Winterkill in Ponds

Winterkill refers to losing fish in winter because oxygen is lacking in the pond. Submerged vegetation and algae create oxygen through the process of photosynthesis. During the winter, oxygen production is often reduced because ice and snow on the lake limit sunlight reaching vegetation. In small, shallow lakes, the available oxygen can quickly be used up by live plants and fish that consume oxygen in the evening, and by bacteria that feed on dead and decaying vegetation. When the oxygen level declines, less tolerant fish species and fish in poor condition overall can begin to suffocate.

Oxygen enters ponds by moving from the air into the water by wind or photosynthesis by the pond plants. Ice cover blocks oxygen movement from the air into the water, and oxygen produced by algae and plants becomes the only source. Thin, clear ice allows oxygen production by plants and algae since enough sunlight can filter through the ice to allow photosynthesis. Suppose the ice remains into mid-winter or thickens slightly. In that case, oxygen levels will begin to decrease because pond plants and animals use oxygen each day, even in winter, as the animals breathe and dead plant material decays. 

When ice cover continues into late winter and thickens, or worse yet, is covered by snow, problems develop. Sunlight reaching the plants and algae is reduced, as is the amount of oxygen produced. Since metabolism and decomposition continue, oxygen levels begin to drop.

Other Factors

The more water volume there is in a pond, the less likely the pond is to experience winterkill. A 1-acre pond 6 feet deep is less likely to have winterkill than one 3 feet deep. The amount of decaying organic matter on the bottom of the pond is critical. A pond bottom covered with leaves and dead plants is more likely to experience winterkill than a pond without such materials. Ponds with a very dense aquatic plant community in summer are most susceptible to winterkill during harsh winters.

The pounds and number of fish in the pond during winter influence the oxygen decrease under the ice. Even though fish metabolism has slowed during winter, they still need oxygen. A pond that contains many fish will consume oxygen faster than a pond with fewer fish.

So, What Can a Pond Owner Do?

Consider installing an aeration system that helps keep a small pond area ice-free. Aeration adds oxygen to the water directly via the bubbles and agitation, and creates open areas that allow for oxygen diffusion into the pond from the air. The aeration system does not need to run continuously all winter, but instead, it should be turned on when ice forms on the pond. Leave it off when the pond is ice-free.

Another strategy is to keep about 30% of the ice free of extended snow cover to maintain enough light to allow sufficient photosynthesis. Be sure that the ice is safe for the proposed method of snow removal.