Aquatic Animal Health: Strategy for shrimp culture during price crisis
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Tóm tắt: Aquatic Animal Health: Strategy for
shrimp culture during price crisis
Aquatic Animal Health: Strategy for
shrimp culture during price crisis
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
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
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
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
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.
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