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second edition • Nebraska Pond Management • 69 enough sunlight penetration for plants. But, ice blanketed with snow allows very little sunlight penetration, so plants are unable to produce a sufficient amount of oxygen. Oxygen levels drop due to plant decomposition and respiration by the various aquatic organisms. If snow cover persists for an extended period of time, the oxygen will be completely depleted, resulting in a fish kill. This usually happens in shallow ponds which have large amounts of organic matter, such as decaying aquatic vegetation or livestock wastes. Winterkills can be severe enough to kill all the fish and other aquatic life, including frogs and turtles, in a pond. Often, bullheads and carp are the only fish to survive winterkills. In these cases, the pond should be renovated with rotenone and restocked with appropriate pond species, provided adequate depth is present. Seasonal Turnover Kill Some ponds that contain relatively large amounts of deep water that annually stratifies can experience a fish kill when the pond turns over, producing conditions similar to an inversion. These ponds thoroughly mix in both spring and fall when surface and deep water temperatures are the same. Wind can facilitate the mixing. Preventing Fish Kills due to Suffocation Most of these fish kills can be avoided if a pond contains sufficient depths to prevent excessive growth of aquatic vegetation and to store enough oxygen during critical time periods, especially winter. Proper pond construction and management can prevent these fish kills. These types of fish kills can also be prevented by installing an aeration or water circulating system that will oxygenate the water year- round. Do not allow the aerator diffuser (air stone) to rest on the pond bottom as this will stir up organic materials, accelerating their decay and increasing oxygen consumption. Algae blooms can result if large amounts of bottom nutrients are carried to the surface as the bubbles rise. Either place the diffuser on a pedestal or in a weighted 5-gallon bucket, or suspend it at least two feet off the bottom. Both systems will maintain an open water area during the winter and facilitate oxygen absorption directly from the atmosphere. Cutting a single large hole in the ice during the winter isn't very effective because not enough water gets exposed to the air. See Appendix D for additional information on aeration. Winterkills can also be prevented by removing snow from the pond. Three inches of ice, covered by five inches of snow, will block 99% of the incoming sunlight. Remove snow from 30 to 50% of the pond surface or just in shoreline areas where submergent vegetation would be located beneath the ice. Fish Kills due to Poisoning Fish kills can be caused by the improper use or spills of many chemicals, including insecticides, herbicides, fertilizers, and petroleum products. Aeration System Aeration System The underwater aeration grid needs to be on a platform to avoid causing turbid or enriched water. Sunlight Helps Plants Oxygenate Water Winterkill Prevention Winterkill Prevention Snow Ice CAUTION: When using snow removal equipment, make sure the ice is at least 8-10" thick.