Are fish freezing to death in your pond?

By Amy Reeher

Ohio Certified Volunteer Naturalist

What happens to the fish in your pond during the winter? Are they freezing to death?

Just as we make adjustments in our everyday lives to accommodate the weather changes, so does the life in our local aquatic habitats.

Many species of fish aren’t likely to freeze, even in the worst Ohio winters since they can move to deep waters in well-built ponds when the surface of the pond freezes. These fish adjust their body temperature and change metabolic rates to match the colder environment.

Some fish, like yellow perch, are well-designed to handle seasonal change.

Some pond owners ask if their fish are still moving about the pond. The answer is likely yes, but they are moving much slower than their speed during warmer weather.

Fish will still look for food and shelter, but they tend to eat less because it takes them longer to digest food.

Other types of fish such as catfish, sunfish and bass hibernate. Many animals hibernate or move into a state like torpor to deal with winter.

Fish that hibernate are likely near the edges of the pond or stream and are buried in the sediment or an area thick with leaves. There they will stay until warmer weather arrives.

Many macroinvertebrates – organisms you can see with the naked eye and which lack a backbone – remain active in the pond due to special adaptations. E. crayfish, dragonflies and damselflies are examples.

Crayfish remain active but slow down and stay in their burrow during the worst winter weather.

Another aquatic creature that may remain burrowed in the mud is the turtle. Snapping turtles tend to settle beneath logs or large amounts of plant debris but have been seen moving about the aquatic habitat during the winter season.

One area of concern after winter is finding fish kills because winter tends to bring a lack of dissolved oxygen, which is critical to the health of the fish.

Most of the dissolved oxygen in the pond water is produced through photosynthesis by aquatic plants. That continues during winter as long as the ice is clear enough to let sunlight through.

If sunlight is blocked, photosynthesis is slowed or stopped. If sunlight is blocked for a significant amount of time, aquatic plants die.

Now, instead of producing oxygen, the dead plant material uses oxygen to decompose. The oxygen is gradually depleted, suffocating fish and other aquatic creatures.

The potential for a fish kill is more likely in shallow ponds with high nutrient levels where it takes only a few days for oxygen depletion to occur.

Aerating the water through winter can help prevent winter kill. Different types will match your aesthetic and oxygen needs.

Controlling excessive aquatic vegetation growth and removing heavy snow from the surface of the pond also can help. Try to remove the snow from at least one-third of the pond’s surface – but be careful on the ice.

If a pond seems to experience winter kill frequently, it might need to be drained and deepened.

If the problem is high levels of nutrients and the resulting excessive vegetation, try to find and eliminate the nutrient source.