What Happens to Fish When a Pond Freezes?

Sometimes during the winter months of January and February, we can get an extended period of freezing temperatures in Maryland, enough to cause a pond to freeze over.  What causes this? Well, as the temperature of water decreases during this time, water becomes more dense.  When it reaches temperatures in the 30’s an unusual thing happens where water begins to expand. Above 32 it behaves like other liquids; it expands as it warms and contracts when it cools.  Water starts to freeze when the temperature approaches 32 and the molecules no longer move vigorously enough to break their hydrogen bonds.  This unusual property of water is what allows ice to float.

Because water freezes at 32-degrees and below, ice is less dense than water.  The reason for this phenomena is that at 32-degrees, water molecules are packed as tight as they can be and any attempt to push them closer by lowering the temperature, only makes the water molecules repel each other stronger.  Water molecules at the freezing point form a crystal structure that is significantly much less dense that liquid water.  This means the density of ice is almost ten times lighter than liquid water.  This property of water is due to the hydrogen bonding in its structure.


Photo by Russ Considine.

When the surface temperature in a pond reaches 32-degrees, ice forms and floats on top of its surface.  This ice becomes an insulating layer on the surface by reducing the heat loss from the water below and thus, enabling life to continue in the lake or pond.  When ice absorbs enough heat for its temperature to increase above 32-degrees, the hydrogen bonds can be broken allowing the water molecules to slip closer together (melting).  If this phenomena did not take place when water turns into ice, then ice would sink to the bottom of lakes and ponds where the marine life would be killed from the ice since it would be formed from the bottom.  Also, the ice would not be able to thaw out, since the energy from the air and the sunlight would not be able to penetrate through thick dark layers of ice way below the depth of the pond.  So ice being less dense than water will form and float at the surface.

During cold periods, water in ponds produce layers of water called strata.  This means you will have different temperature levels of water as you move from the top to the bottom of a pond. Hence, bodies of water freeze from the top down where fish can be sustained under the ice.  The top will be much colder so fish, which are cold-blooded, will move to the bottom of ponds when the water gets too cold, and their metabolism slows down dramatically (lethargy), thus using up less energy, which they will need for their survival.  This is why I don’t fish from December through February, it’s nearly impossible to catch fish.

If a heavy amount of snow accumulates on the ice, the amount of light penetrating through the ice will be reduced.  This will result in less light reaching aquatic plants and/or algae below the ice which is needed to carry out photosynthesis thus, causing the plant to die and be broken down by bacteria.  These bacteria (called aerobic bacteria) will then use up the oxygen and cause a drop in dissolved oxygen in the water.  When this winter oxygen depletion occurs, fish that depends on oxygen may be stressed or die.  As fish die, their bodies decompose and use up even more oxygen, and the depletion of oxygen gets even worse. This is called ‘Winterkill.’

Unlike in freshwater, the presence of salt in the water allows it to get much colder than freshwater before it freezes.  Therefore, saltwater fish have developed antifreeze molecules (proteins) that bind to tiny ice crystals in their blood thus preventing the ice crystals from getting bigger and causing serious cell damage that can lead to death.

Here’s a valuable winter tip:  Do not walk on ice unless it’s 4-inches thick, anything less than that can be dangerous.