The expert visited all the Government facilities where research on shrimp culture is being conducted. His work, however, was confined for the most part to the Marine Laboratory in Bangkok and the experimental ponds at Samut Sakorn. He also visited many of the areas with potential for development, as well as several private shrimp farms.
The expert suggested several modifications in the procedure used to culture algae with which the larval shrimp were fed. These included a medium for algal culture that proved useful in culturing both Chastoceros sp. and Skeletonema sp.; the use of fluorescent lights at night; the use of test tubes for maintaining stock cultures; the use of ultraviolet light to sterilize seawater; and the use of continuous cultures with periodic draw-offs instead of batch cultures.
The Bangkok Marine Laboratory has no seawater available and all the seawater used must be brought from offshore by boat. Due to this lack of water, the expert recommended that only a limited number of larvae be reared in small containers. The expert's counterparts modified the containers suggested by him and a very serviceable container was constructed that has since been adapted by several other groups of Thai biologists who are conducting research on shrimp.
Gravid female shrimp were captured in the Gulf of Thailand and brought to the Marine Laboratory by the Thai Department of Fisheries Research Vessel PRAMONG 4. Spawning and hatching took place in 1 200 1 fibreglass tanks. A small number of newly hatched larvae were placed in 15 1 containers in which they were reared to post-larvae. A portion of the water in each small container was changed daily. Protozoeae were fed algae, either Chaetoceros sp. or Skeletonema sp., or a mixture of the two. Myses and post-larvae were normally fed brine shrimp (Artemia) or minced clam. Using these techniques, the expert's counterparts successfully cultured the following species to post-larvae: Penaeus merguiensis, P. semisulcatus, P. latisulcatus, Metapenaeus monoceros, and M. intermedius.
One experiment was conducted in which larvae of P. merguiensis were reared in water of three salinities; 28, 30 and 32‰. There were no discernible differences in the rate of survival to post-larvae. In another experiment, P. merguiensis myses were fed four different diets; shrimp paste, a processed fish food, a processed chicken food and live Artemia nauplii. Each of these diets was supplemented with a diatom, Chaetoceros sp. Almost all larvae feeding on shrimp paste and fish food died. The water in these tanks became cloudy, probably due to bacteria growing on nutrients leached out of the foods. Survival of those given chicken food was comparable with those fed with Artemia. The processed chicken food was in the form of a meal and it was screened before use to ensure that only small particles were fed. Its analysis was given as protein 38 percent, fat 6.6 percent, residue 4.6 percent and moisture 8.6 percent. Unfortunately, personnel changes within the Department of Fisheries caused the larval culture research to stop after six months.
One experiment was conducted to determine the salinity tolerance of P. semisulcatus post-larvae. The experiment was started with 90 post-larvae, 7.5 mm total length, in each of five salinities 30, 20, 15, 5 and 2‰. Survival was less than 10 percent in all tanks. This high mortality was caused by feeding processed chicken food which caused the water to foul. The biologist who conducted this experiment gained useful experience however, and future results should be better.
The expert assisted generally in laboratory work and advised on procedures used. He also advised his counterparts on the preparation of manuscripts reporting the results of their past work.
The number of shrimp that a pond can support is limited by the amount of food available. The most economical way to increase the amount of food is by fertilizing the pond water.
Government biologists had started an experiment to test the efficiency of superphosphate as a fertilizer before the expert arrived in Thailand. The results of this experiment showed that a 1.4 ha pond treated with superphosphate yielded 148 kg of shrimp, while the yield from a 1.4 ha untreated pond was only 90 kg. The next step was to see if the addition of nitrogen would increase the yield further. Consequently, another experiment was started in which superphosphate was added to one 1.4 ha pond at the rate of 12.5 kg/ha, and a second pond was treated with 62.7 kg of ammonium sulphate and 3.8 kg of superphosphate per hectare. During the course of the experiment, three applications of fertilizer were made to the first pond and two to the second. The ponds were harvested after 60 days. A total of 100.3 kg of shrimp and 4.4 kg of fish was harvested from the pond fertilized with only superphosphate. The pond fertilized with ammonium sulphate and superphosphate yielded 176.2 kg of shrimp and 4.7 kg of fish. After allowing for the additional cost of treatment (U.S.$ 17.00), the added income derived from fertilizing with both ammonium sulphate and superphosphate was over U.S.$ 50.00.
Generally the pond water at Samut Sakorn was poor as regards nitrogen and phosphorus. Levels of nitrogen were usually less than 0.1 ppm (nitrate, nitrite) and phosphate less than 0.7 ppm. At Surat Thani on the other hand, nitrogen values ranged from 7 to 9 ppm (nitrate, nitrite) and phosphate was less than 0.2 ppm on the one occasion when measurements were taken. This variation in nutrient levels at different localities indicates the use of different fertilizers. As was shown in the fertilization experiment reported above, water at Samut Sakorn benefited from a balanced fertilizer containing both nitrogen and phosphorus. It is likely that at Surat Thani, greatest benefits would be obtained with the use of superphosphate alone.
One of the factors which makes the use of fertilizers difficult in Thailand is the leakage of water from ponds. The leaking water carries away nutrients. There are two main causes of leaks; (1) holes in levees caused by burrowing crabs, and (2) leaky sluice boxes. The expert and his counterpart found that leaks caused by crabs could be stopped by digging a trench in the levee and inserting sheets of polyethylene film in the trench. It is not known how long this protection will last. The cost of digging the trench is high, but if longer testing proves the plastic film to be worthwhile, it might be possible to install it in new ponds at a much lower cost. One possible method is shown in Figure 1. A small ditch and levee would be constructed initially, then the polyethelene film laid down, and finally the ditch widened and the plastic film covered with earth.
The expert and his counterpart tried several ways of preventing sluice gates from leaking. They found that a sheet of cellocrete 60 cm wide fastened to the sides and bottom of the sluice box (Figure 2) prevented leaks. The cellocrete should be positioned adjacent to the sluice boards.
In Thailand the amount of fish harvested from shrimp farms is about 32 percent of the total weight, but only 1 percent of the total value (Anon., 1971). The fish either eat shrimp or compete with them for food, so every kilogramme of fish harvested from a pond represents a loss of shrimp. As shrimp are much more valuable than fish, in this case in view of the fact that shrimp brings higher prices, it is essential that as many fish as possible be eliminated from a pond. This can be accomplished by a combination of screening and application of tea-seed cake.
The expert participated in two experiments to assess the advisability of using tea-seed cake. In the first experiment, it was applied at a rate of 25 ppm (25 g/m3) and in the second 10 ppm (10 g/m3). Both treatments were effective, eliminating almost all the fish and not harming the shrimp. The pond level was lowered before treatment, so that the only water remaining in the pond was in the ditch running around the perimeter of the pond. The tea-seed cake was powdered and mixed with water before being added to the pond. New water was pumped into the pond after 12 h to dilute the tea-seed cake and raise the water level. Cost of treatment was relatively low as only a small amount of water was treated.
All indications are that the supply of seed shrimp is too low to support high yields in the shrimp ponds of Thailand. In one of the experiments conducted at Samut Sakorn, the estimated rate of stocking was only 6 250 shrimp per hectare. If all these were grown to a size of 14 cm the yield would only be about 135 kg/hectare. All the shrimp do not grow up. However, as many are eaten by fish or escape from the pond through water leaks, most of the small shrimp pumped into a pond just prior to harvest are also lost through the net during harvesting. The expert and one of his counterparts designed a system of ponds to minimize loss from these causes (Figure 3). Water containing seed shrimp is pumped into a small nursery pond (pond A). A fine mesh screen on the overflow and water gate keep the small shrimp in the pond. After water is pumped into this pond for 15 or 20 days, the pond water is treated with tea-seed cake to eliminate fish, and then the shrimp are transferred to one of the growing ponds (B and C) by gravity flow of water. The process is then repeated, only this time the shrimp are transferred to the remaining growing pond. The next time, water is pumped into pond A until the shrimp in the first growing pond are harvested and the pond is ready for restocking. The process is then repeated with alternate stocking and harvesting of ponds B and C.
Fig. 1 - SCHEME FOR INSTALLING POLYETHYLENE IN BANK OF POND
(using black polyethylene at least 6 mm thick)
Fig. 2 - PLACEMENT OF CELLOCRETE BOARD AROUND WATER GATE
There are several benefits to be derived from this procedure. The first is that the nursery pond is small enough so that all leaks in its levees can be eliminated, thus preventing escapement of post-larvae. Another is that water can be pumped into pond A to obtain seed shrimp, without the water flowing through the growing ponds, after fertilizer has been applied. A third advantage is that the area to be treated with tea-seed cake is small, thus reducing the cost of eliminating fish to a minimum. Another benefit is that all the shrimp in the growing ponds are cultured for a long enough period of time to ensure that they grow to a catchable size, and none would be lost through the net during harvest.
The Thai army requested assistance from the Department of Fisheries to start a shrimp farm. The army has over 80 ha of land that had been used for salt production some years ago. This land borders on an extensive mud flat which extends from 3 to 5 km at low tide. The major problems to be faced were excessive silt in the water and rapid closure of the channel used to bring water to the pump. The expert and his counterpart decided to work with only one small farming unit to determine if these difficulties could be overcome. This unit was divided into two small ponds; one of 2.5 ha to be used as a growing pond, and one of 0.32 ha to act as a settling pond and water storage area (Figure 4). The plan was to flow water out of the 0.32 ha pond periodically in order to flush out the water inlet canal. There was some difficulty encountered initially, because the water spread out instead of cutting a channel through the mud. After the channel was dug deeper by hand this was no longer a problem, and the system worked well. Most of the silt in the water settled out in the small pond and the water was relatively clear when it entered the growing pond. For example, on one occasion water near the inlet in the small pond had a turbidity of 650 JTU (Jackson Turbidity Unit) and at the water gate leading from one pond to the other it was only 38 JTU. The small pond will be cleaned out manually at periodic intervals to remove accumulated sediment.
