Common problems in semi-intensive shrimp farming in Northern Vietnam
Pornlerd ChanratchakoolAquatic Animal Health Research Institute, Bangkok 10900 Thailand
Support to Brackish Water and Marine Aquaculture (SUMA) is a Danida supported part of the Fisheries Sector Programme in Vietnam, advocating the Ministry of Fisheries on marine and brackish water aquaculture development. The aim of the project is to strengthen the administration work and to strengthen the management practices as required by the farmer in order to produce marine aquatic products through environmentally and socially sustainable aquaculture development.
Part of the SUMA reinforce, is to look into possibilities for improving or changing the non-productive salt fields or extensive shrimp into improved extensive (3-5 PL/m2) or semi-intensive shrimp production (5-15 PL/m2). As part of the SUMA input, pilot projects have been set up in two provinces, Nghe an and Ha tinh for above shrimp production as a mean to try to alleviate poverty in the coastal areas.
Since the project started, there were many serious disease outbreaks in the study areas. The most common shrimp diseases are White Spot Disease, Bacterial Diseases and some other management related problems. Causes of the problem and some management recommendation have been developed during some visits as followed
Pattern of disease outbreakThe information gathered from farm visit and discussion with the farmers on disease problem in the area revealed that most of the pond stocked during February-March suffered from White Spot Disease. The outbreak occurred around 30-40 days after stocked. The pond stocked with the seed from the same seed seller at the same time was generally suffered from the disease nearly the same time. The extreme changed weather condition was proved to be the factor that triggers an outbreak. Moreover, the water depth in most of the pond was shallow which allowed the benthic algae growth at the pond bottom. This algae was gradually died within one month after the pond has been stocked and led to the pond bottom deterioration and poor water quality. If the pond has stocked with some infected seed, outbreak of the disease occurred soon after the pond and weather condition rapidly changed. The severity of outbreak depended on the prevalent of the infected seed at the time of stocking. Transmission of virus from the infected pond to the neighboring pond as well as via incoming water was unlikely.
For example, there was one infected pond in the Quynh Loc Commune, Nghe An Province. The shrimp were died since 35 days after stocked and there was some mortality continuing where as the surrounding ponds stocked 2 weeks after were not infected. These ponds were sharing the same water source. However, 2 weeks before the workshop farmers observed some of the white shrimp in most of these ponds were died with white spot on the shell. No black tiger shrimp was died. From the information given by the farmer, there was a cold weather at the end of April followed by hot weather, then the mortality was observed. It was assumed that those white shrimp were being infected from the water source but still survived within the pond. The outbreak was triggered by the bad weather condition. The infection was not via the water or contaminated from the neighboring pond as well as via incoming water was unlikely.
For example, there was one infected pond in the Quynh Loc Commune, Nghe An Province. The shrimp were died since 35 days after stocked and there was some mortality continuing where as the surrounding ponds stocked 2 weeks after were not infected. These ponds were sharing the same water source. However, 2 weeks before the workshop farmers observed some of the white shrimp in most of these ponds were died with white spot on the shell. No black tiger shrimp was died. From the information given by the farmers, there was a cold weather at the end of April followed by hot weather, then the mortality was observed. It was assumed that those white shrimp were being infected from the water source but still survived within the pond. The outbreak was triggered by the bad weather condition. The infection was not via the water or contaminated from the neighboring pond since the black tiger shrimp were not infected at the same time.
Since the disease outbreak is activated by the weather and pond condition as describe above. In order to prevent an outbreak, the farmers should manage the pond well. Following are some management recommendations.
Pond preparation. Before stocking the new cycle, the accumulated waste must be removed in order to reduce the organic matter in the pond. Care should be taken in sandy soil or uncompacted soil since the organic matter may not accumulated on the pond bottom surface but was deep into the soil. There fore, the soil must be flushing or washing thoroughly.
Water preparation. The water from the storage pond proves to be a good source for the culture. The farmers should prepared and stored the water in the storage pond for at least 7 days before preparation. The longer period up to 20 days is required in outbreak area. Water disinfectant was not recommended for the extensive or improved extensive culture since the effectiveness is greatly reduced if the mixing of the chemical in the water was not good, and it was not cost effective. Water must be filter by 2-3 layers of the net (89 holes/cm2) to prevent predator and others crustaceans. The water should be fertilized with organic fertilizer such as chicken or cow manure (50-100 Kg/Ha). The water color should be light green with transparency of 30-40 cm before stocking. The water depth should be maintained at 1.0 meter. If the water has been treated, it should not be used before 4 days after treatment.
Post larvae selection. There are some criteria for larva selection as followed. The farmer should selected the larval at the hatcheries by they own. At the time of visited, it should not see many dead shrimp within the tank. Many dead shrimp in the tank indicated the continuous mortality. The larva should swimming actively against the current, without abnormal swimming or floating in the water. The larva should have uniform color and size. No external fouling and show negative PCR result for WSSV test.
Stocking.Stocked time should be decided base on the weather condition. The pond should be stocked in the period with stable weather condition. Removed the weak and dead larva from the bag before stocking can reduced the number of the infected larva. Therefore, reducing the severity of an outbreak.Water quality monitoring
There are some water quality parameters that should be monitored. Water color, pH and weather condition should be monitored and recorded everyday (3-4 PM). This will help the farmers to make decision on farm management. The water color should be maintain in green color and maintain the pH between 7.5-8.5. Change the water when the pH is out of range or the color change to dark green or brown or clear. Use deep water to prevent benthic algae growth in clear watercolor.Pond soil monitoring.
The mud from the pond bottom should be checked once a week after stocked. Do water exchange when the mud color turn to black or bad smell.
Action plan for outbreak. In case there is mortality in the pond the following action can be done.
1. Count the number of dead shrimp everyday to look at the increasing mortality rate.2. If the number of dead shrimp rapidly increase exponentially, consider as an outbreak i.e.5 at day 1, 20, at day 2 and many more in day 3. The farmers should decide to harvest or break the culture cycle.3. During harvest the water must be retained in settling pond 7 days before release. 4. Incase the farmer stop culture, the water must be kept within the pond and continue to record the mortality.5. If the mortality stop within one week and there are many shrimp left, continue feeding them.6. If the mortality is continuing, wait until it stopped, waiting more than 14 days before release the water.
There were some other management-related problem identified and recommendation was made as followed,
Most of the farms visited only one pond was faced the problem with slow growth. This caused by the acid soil in the pond, no natural food available. It was recommended that the pond should be limed and fertilized. The fresh fish can be used to mix with the feed in order to improve appetite.
Tail rot and missing antenna.These abnormality were found in some ponds. This was also due to deteriorated pond bottom. The pond soil condition should be checked and pellets should be placed in clean area only. Feeding the shrimp only in deeper part of the pond when the temperature is high in the afternoon or evening.
Most of the farmers in both areas were feeding the shrimp 3-4 times/day. The feeding schedule was 6.00, 18.00 and 24.00 for 3 times and 6.00, 12.00, 18.00 and 24.00 fir 4 times. It was found that the farmer usually feed more at night. Since the temperature is change very rapidly, therefore the water temperature should be checked and the feeding rate is adjusted accordingly. The amount o feed should be reduced 20-30% when the temperature below 24 c and higher than 33 c. In sunny day, the feed in the evening can be reduced or trying to feed in deeper area only. The amount of feed increase should not more than 1 kg/day, for 100,000 shrimp. It was also recommended that the survival rate should be estimated and the feed adjust accordingly. The farmer should estimate the survival could be checked again at day 5 or 7 after stocked. This will give a rough idea on the survival and can be used for feeding adjustment. Around one month after stocked, the shrimp should be sampling and average body weight should be checked as the initial size for comparison. Feeding rate can then be adjusted again using the size and estimate survival rate. Sampling the shrimp again within 7-10 day for the size checked. Then feeding could be adjusted according to the growth of the shrimp again comparing with the last sampling. Routinely check and adjust the feed until harvest. In general, the maximum feed increment for the early stage of culture (40-70 days post stocked) is 500 gram for 100,000 survived shrimp per day could be used after 70 days until harvest.
Using fresh feed such as trash fish or mollusk was recommended as a substitute feed, at the ratio of 2 or 3 kg fresh feed to 1 kg pellet for 1-2 time per day. More water should be changed if the fresh feed has been used.
Most of the ponds were shallow with the water depth ranged from 60-70 cm. If possible the water depth should maintained at least 80 cm. or provided some deeper area in the pond.
In case the water parameter was not in the optimum range, pond water should be drained and filled in with stored water from the reservoir, which must be stored at least 5 days before being used. When the water pH became high as well as high plankton density, the water should be changed and lime could be used at the rate of 30 kb/ha at night. No chemical is adviced for decrease the water pH since the chemical may cause the dead of phytoplankton.