Problems in shrimp culture during the wet season
Recently, many farmers have faced the problem of fluctuations in salinity and temperature causing shrimp diseases; particularly yellowhead, white spot and luminescent bacteria that generally cause farmers to lose their crops.
They have also had problems with stunted growth or black gill. Many of these problems can be overcome. Most farmers generally prefer to stock shrimp in the wet season, as they believe there are fewer problems than in the dry or cold seasons. However, culture in the wet season also brings various problems that I would like to discuss to help farmers plan their future crops and take adequate precautions.
1. Problems in pond preparation: In acid sulfate soil areas, heavy rain will cause acid sulfate to leach from the pond bottom and dike if the pond has been dried too long in dry season.
Therefore the pond should be limed or heavily flushed at least once until the water pH is higher than 7. Fertilizer or lime should then be further applied.
2. Problem in salinity: Due to the different salinities among shrimp farming areas, farmers should report the actual pond salinity to the hatchery or nursery so that they can adjust, in advance, the salinity of postlarvae close to the farmers’ requirement. For safety and high survival, postlarvae should be also nursed in small enclosure with 4-5 ppt salinity if pond salinity is extremely low.
3. Problem with predators: If the pond and water has been prepared too early for stocking, predators such as metapenaeus shrimp, dwarf prawn or finfish may be observed as they grow faster in the rainy season. If these are present they should be eliminated or the pond should be re-prepared as such animals may cause poor shrimp survival or introduce diseases including viruses.
4. Problems with rain during stocking: It is commonly raining in the afternoon or evening during the wet season and this can flush acid sulfate from the dikes into the pond. This acid water will cause high mortality to newly stocked postlarvae, which are generally weak after transportation and acclimatization. Therefore stocking of postlarvae in the morning can avoid the problem with rain. Regular liming with CaCO3 on pond dikes will minimize this problem.
5. Problems with shrimp floating after raining: After heavy rain, shrimp are observed on the surface of ponds, particularly in acid sulfate soil areas or in old or deep ponds that have poor water circulation. Flushing of acid sulfate from dike into pond can cause low water pH, which subsequently increases toxicity of hydrogen sulfide gas accumulated at the pond bottom. This causes shrimp weak and float to the surface. To solve this problem, bottom water should be drained and lime solution should be spread all over the pond in order to increase water pH over 7.5. Feeding amounts should then be reduced until the shrimp are observed in feeding trays as normal.
6. Problems with clear water after raining: This problem generally exists in acid sulfate soil or sandy soil areas. It is mainly caused by the rapid change of alkalinity and carbon dioxide level in pond water after heavy rain, which suddenly reduces the phytoplankton population. To solve this problem, pond watershould be renewed or green water containing dense phytoplankton from nearby pond or drainage canals should be added. CaCO3 lime should be subsequently applied daily or every two days at the rate of 125-187 kg/ha together with fertilization. In general, application of CaCO3 lime or dolomite at the rate of 125-187 kg/ha every two days during the first 50 days after stocking can improve water color. If water is still clear and lab-lab (algal mats) are developing, artificial color may be applied in order to reduce light intensity.
7. Problems on floating after water exchange: This may happen in the farms close to canals or river mouth where early rain may flush acid sulfate developed in dry season from upstream. Therefore it is very risky to conduct heavy water exchange during early raining period. The best solution is to stop water exchange during the first 1-2 days of spring tide. Water from outside can then be added to acclimatize shrimp in the pond before draining water the next day. In order to check water quality before pumping into growout ponds, 5-10 shrimp from growout ponds stocked in net cages at the inlet canal should be a good living indicator.
8. More suspended solid after raining: In sandy or sandy soil area, there will always be more hanging colloidal particles in the pond after heavy rain. In order to remove these suspended solid particles, water should be heavily drained and followed by application of lime at the rate of 62-125 kg/ha/day without aeration (airjet type) at daytime. If these particles still remain within 2-3 days, flocculant should be applied before water exchange. During this treatment, feeding should be reduced approximately by 20-50% because flocculant may affect feeding of shrimp.
9. Problem with softshell and abnormal walking legs: In acid sulfate soil and low alkalinity (less than 50 ppm) areas, particularly using water from canals, shrimp may have softshell, be unable to moult and have abnormal pereopods. Shrimp are not able to feed due to the unbalance of minerals. Application of CaCO3 lime or dolomite at the rate of 125-187 kg/ha every 1-2 days during the first 50 days of stocking is recommended.
The above is only a part of general problems and solutions for each problem will be different in each location. Therefore, farmers should regularly monitor shrimp health and water quality and immediately solve the problems. Proper position and management of aerators for cleaning feeding areas of pond bottom could also reduce shrimp mortality.
The key principle for solving the above problems is efficient water management by having a reservoir (25% of farm area, with 3 m. depth) attached to growout pond. If necessary, chemicals or disinfectants could be applied in this reservoir before introducing to growout pond. Proper stocking density (less than 50 PL/m2) can reduce organic loads in the pond and improve water and sludge treatments.
The above prevention and treatment are simple management, which are the basic and key instructions for shrimp culture. If farmers could keep pond bottom clean, water colour constant and water exchange with care, these problems will be minimized. Finally, the author hopes that this article will assist farmers in understanding the background of the problems, in precaution and treatment at a certain steps and also wishes them successful crops in the next raining season.
Source: Dr. Pornlerd Chanratchakool, Aquatic Animal Health Research Institute, Department of Fisheries, Thailand – Photo: fishconsult.org.