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11.1 Indicators of problems in a pond

11.1.1 The presence of dead shrimp or fish.

11.1.2 A die-off of algal growth. This can sometimes cause milky coloured water. The products of decomposition can be harmful.

11.1.3 An overgrowth (bloom) of phytoplankton can cause oxygen depletion. This can be especially harmful on cloudy days that follow several days of bright sunlight. Caution should be exercised if one cannot see a white coloured object 25 cm deep in the pond. Check in the early morning to see if shrimp or fish are swimming erratically at the surface, frequently breaking the water surface.

11.1.4 Active swimming of shrimp at the water surface during day-light hours. This indicates the shrimp are in stress. The usual cause is low oxygen and/or high temperature.

11.1.5 Active swimming of shrimp around the edge of the pond during daylight hours, but not at the water surface. This can indicate a lack of food in the pond.

11.1.6 An abrupt change of water colour. If the water becomes clear, it means the phytoplankton died and a shortage of natural food will develop. If the water turns a reddish or bright-green colour, types of algae might be present which give off toxins which can kill shrimp. A milky colour can indicate a die-off of algae as noted above.

11.1.7 Bad smell. A smell of sulfide or rotten eggs is caused by hydrogen sulfide. This is produced by decomposition and it can indicate that an accumulation of organic matter has occurred on the pond bottom. The most frequent cause of this is lab-lab which floated to the surface and was carried to a corner by wind. During night hours it sinks to the bottom where it decomposes. If the smell comes up from the bottom mud while someone is wading in the pond, the bottom is bad.

11.1.8 Gobies swimming in stress and/or concentrated on the sides of the dikes can indicate low dissolved oxygen in the pond water.

11.1.9 Snails climbing out of the water can also be an indication of low oxygen in the water.

11.1.10 A heavy concentration of the rotifer Brachionus or other form of zooplankton in the pond water can indicate either a build up of organic matter in the ponds as a result of decomposition of other food organisms or a heavy growth of bacteria.

11.1.11 Shrimp with black gills. This condition can be caused by disease or by the shrimp burying in mud made black by decomposition. Place the shrimp in clean water in an aquarium. If the black colour goes away after one or two days, it is accumulated debris, if the colour remains, it is disease.

11.1.12 Shrimp with white discolouration on their tails. This can be caused by disease or by the stress of low dissolved oxygen and high temperature. In the last case, the shrimp are usually swimming actively and show signs of stress, some may even be jumping out of the water. Some of these shrimp will lose the white spot if placed in well-aerated water for a day.

11.1.13 Shrimp with papery shells and body that pushes in easily. This is usually caused by lack of food.

11.1.14 Abrupt lowering of salinity in a pond, especially when caused by heavy rain. The freshwater floats on top of the saltwater. This forms a barrier and the bottom water often can become deficient in oxygen.

11.1.15 Temperature above 32°C. A higher temperature of pond water is dangerous and can lead to increased mortality.

11.1.16 Low pH. In brackishwater, a pH of 8 to 8.2 is normal. A pH lower than 7 is a cause for concern in that it indicates some abnormal condition in a pond. A high pH is usually associated with a good growth of phytoplankton. It is not a cause for concern unless it rises above 9.5.

11.1.17 Low levels of dissolved oxygen in pond water by measurement usually during early morning hours before sunrise.

11.1.18 Bottom mud containing a large number of chironomid worms and nothing else. Chironomid worms are an indicator of pollution. They can live when dissolved oxygen levels are very low, and when everything else dies their numbers increase. They are very small red worms.

11.1.19 Numerous shrimp with black spots that look like an old injury. This is caused by a bacterial disease, and is usually associated with water that has a high organic content.

11.1.20 Shrimp with fuzzy growth on outside shell. This can be caused by bacteria, protozoans, or algae. The first two are associated with water which has a high organic content. In any case, they are an indication that growth is slow and the shrimp are not molting.

11.1.21 A foam is formed on pond surface by waves during high winds. This happens to water with a high amount of dissolved organic matter.

11.2 Remedial action

11.2.1 Water exchange. Changing water is a general preventative and/or remedy for most of the conditions listed above: it introduces new oxygen; dilutes waste products or phytoplankton that may have built up too high; introduces new food organisms, trace minerals and organics; dilutes disease causing organisms. It is important that water be exchanged as soon as possible in cases where low dissolved oxygen is the problem. Oxygen depletion usually occurs near the bottom and it is best to drain and replenish water from the bottom most of the time. If a pump is available water should be flowed through the pond. In the case of low salinity of the surface layer, caused by heavy rains, water should be drained from the top and replenished from the bottom. Sometimes it may be necessary to exchange water for several days in a row before pond conditions improve.

11.2.2 Mechanical mixing. Water can be mixed to supply oxygen or to break up a layer of freshwater. Mechanical agitators are sold for this. In an emergency an outboard engine can be used. Windmills have proved practical.

11.2.3 Addition of chemicals. Addition of chemicals like potassium permanganate could be a useful remedy for low dissolved oxygen levels. This would be most practical in small ponds where shrimp are grown at high density. Hydrated or quick lime applied at rates from 200 kg/ha has been used to relieve milkfish from stress caused by low dissolved oxygen (Padlan, personal communication). The same treatment might prove useful for shrimp. Quick lime is a special activated type of lime and should not be confused with agricultural lime, It is caustic to handle and bulky. Workers are cautioned against becoming burned if it should get in their eyes or get wet on their skin (Anonymous, 1976b).

11.2.4 Raise water level. In ponds with high water temperature, the water level should be raised. In some cases, it may be necessary to provide shade. Rafts of bamboo supporting banana leaves would be inexpensive.

11.2.5 Stop feeding or fertilization. Any time shrimp appear to be in stress, or pond conditions are poor, supplemental feeding or fertilization should be postponed until the situation is corrected.

11.2.6 Remove dead fish or algae. This should be standard procedure. Any time dead things are observed in a pond they should be removed. If “lab-lab” piles up in a corner, it can be removed with a rake or scoop.

11.2.7 Add feed. If the shrimp give signs of being undernourished or hungry, it might be useful to supply extra food until a new growth of natural food can be produced.

11.2.8 Transfer shrimp. In some cases where shrimp in a pond have stopped growing, growth resumed when they were transferred to another pond with a good crop of natural food.

11.2.9 Harvest shrimp. Total harvesting is advised only as a last resort when a large percentage of shrimp in a pond are diseased or they are dying from bad pond conditions and there is no way to remedy the situation. It is better to receive a low price for undersize shrimp than it is to wait too long and have most of the shrimp die. Partial harvesting can be utilized when there is evidence of slow growth caused by lack of food. Selective harvesting can be used to reduce the number of shrimp in the pond by cropping large individuals and leaving the smaller ones to grow larger.

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