When ponds and lakes freeze over 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.
If you want to find out more about how fish survive during the winter then I have some great information below.
Unlike mammals, fish are cold-blooded animals and are unable to regulate their body temperature. The body temperature of a fish is influenced by their surroundings and heat cannot be made inside the body.
Fish live in a variety of habitats and temperatures. Fishes living in the seas of the Arctic have a very low body temperature, almost close to the freezing point of water.
Other tropical fish living on coral reefs have a higher body temperature, around 27 celsius.
The way that fish live with body temperature the same as that of their surroundings have a few important 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.
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 edge of the bank or in 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. With the internal organs functioning at their best, the fishes will become more active.
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 4 celsius, it starts to expand. Once it reaches freezing point it will stop expanding. When the water contracts on top of the water, its density contracts, and the ice will sink to the bottom.
Once the water drops below 4 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 freezing point and 4 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 then the fishes can survive in the unfrozen water.
In ponds or streams which are shallow, fish have a harder time surviving as there is not enough unfrozen water to live in. Water that has a large amount of plant growth can also be harmful to fish in winter.
If the water freezes with a layer of ice and then it snows, the light is reduced from the sun to the water. Because of this, the plants produce more carbon dioxide than oxygen.
If there are too many fish in the water and there is not enough oxygen for them to survive due to the amount of carbon dioxide then many may die.
Although fishes in ponds cannot 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 deepest parts of the river, where the water is slightly warmer. Fish will also move to the deepest parts of lakes in the winter.
Do Fish Eggs Survive Freezing Waters?
Salmon spawn their eggs in winter, laying them in the gravel. By using shallow, running water, the eggs develop in temperatures close to freezing point. As the development is slow, between 6-20 weeks to hatch, the young are born in spring and are able to take advantage of the ready availability of food.
The amount of oxygen and carbon dioxide in the water can make a huge difference to the survival rate of the salmon spawn. 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.
How Do Fish Survive At Sea In Winter?
Due to the immense size of the sea, it retains 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 to fresh water due to the salt.
During severe weather, however, fish that usually live in the shallow river mouths and inshore waters move into deeper water to escape the cold.
Many fish live in shallow water, and many of these will die in the winter if they do not find deeper, more suitable waters. Much of this is due to their food supply being decimated by the cold, reducing the algae and invertebrates available to them.
Some fish are more sensitive to the cold than others. Flatfish such as plaice and sole congregate in the deeper parts of the sea as they fall foul to the cold temperatures due to their body shape.
Fish need to make several adaptations to survive the cold water. By feeding less, moving less, and putting fewer demands on their bodies they are able to live through severe winters.