by
Somsuk Singholka 2
National Freshwater prawn Research and Training Centre
Freshwater Fisheries Division, Department of Fisheries
Ministry of Agriculture and Cooperative
Bangpakong, Chachoengsao
Thailand
1. INTRODUCTION
About 20 years ago, prawn from the Gentral Region of Thailand were adequate for the local demand, At the present time, because of increase in human popuition, construction of roads and dams, the migration routes of the species were blocked. This reduced the feeding ground and spawning areas for the prawn. Especially in the brackishwater zones, many factories were built which polluted the spawning grounds, the demand for Macrobrachium in the market continues to be high, resulting in high prices of approximately 120 – 150 Baht 3 per/kg.
The Thai Government realized this situation and put up a budget for the establishment of two stations for Macrobrachium. One is at Bangpakong in Chachoengsao Province, and the second one in Songkhla Province. Besides this government activity, private farmers who have taken training in the Songkhla station built their own hatcheries. Most of these are located in greater Bangkok.
2. CONSIDERATIONS IN SITE SELECTION
2.1 Water supply
Fresh water supply is obtained from tap water source while one of ten hatcheries is known to obtain its water from canals or klong water. The sea water required was transported from the sea by using plastic containers.
2 Author is Centre Director and Co - Manager, FAO/UNDP/THA/75/008
2.2 Availability of stock
The majority of the hatcheries get gravid females from their own ponds. Some of the newly-constructed hatcheries obtain spawners from natural waters. The gravid females are obtainable all year round but with a peak during November, December and January.
2.3 Availability of technology
The farmers that train in the Songkhla station can successfully build and manage their own small - scale hatchery after their raining This shows that the techniques learned are effective. There is available technology offered by the Government which the prawn farmer can use in his project.
3. DESIGN OF THE FAMILY - TYPE HATCHERY
The design of the hatchery is simple and depends the decision of the individual farmer. This design may not be based on engineering principles, but usually the farmer considers convenience in the future management of his project.
3.1 Facilities
A typical small - scale prawn hatchery may consist of hatching tanks (either round or rectangular), sea water containers, plastic reservoirs, water pump (usually the smallest otbainable - submersible, 1/8 HP, with 1 - inch pipe) and transparent plastic pipes.
3.1.1. The brackiswater reservoir
A farmer mixes fresh and sea water in these reservoir to a salnity of about 12 ppt and allow this mixture to stand for 1–2 days for through and setting of suspended particles.
3.1.2 Plastic pipes
These pipes are made of plastic, flexible and transparent for use in siphoning any detus or extraneous particles (1 left over food) that have settled at the bottom of the hatching tanks.
3.1.3 Fine-meshed dip net
This dip net has a round rim handle usually with 20–25 cm in diameter. This handle is fitted with fine-meshed nylon net to form adip net which is used to collect larvae which may be carried by the water during the spihoning process to clean the tank.
3.1.4 Air pump
Small vibrator pumps wuch as those unsed for aerating glass aquaria for ornamental fish are used to aerate the hatching tank. These aerators are electrically-run and are also used to aerate continuously the fresh and sea water reservoirs as the brackishwater supply resulting from the mixture of sea water and fresh water.
32 Layout
There is no fixed pattern of layout fro those small-scale prawn hatcheries. The arrangement is fully based on the decision of the individual farmer.
3.3 Construction
For the round tanks, ready-made circular concrete cylindrical pipes (1.5 m diameter × 0.5m high) of low quality, used in constructing septic tanks in rural areas are usually purchased for this purpose. So that these can be used, one end is sealed by concrete on a hard level surface. Normally the hatchery space is first floored with concrete.
The rectangular tanks are made of hollow blocks and have dimensions of about 2 m × 0.5 m. The inner side of the tank is adequately cemented to pervent leaks and seepage. No special treatment or waterproofing these tanks is dond because of the expense involved.
The reservoirs for sea water, fresh water and brackishwater (mixture) are likewise made of hollow blocks. The size of these reservoirs varies from 3–5 m3, with dimensions usually 2–3 × 1.5 m × 1 m. Newly-built reservoirs or tanks are filled with water and allowed to stand for one month. These tanks are usually washed three or four times this period.
A hatchery set-up is usually placed under a specially built roof made of corrugated zinc or asbestos sheets.
4. MANAGEMENT
4.1 Techniques
The prawn farmers obtain the berried females and place them in the hatchery tanks. Spawners with eggs that have turned grey from orange colour are selected as these are in pre-hatching stage. The depth of water in the hatching stage is about 30 cm. After hatching, the spawners are removed from the tands. After two days feeding of the larvae commences using Moina sp. or pulverized hard boiled egg yolk. The larvae are fed about 4–5 times a day. No definite amount is used but the farmer must see to it that the food is adequate and to amount is wasted to settle in the bottom.
After five days, cleaning the bottom by the siphon is done every day until postlarvae are removed from the tank. This takes about 30–40 days. Each day the amount of water siphoned and evaporated is replaced with new brackishwater. The farmer will wait until majority of the postlarvae become juveniles and the farmer replaces the brackish water with fresh water within a period of two days. These juveniles produced are ready for the maket. Prevailing prices at present are about 0.5 Baht per juvenile.
5. ECONOMICS OF OPERATION
This is based on data from three small-scale hatcheries. These hatcheries use from 6–19 hatching tanks with capacity of 0.5 m3. The number of juvenile production per month from each hatchery is about 25,000. This means an income of 12,000 Baht and with about 5,000 Baht for operational expensesper hatchery (80% cost of obtaining sea water and 20% for food and electricity. The labour is done by the famer's family). The income per month is about 7,500 Baht (U.S 375) if all juveniles can be sold. Usually about one-half is sold and if the farmer has his own growout ponds, he can use the unsold stock for these ponds. However, some famers do not have ponds and depend on the demand from other farmers.
6. CONCLUSIONS AND RECOMMENDATIONS
Under present conditions of management, the survival of juveniles from hatching is 5–10 percent. This is very low. There is a possibility that the survival rate can be considerable increased by the following innovation
6.1 Hatcheries at present use water supply of low quality for hatchery work. For instance, the fresh water water supply using tap water is often affected by old G.I pipes which have been used for a long time, thus containation the hatchery water supply. Likewise, fresh water from canals are supplied and allowed to settle but no disinfection for microorganisms harmful to the young prawn is being done. Sea water that contain small medusae can harm the small prawns.
The above constraints can be remedied by the installation of simple filtering devices like sand/charcoal filter.
Tap water should not be used but ground water of good quality, if avilable, should be preferred.
6.2 To improve this simple and inexpensive hatchery system, engineering knowledge should be employed, to systematize and arrange this hatchery to make it more efficient without affecting its simplicity.
6.3 No attention is being given to the disease and parasite problem of these small prawn hatcheries. Mortalities through diseases and parasites are common. Proper treatment and filtering of the water may reduce this danger. The amount of water supply should also be increased over present levels.
6.4 Present practices make use of shallow water in the tanks resulting in big range of temperature fluctuation. The depth of water in the tank will reduce the temperature fluctuation and increase survial.
5.5 New hatcheries to be developed would be better located if near the sea. In general, fresh water would be easier to obtain even along the coast while sea water will be difficult to transport in hatcheries for inland fisheries.
5.6 Prawn farmers operating hatcheries should have their own growout ponds so that any juveniles not sold can be stocked in his own pond and he can derive greater profit.
