fish live in shallow water and will die in the winter if they do not find more deep, suitable waters. Much of this is due to their food supply being decimated by the cold, reducing the algae and invertebrates available.
Some fish are more sensitive to the cold than others. Flatfish such as plaice and sole congregate in the sea’s deeper parts as they fall foul to the freezing temperatures due to their body shape.
Fish need to make several adaptations to survive the cold water. They can live through severe winters by feeding less, moving less, and putting fewer demands on their bodies.
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Bryan Harding is a member of the American Society of Mammalogists and a member of the American Birding Association. Bryan is especially fond of mammals and has studied and worked with them around the world. Bryan serves as owner, writer, and publisher of North American Nature.