Enaca, 4/11/2004

Authorized by : SimonWilkinsonArticle ID : 26

Audience : DefaultVersion 1.00

Published Date: 4/11/2004 15:04:16Reads : 227

By Pornlerd Chanratchakool, Aquatic Animal Health Research Institute, Thailand. This article is also available in Thai.

The low production capacity in the past few years, the strict product standards from buyers and the production increase in other countries have caused a price crisis for the shrimp industry in Thailand. The main problems that are affecting the production capacity of P. monodon culture are summarized below, together with some suggested measures to mitigate against price pressures:

1. In the past 3-4 years, slow growth in shrimps, the cause or causes of which cannot be clearly identified, have occurred everywhere and in every culture system. However, there is a possibility that shrimp stunting may be caused by some of the following:

1.1 Hepatopancreatic Parvo-like Virus or HPV may cause this slow growth by damaging the cells of hepatopancreas whose main functions is the production of enzymes for digestion and food storage. HPV is always diagnosed in the hepatopancreas of stunted shrimp while MBV is found occasionally. However, in normal shrimp, HPV will not be available while MBV may be found. This points to HPV as a more probable cause of stunting in shrimp than MBV.

1.2 Apart from HPV and MBV, the hepatopancreas may be damaged by other causes such as from handling during the postlarval stage before stocking in growout pond, bacterial infection, and shortage of feed or presence of toxic substances in feed. Therefore, farmers should observe the hepatopancreas of postlarvae through a microscope. Stunted shrimp can be prevented if PL are not stocked whose hepatopancreas is 20% less than normal, or if 10% of the total number of the fry have no hepatopancreas.

1.3 Slow growth in shrimp may be caused by the following culture practices:

1.3.1. Stocking of postlarvae that are less than than1.0-1.2 cm. This often happens during the period of high demand for postlarvae when the climate is good for stocking (eg. summer). If there is a shortage of natural feed in growout ponds, postlarvae that are too small to feed on artificial pellets will starve.

v1.3.2. Overstocking may lead to overfeeding followed by accumulation of feed waste and deterioration of the feeding area on the pond bottom. Shrimp juveniles will become stunted and weak compared with those that are stocked in ponds with a clean bottom.

1.3.3. Juveniles will stunt if the natural food (plankton) in pond is not well prepared. This usually occurs 1-2 weeks after the application of chemicals for pond water treatment, which kills almost all living organisms in the pond, including plankton parental stock. Therefore it is difficult to boost phytoplankton after fertilization during water preparation.

1.3.4 Shrimp may stunt at the end of culture period. This probably relates to the stocking of post larvae that are too small, which usually leads to a longer culture period. In closed systems organic wastes that accumulate on the pond bottom will cause pond water deterioration followed by less feeding, weakness and some mortality. To avoid this problem, the current practices in pond preparation, selection of postlarvae, water and feed management should be modified to suit the pond and shrimp conditions.

2. In the past 5-6 years, survival of shrimp has become lower, particularly in low salinity or freshwater areas. The major problems include:

2.1 Poor survival at early stage during the first two-months may be caused by water quality. Salinity and alkalinity of pond water during stocking may be too low. Therefore, farmers in freshwater areas should maintain pond salinity and alkalinity during stocking at levels not lower than 5 ppt and 80 ppt, respectively. Later it can be diluted. If the water quality of the nursery enclosure is different from that of the growout pond, juveniles cannot acclimatize if the period given to them to adjust to the different water condition is too short. Farmers should keep salinity in the growout pond to not less than 2 ppt and allow 3-4 days for water adjustment before release into growout ponds. In low salinity areas, there are a lot of water bugs and insects particularly dragonflies that feed on shrimp fry. Juvenile insects and water bugs in ponds should be dragged out by mosquito net before stocking of shrimp seed.

2.2 Poor survival at the adult stage occurs in highly intensive ponds with closed systems or low water exchange systems, which causes the pond environment to rapidly deteriorate. In this case, shrimp growth and feeding requirements decrease after 60-70 days. Unhealthy shrimp with external fouling are usually observed at the shallow parts of the pond. These shrimp will gradually die and lead to poor production. It is difficult to solve this problem. Therefore farmers should have good planning and preparation for each season.

The above problems are commonly found in the Thai shrimp culture industry. However these do not include the market problems related to GSP in EU, anti-dumping charges, environmental issues and others. In order to reduce the above problems, the culture practices should be adjusted to match with the market requirements as well as to suit each season/environment. These would help avoid problems associated with shrimp health and farm management. The following are guidelines for adjustment of culture practices:

1. Selection of shrimp size and yield of production. The price for small sized shrimp (less than 50 pieces/kg or 20 gm each) is now very low and it is difficult to make any profit. Therefore the production of shrimp larger than 20 gm within 130 days should be the trend for the future. It is not possible to use the existing culture practice that stocks small postlarvae with a high density. For this new practice, farmers should have good planning that matches with the existing farming system by focusing on good preparation of pond and natural food; selection of healthy and larger fry (over 1.2 cm); lower stocking density (31-38 PL/m2); and suitable water management, aeration and pond bottom management systems.

2. Plan for stocking at the right season. This is to avoid disease outbreaks. From scientific and field information, it is clear that the outbreak of severe diseases in P. monodon relates to the susceptibility in shrimp fry and stocking seasons. It has been observed that white spot disease outbreaks occurs most severely during the period of weather change, eg. October to January. Therefore, farmers should plan to stock shrimp fry in February or March so as to harvest before June and then start the second crop from July to October before the cold season. However, it is still risky for disease outbreak if shrimp sizes of the second crop do not reach expected marketable size within 120-130 days. The culture period has to be extended until the cold season. There is also risk for the first crop in summer when temperature and evaporation rate are too high in March and April with high sunlight intensity. If there is no proper feed management and water exchange pond water may deteriorate triggering infection of yellowhead or bacterial diseases.

Therefore, it may be difficult to maintain two crops per year for production of larger shrimp in the farms that are not well equipped with aerators, water pumps and reservoirs. The solution for this is to shorten culture period in growout ponds by stocking PL in nursery pond for 30-45 days, in other words having larger PL.

Another alternative is to maintain only one crop a year by stocking at the best season in May-June when salinity and temperature become lower from rain. Higher stocking density (37-50 PL/m2) can be done in this case. After 110-120 days the pond can be partially harvested by 30-40%; the final harvest for larger shrimp would then be in October. This will help to avoid the outbreaks of yellowhead disease in early summer and whitespot in the cold season. Increasing shrimp size by lowering stocking density can be successful when postlarvae are healthy and large enough. Therefore selection of healthy shrimp fry should be a critical factor for farmers to increase production yield and profit margin.