Most of the ponds used for shrimp culture in Thailand are not deep enough. On some farms a portion of the area designated as ponds is not even under water. The farmers should build their levees higher, so that a water depth of 60 cm could be maintained. The deep water will help keep summer temperatures from becoming too high and minimize the effects of salinity increases caused by evaporation. In addition, weeds cannot grow in such deep water. The ditches next to the levees should be made wider and shallower to help prevent water in these canals from becoming stratified. If the water becomes stratified only the top layer is circulated by the wind and the bottom layer can become depleted of oxygen.
Many shrimp farmers still do not use pumps and as a result they could not maintain an adequate level of water in their ponds even if they built higher levees. These farmers should be encouraged to install pumps.
Fig. 3
EXPERIMENTAL
SHRIMP FARM AT
SAMUT SAKORN
Fig. 4 - SHRIMP FARM AT BANG POO
Most farms which do pump water, use a wooden water-wheel pump. This type of pump has several disadvantages; the volume of water that can be pumped is small and water cannot be lifted high enough to maintain an adequate level of water in the ponds. In addition, because water cannot be lifted to a great height, it can be pumped only on a high tide and as a result there are many times when water cannot be pumped for several days at a time.
The best way to supply water, especially for large farms, is with a central water supply canal. The canal should be elevated to a height great enough to allow water to flow into the ponds by gravity. One large pump would supply water to the central distributing canal continuously, even on a low tide.
A system capable of supplying water any time it is needed would have benefits in addition to being able to maintain adequate levels of water in a pond. Changing water in a pond rapidly is one of the best ways of controlling excessive blooms of algae or replacing polluted water. This is especially important when fertilizer or supplemental foods are used. Any time a farmer notices dead shrimp, or shrimp swimming at the water's surface, the best way to save the shrimp is flowing water through the pond as soon as possible. Also, evaporation of pond water causes salinity to rise and it is important to change water periodically to keep the salinity from becoming too high. Lastly, the more water is pumped the more seed will be collected.
A high volume centrifugal pump would probably kill many of the shrimp that pass through it. A solution to this is to capture the shrimp before they reach the pump. This could be accomplished by placing fine mesh nets in the water canal leading to the pump. The seed shrimp would be removed periodically and held in a small pond or tank until they were treated with tea-seed cake and released in a pond. A modification of a type of net that has been used by the Thai Department of Fisheries to sample post-larval abundance would serve admirably for this purpose (Figure 5).
One of the major costs of shrimp culture in Thailand is cleaning sediment from the interior canals of the ponds. This is done by manual labour once or twice a year. The pond is drained and workers scoop out the sediment and throw it on top of the levee. Over a period of years this should cause an appreciable rise in the height of the levees, but it does not because many of the levees have no vegetation to hold the silt in place. As a result, most of the material simply washes back into the ponds after several hard rains. The farmers should be encouraged to let vegetation grow on their pond levees instead of removing it. A vegetative cover on the levees would also minimize damage caused by wind waves.
It has been demonstrated that the application of fertilizer to pond water increases the yield of shrimp from ponds. The most immediate need is to determine what kinds of fertilizers work best in Thai waters, and what are the proper rates of application. It is probably not only the amount of algae growing in a pond which is important, but also the kind and quality of algae (the type of algae present in a pond is dependent on the level of nutrients). Mobile algae that colour the water deep green or red frequently cause mortalities of shrimp. Diatoms (Diatomae) which make the pond waters appear yellow-brown are much more desirable. The latter ordinarily require a high ratio of nitrogen to phosphorus, usually in the order of 20:1, while several other less desirable algae prefer a much lower ratio. By measuring the levels of these nutrients in pond waters it should be possible for biologists to calculate how much of each must be added to encourage the growth of favourable types of algae. Sufficient nutrients should be added to maintain the bloom of algae at a density sufficient to give a turbidity value falling within the range of 80 to 100 JTU.
The second step should be to train shrimp farmers in the use of fertilizers, for unless fertilizers are applied in the right way it is useless to apply them. If farmers use fertilizers without proper instruction there is a great danger that they may encounter the following problems. When too much fertilizer is added to a pond the algae will grow too thick and shrimp will die. On the other hand, if fertilization is started and then discontinued or applied at an insufficient rate, the pond will have more shrimp than the food supply warrants. When this happens the shrimp will stop growing and then begin to eat each other. If this happens both the farmer and his neighbour may never use fertilizer again.
Fig. 5 - NET FOR COLLECTING POST-LARVAL SHRIMP
The expert heard of several cases where farmers complained that their ponds became polluted after especially heavy rains and many shrimp died. Unfortunately, the expert was not able to observe this himself. This condition is probably caused by stratification of water within the pond. Rainwater is less dense than seawater, and during a storm with heavy precipitation and little wind it would tend to float on top of the saltwater layer. When this happens, the bottom layer of saltwater would not receive oxygen from the air. When oxygen in the saltwater is used up, the shrimp would either die from lack of oxygen or swim to the surface where they would die in the freshwater. Two methods of correcting this problem are suggested. The first is to drain off the top layer of water; the second is to stir up the pond water so the two layers mix. Probably a combination of these two procedures would give the best results.
Supplemental feeding is not really necessary at this stage of development in the traditional practice of shrimp farming in Thailand. One reason for this is that the density of shrimp in ponds is usually so low that lack of food should not be a problem with an adequate programme of fertilization. Another reason is that there are no appropriate foods available. The food conversion ratios obtained when feeding fish or other natural foods is so poor that large amounts of food must be added to a pond. This causes the pond water to become polluted, and in most cases the shrimp will die if the water is not changed rapidly. In addition, shrimp culturists are becoming aware of the importance of fungus infections in shrimp. Many shellfish and some fish are known to harbour fungi, and it is possible that by feeding unprocessed foods, a farmer would be needlessly exposing his shrimp to a source of infection.
There appears to be no standard method for managing a pond in Thailand. A survey conducted by the Thai Department of Fisheries (Anon., 1971) showed that the number of days farmers harvested their ponds varied from 6 to 285 days per year. Some farmers dried their ponds for only one month, others for three months, and the number of times the ponds were dried varied from one to three times per year. Such a great variation means that many farmers are practising poor management. Efforts should be made to determine what constitutes good management, i.e., the optimum number of growing days per crop; whether it is better to harvest a crop all at once or over an extended period of time; how often water should be changed; and the number of days a pond must be dried each year. A great deal of this type of information could be determined by collecting more accurate statistics and correlating management practices with yields.
One element essential to good pond management is a knowledge of the seasonal abundance of seed shrimp. This information would enable a farmer to know whether or not to harvest a crop in order to get more seed, while the shrimp were abundant, or to grow them for a longer period of time because there were too few shrimp available for restocking. The Department of Fisheries should initiate a programme to sample post-larval shrimp in each of the major shrimp farming areas.
A more accurate picture of seasonal price fluctuations would also be useful. For example, many shrimp farmers and biologists believe it is desirable to culture only shrimp of the genus Penaeus, and that the smaller Metapenaeus are not valuable. The Fish Marketing Organization, Yanawa District, Bangkok supplied the expert with a summary of the maximum and minimum monthly prices paid for shrimp in 1971 (Table 1). These data show that the price of large Metapenaeus monoceros increases greatly in October and November. In the area near Bangkok, pond salinities, from August through January, are low and very few Penaeus merguiensis are produced, but M. monoceros is abundant. The farmers would probably realize a greater value for their crop by stocking their ponds in July or August and holding them until October or November, allowing the M. monoceros to grow to a large size.
Table 1
MAXIMUM AND MINIMUM PRICE (BAHT PER KG) PAID FOR SHRIMP BY MONTH IN 1972
(large, medium, small)
| Species | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sept | Oct | Nov | Dec | ||||||||||||
| L | H | L | H | L | H | L | H | L | H | L | H | L | H | L | H | L | H | L | H | L | H | L | H | |
| Large P. merguiensis | 35 | 40 | 36 | 38 | 35 | 38 | 38 | 38 | 38 | 40 | 38 | 42 | 38 | 40 | 38 | 40 | 38 | 42 | 58 | 64 | 55 | 60 | 55 | 60 |
| Medium P. merguiensis | 24 | 25 | 24 | 25 | 25 | 26 | 22 | 23 | 17 | 22 | 20 | 23 | 22 | 23 | 22 | 24 | 22 | 24 | 28 | 30 | 26 | 30 | 23 | 26 |
| Large M. monoceros | 16 | 20 | 16 | 18 | 16 | 18 | 18 | 20 | 20 | 28 | 16 | 18 | 18 | 20 | 20 | 28 | 18 | 28 | 30 | 32 | 32 | 35 | 22 | 35 |
| Misc. small shrimp | 10 | 12 | 10 | 12 | 10 | 12 | 10 | 12 | 9 | 12 | 10 | 12 | 10 | 12 | 10 | 12 | 8 | 12 | 10 | 12 | 9 | 12 | 9 | 12 |
The problem of an inadequate supply of seed shrimp has already been discussed. It would be advantageous if farmers were able to supplement seed obtained from natural waters with post-larvae reared in a hatchery. Consequently, the expert recommends that the Department of Fisheries proceed rapidly to develop hatchery techniques suitable for indigenous species of shrimp. This development must not concentrate only on highly valued shrimp, but on those species that grow well in ponds. For example, it would be inadvisable to stock ponds near Bangkok with P. monodon or P. semisulcatus in August when pond salinities are so low the shrimp would not grow. This means that physiological studies to determine the tolerance of various species to environmental conditions must be carried out concurrently with hatchery development.
More skilled management is required of the farmer as the density of shrimp in a pond increases. With a stocking rate of 5 000/ha, the pond will probably produce sufficient food to feed the shrimp and no real management is needed. If the rate of stocking is increased to 50 000/ha, both fertilization and supplemental feeding will be necessary to rear the shrimp to a marketable size. When supplemental feeding and fertilization are employed, the risk of catastrophic mortalities is heightened. Most traditional farmers have a low income and any such loss would affect them greatly. Consequently, it is of prime importance that pond management skills be developed before additional seed shrimp are provided to farmers.
If seed shrimp are provided, the number supplied should bring the total number of shrimp in the pond to no more than 11 000/ha and with proper management this should produce 500 kilogrammes. This is about the maximum yield that can be produced without supplemental feeding.
The ponds used by biologists for conducting experiments on shrimp culture at Samut Sakorn are not well suited for research. There are four 3.2 ha ponds leased on an individual basis. The ponds are too large, and the water supply is not adequate for dividing them into smaller units. The ponds are converted salt pans and they are still very shallow. The owners also frequently object to modifications suggested by the biologists. The ponds are 46 km from Bangkok and due to the difficulties involved in travelling, biologists cannot visit the ponds on a daily basis.
The Department of Fisheries should construct their own ponds. It is recommended that there should be ten ponds of 0.2 ha each. Particular attention should be given to constructing a water supply system so that the incoming current canal is sufficiently deep to permit pumping water at low tide. Adequate laboratory and storage facilities should be provided at the pond site. Biologists assigned to work at these ponds should live close enough to visit the ponds on a daily basis.
The technology necessary to bring about significant improvements in the yield from existing shrimp farms is already known. Such things as the use of screens and tea-seed cake to eliminate fish, growing vegetation on levees, replacing leaky water gates, and the use of pumps could be adapted with very little capital expenditure. Other practices such as raising the water level in ponds or repairing holes in levees would require greater expenditures. The Department of Fisheries should exert every effort to educate the shrimp farmers in new technology, for as the farmers make the initial rudimentary improvements their incomes will rise and they will be able to afford the cost of more expensive improvements. To this end, the Government should employ and train extension biologists to work among the farmers.
Demonstration farms should be constructed in areas where the Government desires shrimp culture to become established. One of the farms at Samut Sakorn should be retained as a demonstration unit. An extension biologist, well trained in shrimp culture, should be assigned full-time to each demonstration unit.
The Government should start a programme of research to determine what will be the effect on the adjacent environment when mangrove forests are cleared for the construction of shrimp ponds. The mangroves are a nursery for many species of marine life and it is probably essential that they are not all eliminated. An intelligent decision on their most productive use cannot be made until their role in the overall ecological system is better understood. It is not necessary that development of all virgin areas be prohibited. Instead, the area that will be converted from mangrove to shrimp farms should be limited while the necessary studies are being made. By setting such a limit, the Government will ensure that destruction of the mangrove does not proceed too far, while still allowing shrimp farms to develop in new areas. It is suggested that no more than 10 percent of the original mangrove in any area be converted to other uses. No additional development should be permitted in areas where more than 10 percent of the area has already been reclaimed. Standards should also be set to ensure that a border of mangrove be retained adjacent to coastlines and major rivers and canals to prevent erosion and storm damage.
One of the aims of the Government is to produce more shrimp for export. Shrimp for export must meet increasingly rigid quality standards. The Department of Fisheries, with the assistance of FAO, is trying to upgrade the quality of frozen shrimp and this work should continue. Additional work is needed to ensure that shrimp processors receive a good product. Shrimp farmers should be encouraged to use ice, and they need to be shown how to pack correctly. The few times the expert observed ice being used it was placed on top of a basket of shrimp, which was of little value. In many western countries, shrimp are marketed without their heads. The possibility of having large shrimp suitable for export beheaded at the farm should be investigated. This would ensure a higher quality product.
