GIANT FRESHWATER PRAWN FARMING IN SUPANBURI, THAILAND
National Freshwater Prawn Research and Training Centre
Inland Fisheries Division, Department of Fisheries
Ministry of Agriculture and Cooperatives
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GIANT FRESHWATER PRAWN FARMING AT SUPANBURIa
Amongst all the Thai people who earn their living from rearing aquatic animals, to be able to culture freshwater prawns has been a long present hope. The obstacle that impedes business development in this field is a lack of technical understanding; there is, for instance a belief that:
cannibalism during pond rearing will cause low yields
prawns will not grow in ponds
prawns will not have a chance to grow because predatory fish and other predators will attack them in the early stages of rearing.
However, the development of prawn culture is greatly and genuinely handicapped by lack of experience and the difficulties in obtaining sufficient juveniles and mature prawns for culture purposes from their natural breeding grounds.
Recently, the Department of Fisheries of the Royal Thai Government established a programme to determine the necessary data for producing juvenile prawns under controlled conditions. The Chacheongsao Fisheries Station is one of the stations conducting these experiments. During 1977, freshwater prawns were hatched and reared there so that some juvenile prawns could be stocked in private ponds in some 23 provinces in order to collect and study data on growth and survival rates. Supanburi is one of the provinces where many inhabitants are extremely interested in freshwater prawn culture. Some already had experience in fish culture, which is considered advantageous for those wishing to culture prawns. Being a new enterprise in its infancy there is a considerable amount of work yet to be done to bring prawn culture to full maturity. However, the successful result obtained by the culturists of this province may serve as a guide for those who are interested and at the same time serve as a basis for promoting deeper interest amongst them. It is hoped that these annotated results will encourage them to devote themselves to prawn culture without hesitation.
a Translated from a simple leaflet for farmers, distributed from Chacheongsao Fisheries Station
b Chief, Chacheongsao Fisheries Station
As with the rearing of other aquatic animals, water is an essential factor in selecting a site for the construction of ponds for rearing this type of prawns. The ideal place is anywhere which has plenty of clean water at all seasons which can be introduced into the ponds throughout the rearing period. During the early stages, soon after stocking, there is no need to change water so often because the prawns are too young and because the quantity presented is so small that the uneaten feed will not cause water pollution. After prawns have been in the ponds for about 2–3 months, it is necessary to examine the ponds carefully at regular intervals to see whether the water has turned to dark green or black. Notice also whether the water at the bottom of the ponds is dirty and polluted through the decomposition of uneaten feed. If this has happened, drainage, disposal and replacement of water must be carried out immediately.
Apart from water, the quality of soil is important; for instance the various types of soil, such as acid, alkaline, etc., have to be taken into account. Other factors will include those routine practises which have been developed for culturing fish.
In the case of Supanburi province, where these rearing trials were conducted, the water used for prawn culture comes from irrigation channels of the Photiphya and water is abundant almost throughout the year. The bunds of the ponds are constructed so that they are high enough to prevent water spillage in the rainy season when flooding occurs. The quality of the soil is perfect for prawn culture; it has a pH of about 7 and the soil holds water very well, even in newly dug ponds.
At Supanburi, one-time fish ponds are mostly used with a depth of 1.2–1.5 m. Some are newly dug ponds 2 m. deep with a bund height above surrounding land of 1.2–1.5 m. The area of the ponds was from 0.5–2.0 rai (800m2 – 3200m2). Five ponds were used in the trials.
Before introducing juvenile prawns into ponds, the Fisheries Station will generally ask the owners to drain the ponds completely and to eradicate various types of predatory fish. In addition, green plastic nets should be placed around the perimeter of the ponds in order to protect prawns from being attacked by predatory animals such as catfish, snakehead fish and other crawlers. Moreover, water inlets should be carefully screened by fine mesh plastic netting to prevent the entry of juveniles and eggs of predatory fish which may flow in with the intake water. Additionally, where water inlets are below or close to the water surface, this netting retains young prawns in the pond. However, in spite of all these precautionary measures, many fish will be found in prawn ponds when they are drained. This does not mean that these preventative measures are not worth taking. The main objective is to ensure that juveniles are stocked in the ponds they will be protected from the larger fish that could attack them. However, fish eggs and very small fish which flow in with the water supply passing through the fine mesh netting on the intake will be too small to be able to attack juvenile prawns. Prawns and predators will grow together at the same rate in such a manner that the fish can do no harm to the prawns; indeed they may contribute to the feed available.
When newly dug ponds are emptied of water and fish, the next thing to do is to sprinkle lime on the bottom of the ponds to prevent the soil from being too acid. The ponds should then be refilled with water.
Stock of young and mature prawns suitable for culture and breeding purposes may be collected from three sources:
private hatcheries that specialise in prawn breeding
natural habitats; young prawns measuring about 3 cm. in length can be collected from many water resources for pond rearing
official breeding stations that distribute juvenile prawns free of charge, at least in the early stages of a new venture.
Prawn culturists in Supanburi, whose work is described in this leaflet, obtained their juvenile prawns from the Chacheongsao Fisheries Station. Stocking rates varied as follows:
2 ponds at 6/m2
1 pond at 5/m2
1 pond at 3/m2
1 pond at 2/m2
The area referred to above is area of water surface.
The size of juvenile prawns obtained for rearing was about 1.5 cm in length; they had been reared in freshwater for 15 days in the hatchery.
Prawns are fed with broiler starter feed for the first two months. After that period they can be fed chicken feed, or ground fish flesh mixed with cooked broken rice can be cast into the ponds. Some culturists feed the juvenile prawns with calf's bones whose flesh has already been removed but in such a way as to still leave some flesh adhering to the bones; the latter are available from abattoirs. After two weeks, the bones are collected from the ponds and replaced by new ones. In addition to the food mentioned above, fish taken from the rearing pond itself can be chopped into pieces to provide a supplementary prawn food.
In foreign countries, food is given to the prawns at a rate of 5% of their body weight. However, in Thailand, many fish are spawning in the ponds resulting in a large biomass of fish, so that the feeding rates used abroad will not be sufficient. Thus has developed the practise of observing whether prawns have enough to eat and adjusting the amount of food presented each day; food left over from the previous day indicates that the prawns were overfed that day. The amount of food given the following day will have to be reduced. If no food is left it may mean that the food given the previous day was just right or that it was insufficient. Feed more than yesterday and observe again the day after. The most important thing is to pay attention to the particles of food that are left in the ponds. Unfinished food particles should be removed by siphoning each day since they will cause water pollution. Feeding must be done in the evening once a day by casting food around the edges of the ponds. The advantages of feeding at evening time are: a) it takes advantage of the habit of prawns to move up close to the edges of the water or to some shallow areas at night and b) fish will have less chance to take the food away from the prawns at night.
During the first 1–2 months, prawn culturists will seldom see the juvenile prawns which they have stocked. This is because the newly metamorphosed juveniles are small and transparent and are difficult to see because they spread all over the place. By the third month, the juvenile prawns will show their existence again.
The total rearing period in the five ponds described here was as follows:
Ponds 1 and 4 (stocked at 2/m2 and 6/m2 respectively): 7.5 months
Pond 2 (stocked at 3/m2): 7.0 months
Pond 3 (stocked at 5/m2): 6.0 months
Pond 5 (stocked at 6/m2): 8.0 months
The prawn culturists originally expected to rear the prawns for a longer period than they actually did; however, shortage of water expedited the harvesting season when some dead prawns were seen floating around the edges of the ponds. These ponds had therefore to be drained at once and the remaining prawn transferred to other ponds.
Prawn culturists in Hawaii have tried culling prawns every month after six months of rearing. A monofilament net is used for seining the prawns. Marketable prawns are taken first and non-marketable sizes are retained for further rearing until they are big enough. In this manner, prawn culturists will achieve high yields. Prawn culturists in Supanburi were confronted with the problem of water shortage so the method mentioned above was not applicable.
Harvesting can also be done by draining the ponds. All the prawns in the pond are then caught, marketable sizes selected and the smaller ones re-stocked into another pond. The harvested pond should then be sun dried until the soil is hard and ready for the next rearing cycle. The latter method of harvesting is not considered satisfactory for the following reason. After the pond is drained the culturists will have to climb down to the bottom of the pond to catch the prawns. The footprints of the workers cause small pits in the sludge and prawns tend to retreat gradually into these pits, which still retain a small amount of water, and hide themselves. The following day, a large number of dead prawns will be found in these pits. A better method is to reduce the water level in the pond, about 0.3 – 0.6 m. first. Then reduce the water level further gradually, with the same rhythm as prawns are caught. Start by catching prawns that climb up on the edges and then go further and further into the ponds until neither water or prawns are left. This method will prevent footprints in the pond mud in which prawns can hide. However, this is still not the best method of harvesting; it is better still to use the seining technique mentioned above.
The pond which was stocked at 2/m2 had a production of 155 kg/rai after 7.5 months. 95% of the prawns caught were of marketable size. The average weight was 65.9 g. or 15 prawns/kg. Survival was 72%.
The pond stocked at 3/m2 produced 104 kg/rai in 7 months. 7% of the prawns were of marketable size and the survival rate was 52%. Additionally some 109 kg. of fish were harvested with the prawns. The average weight of the prawns was 54 g. or 18 prawns/kg.
The pond stocked at 5/m2 gave a production of 154 kg/rai in 6 months and a survival rate of 77%. 87% of the total weight consisted of marketable sized prawns. The average weight was 49.4 g. or about 20 prawns/kg.
Two ponds were stocked at 6/m2. The one that had a rearing period of 7.5 months produced 129 kg/rai with a survival rate of 31%. 81% of the prawns were of marketable size and the average weight was 55 g. or 18 prawns/kg. The other pond, which had a rearing period of 8 months gave a yield of 126 kg/rai and a survival rate of 66%. 96% were of marketable size and the average weight was 32 g. or 31 prawns/kg.
More details are given in Tables 1 and 2, as follows:
|Pond No.||Farmer's Name||Stocking Rate|
|Rearing Period||Production Rate|
|1||Tamchart Punpruck||2||7 mo., 14 days||155.04|
|2||Chaiyasit Kraikunasai||3||7 months||104.13|
|3||Tamchart Punpruck||5||6 months||154.2|
|4||Chaiyasit Kraikunasai||6||7 mos. 13 days||129.5|
|5||Tamchart Punpruck||6||8 months||126.0|
|Pond No.||Stocking Rate (m2)||Average Weight (g)||Marketable Prawns (% of total)||Survival Rate (%)|
The data obtained from the prawn farms in Supanburi has not been subjected to statistical analysis because it was not possible to keep all factors other than stocking rate the same in each pond. However, the data obtained leads to useful conclusions as follows:
Two crops of prawns could be cultured each year if the ponds are well managed and if enough water is provided.
The yield produced in a two crop year should be at least 250 kg/rai.
The data was obtained at the initial stage of prawn culture in the province; it should not be assumed that it represents the highest possible yield. The culturists do not have enough experience yet in prawn culture and there is a considerable amount of work yet to be done to improve the technique of prawn culture.
Survival rates of 50% or even higher, depending on the efficiency with which the ponds were prepared in the early stages and on the efficiency with which carnivorous fish are removed, can be achieved.
It is worth remarking that during the first 3 months, the actual increase in prawn weight seems to be very slow. If the percentage weight increase is taken into account however, it will be clear that the actual rate of growth is very fast. After 4 months the prawns' weight will increase rapidly.
While rearing prawns in these trials it was observed that juvenile fish and small prawns appeared in the ponds in spite of all preventative measures. This fact irritated most of the prawn culturists since they believed that young fish and small prawns take away food which should be used by the animals being reared. In fact, after four months rearing, the quantity of small fish and prawns will be rapidly reduced and when the ponds are drained for harvesting it becomes evident that almost all the fishes and small prawns have disappeared. This can be explained by the fact that when juvenile prawns grow up to a certain size they are able to catch small fish and prawns for food. The animals which begin as enemies end as food. Therefore, in the future, prawn culturists should not mind young fish and prawns and should not consider them as predatory animals.
In the rearing trials in Supanburi, it was noticed that prawns were not fed enough. Under good water conditions and with an ample supply of rich food, the production will certainly be higher next time.
Guidance for high-yield prawn culture can be given as follows:
The experience gained by the culturists from the first rearing period will encourage them to invest more in an ample supply of rich food and in buying a large quantity of juvenile prawns for stocking.
The experience that prawns are able to catch small fish for food gives the idea of culturing suitable species of small fish in polyculture with the prawns specifically for the purpose of providing sufficient quantities of additional food all through the period of rearing, which may prove economical.
Efficient ways to get rid of large carnivorous fish should be studied. Efficient ways and means to fully utilise the ponds all year round without having to drain them must also be found by introducing a continuous culling and restocking programme. The stocking rate should also be studied so as to optimise production rate in the next crop.
Provision of water lilies such as ‘Paktobjava’ (a Thai aquatic plant) in the rearing ponds in sufficient quantity will help in providing shade and hiding places for prawns and also prevent the water becoming so easily polluted. The roots of these plants will also serve as additional food for prawns.