Late Ice Contributes to Natural Phenomenon
"As winter ice has started to melt away, we are receiving calls from concerned pond owners reporting large numbers of dead fish in their ponds," said DEC Regional Fisheries Manager Michael Clancy. "In most cases, fish kills that become evident when the ice melts can be attributed to a natural phenomenon known as winterkill."
Winter die-off of fish, or "winterkill," is caused by oxygen depletion in the water through the winter. Ice that accumulates on ponds or lakes prevents wind action from adding oxygen to the water. If there is significant winter snow accumulation on top of the ice, sunlight is prevented from reaching plant life in the pond. Without adequate sunlight, the oxygen-creating process of photosynthesis cannot occur.
Shallower ponds are particularly susceptible to winterkill, due to their low storage capacity for oxygen. With the exception of extreme situations, it is rare that all fish in a pond will die as a result of winterkill. Typically, larger fish are more susceptible than smaller fish.
"Sensitivity to low oxygen levels varies by fish species," Clancy said. "For instance, catfish and carp are more tolerant of low oxygen levels than species such as sunfish, bass or trout."
If desirable fish species are completely eliminated from the pond due to winterkill, replenishment by stocking may be necessary. Please remember to obtain applicable stocking permits from DEC.
A long-term approach to avoiding winterkills is to deepen the pond. Pond depths exceeding 12 feet are recommended since ponds will gradually fill in over time. Removing some of the organic substrate (decaying plant material) that accumulates within the pond can also help combat oxygen depletion. As a pond ages, organic materials are deposited on the pond bottom; these decaying materials have a high metabolic demand for oxygen.