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second edition • Nebraska Pond Management • 67 vegetation. Pond drawdown, especially during the winter, can dry out plants and expose them to harsh conditions, including freezing temperatures, wind, and sediment compaction. For winter control to be effective, the bottom mud should freeze to a minimum depth of four inches for at least a month. Dead plant material should be removed from the pond basin as soon as possible, definitely before refilling, so their nutrients are not released back into the water. Make sure the pond still has sufficient depth (10 to 12 feet) during the winter months to prevent a fish kill. Cattails can be effectively controlled by lowering the water level, cutting or shredding the plants, and then flooding them during the winter. They will drown, provided the roots are kept completely submerged for an extended period of time, especially during the following growing season, and there is no attached top growth remaining to funnel air down to the roots. Dredging and Deepening Many vegetation problems can be eliminated by deepening shoreline areas. Creating a 3:1 slope to a depth of 4 to 5 feet, and then tapering off rapidly into greater depths will limit where vegetation can grow. At least 25% of the pond's surface area should contain a depth of at least 10 feet, 12 feet in northern and western parts of the state. Dredging, or partially draining the pond and excavating, reduces vegetation problems directly by removing the plants and bottom sediments, and their associated nutrients. Shading Limiting the amount of sunlight available to aquatic plants with dyes that stain the water is another vegetation control method sometimes used by pond owners. Too much chemical shading, however, can reduce the pond's overall productivity, including fish production. To be effective, the dye must persist in the water for several weeks. Ponds with constant flow may not be good candidates for this technique. For best results, dye should be used early in the spring before the vegetation starts growing. If the plants have already grown to the surface, it's too late to add dye. Floating objects such as piers or swimming platforms may shade a small area of the pond and prevent vegetation growth. Or, pond owners can anchor black plastic sheeting on styrofoam floats around swimming areas or boat launches to shade specific areas. Fish Kills Fish populations in a pond are constantly experiencing mortality. Some can have an annual mortality rate as high as 50%. Most of these deaths can be attributed to natural causes, with predation being the most common. It is also common to see some dead fish along shoreline areas. These fish likely died as a result of old age, minor disease outbreaks, handling, or spawning stress. Contrary to popular belief, fish kills are rarely caused by an overpopulation of fish. A pond will naturally stay within its capacity to support fish under normal conditions. Common causes of fish kills are suffocation due to lack of oxygen, poisoning, and disease or parasite infestations. Knowing about these causes can help pond owners prevent fish kills. Fish Kills due to Suffocation Most of the dissolved oxygen in a pond is a product of plant photosynthesis. Oxygen can also enter the pond by absorption through the water surface, especially when there is wind and wave action. Dissolved oxygen levels can vary significantly throughout the year or even during a day. Critically low dissolved oxygen levels can result from certain combinations of environmental conditions and pond characteristics. Low dissolved oxygen is the most common cause of fish kills in ponds, often occurring in summer, winter, or as a result of seasonal water column turnover. Once levels When large numbers and a variety of sizes of different species are found dead, a major fish kill has occurred.