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Ken Semmens As you walk around your pond one summer day, you see a dead fish, then another, and then many more. Dead fish ring the pond or have accumulated in a raft of debris on the downwind side. You wonder what happened and what you should do. Your pond has suffered summerkill. Summerkill is a type of oxygen depletion. In short, lack of oxygen causes the fish to suffocate. Instead of an oxygen concentration between 4 and 10 parts per million (ppm), the amount of oxygen dissolved in the water is less than 2 ppm and perhaps less than 1 ppm. The oxygen depletion need only occur for a short time to kill the fish. It may occur at night. Chances are that the oxygen concentration will have already increased to normal levels by the time you discover the problem.
If oxygen does not immediately become available, the fish will die. Large fish tend to die before small fish. Small fish, which require less oxygen, frequently find some relief in shallow areas where large fish cannot go. Many dead fish will sink and float to the surface several hours later. The day after a fish kill, the amount of fish lost appears greatest as the dead fish bloat and float. This is a good time to determine the number, the kind, and the size of fish lost. Eventually, the dead fish will "melt" away and disappear. The potential for summerkill depends on the presence of a layer of water on the bottom of the pond that has no oxygen (see the figure). This bottom layer is cooler and may have a dark color or an unpleasant smell. If the water on the bottom is "bad," fish will avoid this layer even if they prefer the cooler temperatures (trout, for example). Bacterial decomposition of plant or animal matter causes this "bad" water. This condition occurs more frequently in deep ponds where excess water is discharged from the surface layer. The bottom layer slowly accumulates while any overflow comes from the top layer. A weather event usually triggers summerkill. The event, which may be a strong wind, a hard rain, or a quick flush of water, causes the top and bottom layers of the pond to mix. Mixing occurs more easily in the spring or fall when the temperature of the top and bottom layers are similar. After the mixing, the pond color may change from green to brown. The oxygen depletion may happen a day or two following the weather event. If you discover an oxygen depletion as it is happening, the obvious course of action is to add oxygen to the water. Increasing the flow of oxygenated water to the pond is one strategy. Another is to aerate the water in the pond. Aeration may consist of stirring, spraying, or using another way of exposing water to the atmosphere. Fish farmers have discovered that more oxygen is transferred when large volumes of water are exposed to the air rather than bubbling air into the water. Once the oxygen levels increase, the behavior of the fish will return to normal. If the sun is shining on plants in the water, photosynthesis may save the day. You may recall that oxygen is a by-product of photosynthesis. These plants may be microscopic algae (phytoplankton) that make the water appear green. Rooted plants or filamentous algae growing below the water surface will add oxygen also. Plants floating on the surface (for example, water lilies) and other plants whose leaves are exposed to the atmosphere will not contribute significantly to the oxygen supply below the water surface. It is possible for the oxygen depletion to return each night for several nights until the pond becomes stable again. For this reason, it may be prudent to aerate each night for several nights, especially if plants in the pond are not producing much oxygen. In the long run, the best strategy is to minimize the accumulation of "bad" water on the bottom of the pond. This can be done by drawing the overflow from the bottom rather than from the surface. For ponds with an overflow pipe in the deep part of the pond, this is a simple matter. Placing an oversize pipe over the overflow pipe so one end is slightly above the water surface and the other end is near the bottom will force overflow from the bottom of the pond. It usually is unnecessary to restock the pond following an oxygen depletion. Although many of the larger fish may have been killed, their seed remain in the pond and will grow to replace them. Restocking grass carp or hybrid striped bass would make sense since they do not spawn effectively in ponds and would not be replaced by recruitment. Summerkill is one type of oxygen depletion. Other types include a "bloom crash" and winterkill. These topics and many others are discussed in the Ohio Pond Management Handbook published by the Ohio Department of Natural Resources (ODNR Public Information Center, 1952 Belcher Drive, Columbus, OH 43224). The figure accompanying this article appears on page 37 of that handbook. |
If
you discover the oxygen depletion as it happens, you would see fish swimming head up at
the water surface, opening and closing their mouths. Called "piping," this is a
way for fish to obtain oxygen as it dissolves from the atmosphere into the water surface
film. Fish also will seek out oxygen in water sources like streams or, possibly, springs.
The more severe the oxygen depletion, the more desperate the fish’s response. Muscles
used for swimming require oxygen to work, so many of the piping fish eventually will swim
to the edge of the pond and rest on the bottom. They continue to breathe heavily in an
effort to obtain as much oxygen as possible.