Winter Fish Kills To Be Expected

As the ice melts across the state, the New York State Department of Environmental Conservation (DEC) expects to get increasing reports of fish kills in small ponds. Reports of fish mortalities have already been received from some small waters in the southeastern portions of the state.


Whenever large numbers of dead fish are observed, there is concern that a pesticide spill or disease caused the mortality. However, in most cases fish kills that become obvious when the ice melts can be attributed to "Winterkill," a natural phenomenon that occurs when waters rich in nutrients, algae, and other aquatic plants are covered with ice and snow for long periods of time.

Winterkills occur when ice and snow prevent sunlight from entering the pond and prevent aquatic plants from producing oxygen, necessary to maintain life in the pond. The ice cover also prevents oxygen from mixing into the pond's waters from the atmosphere. Instead, the decomposition of organic matter and respiration of aquatic organisms in the pond cause a steady decline in oxygen. If the snow and ice cover persists long enough, as was the case in some state waters this year, fish mortalities can occur. Once the ice melts, hundreds of dead fish can be found floating at the pond surface. Winterkills are most common in small, shallow, nutrient-rich ponds with plentiful decaying aquatic vegetation. Winterkills are rare in waters over 20 acres in size and do not occur in larger lakes which have sufficient volumes of oxygen rich water to maintain aquatic life through even the worst of winters.

Winterkills are rarely complete as different fish species and sizes of fish have varying tolerances to low oxygen levels. Some fish also find isolated locations of sufficient oxygen in ponds to hold them through low oxygen periods. Fish populations in these waters often rebound a few years after the fish kill occurred.


Anyone noting a fish kill involving a substantial number of fish that they believe cannot be attributed to Winterkill should contact their local DEC regional office.

For more news on the environment, including other fish kills, click here.

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