1.1 Fish is an important source of animal protein in the regular diet of the Thai people. Like the Philippines, the marine fishing industry is still the major supplier of fish. The total production of aquatic products in 1972 consisted of 1 540 157 tons from marine fisheries and 131 303 tons from fresh water fisheries (Rabanal, 1974). With anticipated extension of national fishing limits in the near future, it is expected that the marine fish production in Thailand would be considerably reduced. Emphasis must therefore be placed on the exploitation of other resources and intensification of inland fisheries development, including aquaculture (Menasveta et al, 1975).
Freshwater fish production in Thailand come from rivers, paddy fields and irrigation canals, lakes, reservoirs and fishponds. The former are mostly fisheries without husbandry, but with stocking assistance from the Department of Fisheries. Areas of inland waters used for aquaculture production are identified in Table 3.
Table 3. Thailand inland waters and areas used for aquaculture production (FAO/UNDP, 1975)
| Type of water | Area actually used for aquaculture (ha) | Total potential area (ha) | |
| 1. | Ponds (freshwater) | 2 238 | - |
| 2. | Water ditches of vegetable and fruit farms | 50 | 5 000 |
| 3. | Paddy fields (irrigated) | 200 000 | 4 000 000 |
| 4. | Irrigation tank | 5 000 | 30 000 |
| 5. | Large impoundments | - | 215 000 |
| 6. | Rivers and canals | - | 120 000 |
| 7. | Mangrove and tidal flats | 9 504 | 152 000 |
Fish culture is an old practice in Thailand; however, intensive fish culture has been developed only during the past two decades. The Department of Fisheries has made a significant effort on the development of aquaculture resulting in the substantial increase of fish culture activities in the country. In earlier days, farmers were encouraged to grow fish in paddy fields, and emphasis was placed on the rearing of herbivorous and omnivorous species (Nile tilapia, Tilapia nilotica; Thai carp, Puntius gonionotus; common carp, Cyprinus carpio; and sepat siam, Trichogaster pectoralis.)
Commercial-scale aquaculture was developed some time later. The species raised include catfish (Clarias batrachus), Pangasius sutchi, sepat-siam, snakehead (Ophicephalus striatus), grass carp, bighead carp, silver carp, common carp and Japanese eel (Anguilla japonicus). Among these species, the culture of Clarias spp. gives good returns (approximately 174 tons/ha/year). Snakehead culture also yields promising returns (Menasveta et al, 1975).
1.2 There are very limited data available on the nutrient content, chlorophyll “a” and other water quality parameters as related to fish production in the country. A promising research institution is under construction, the National Inland Fisheries Institute (NIFI) - a bilateral Thai-Canadian Project, which will make it possible to fill the gaps in this field. One of its aims is to perform studies for increasing fish production and reducing losses which occur in overstocked fishponds. In view of the acuteness of the problems associated with Clarias mortalities in fish ponds, the author concentrated on this phenomenon during his stay in the country.
Some aspects of the biology of Clarias batrachus (local name pla duk) make this species highly suitable for pond fish culture. The fish has a rapid rate of growth and attains a marketable weight of 100–125 g in 2.5–3 months from the fry stage. Two or three fish crops can be produced per pond per year. Due to an accessory air breathing organ, Clarias can tolerate water of low quality and particularly low dissolved oxygen content. It requires, however, a diet containing substantial amounts of protein for its growth, which is supplied with the use of trash fish, rice bran, cooked rice and hog supplement, applied twice daily at a rate of 10 per cent of body weight per day. The market price for Clarias is high and more stable than the price for other pond cultured fish.
Typical earthen ponds are 100 to 1000 m2 in area and 1.5–3 m deep. There are reportedly about 10 000 of these ponds in the area north of Bangkok. Typical stocking densities are 80–100 fish/m2; typical recoveries range from 10 to 70 per cent. A normal good yield is up to 50 tons/ha/crop (5 kg/m2). The ponds are supplied by irrigation water from the adjacent river (Chao Phraya). The water in the ponds is stagnant, but can be partially renewed for new water from the irrigation canal.
Whitaker (1974) analyzed the management and water supply of the Clarias culture ponds and specified problems which markedly reduce production and endanger the economic success of the operation. He considered water to be the limiting factor to Clarias production, and the amount and quality of irrigation water available in this area determines the number of ponds and the amount of fish that can be produced. When the amount of fish stock exceeds the carrying capacity of the water supply, water quality and the condition of the fish deteriorate and mortality increases due to rapid spread of protozoan and bacterial diseases and parasites.
There are two major causes for deterioration of water quality in catfish ponds. One is the mechanics of supply of the irrigation water. A single ditch from the Chao Phraya River serves for both supply and discharge. It is used as a source of high quality water to fill and refresh the ponds and also serves as a means of disposal for low quality water which is pumped out of the ponds. Hence it can happen that one producer would be filling his ponds with water that has been discarded by his upstream neighbor. Such a situation is conducive for the transmission of diseases among all producers in the region. Another cause of pond water deterioration is the type of food used and the feeding regime. When fed at current rates, much of the trash fish-rice mixture is not eaten and sinks to the bottom of the pond. Besides representing economic waste, this layer of decomposing food reduces the dissolved oxygen content of the water due to its high BOD, and leads to the production of potentially toxic compounds like hydrogen sulfide. Pond water often contains no or extremely low dissolved oxygen and dangerous levels of hydrogen sulfide (0.5 mg/l as total sulfide). While this H2S concentration is not in itself lethal, it may seriously affect the state of health of the fish and facilitates the spread of disease.
It is obvious that small-sized ponds with extremely high stocking densities, high feeding rates and without adequate water exchange must result in drastic deterioration of water quality. Fortunately, Clarias spp. can theoretically survive in water without any dissolved oxygen due to its air-breathing organs. Oxygen depletion is therefore not the primary cause of catfish mortalities, nor is it high hydrogen sulfide content, which at normal pH levels of about 7 (Potaros personal communication, 1975) in a non-toxic dissociated form. This is, however, only an assumption, as sufficient data on these parameters were not yet available. Accumulation and bacterial decomposition of organic material in ponds can however seriously affect the state of health of fish and increase their susceptibility to diseases. Water quality in this case, appears to be a secondary factor creating conditions for fish mortalities and not the primary cause as it was in Laguna de Bay, Philippines. Nevertheless, attention has to be paid to this phenomenon within the NIFI research programme and basic information on water quality and phytoplankton of the Clarias ponds must be obtained as a means to check the proper manipulation of the ponds for high production.
The author is in full agreement with the analysis of the problem and possible remedies as proposed by Whitaker (1974). These can be summarized as follows:
In order to increase survival rates and production, water quality in catfish ponds must be improved by: (1) Reducing the number of ponds and subsequently the fish stock to a level within the carrying capacity of the available water supply, and (2) by changing the system of water supply so that one ditch from the Chao Phraya River serves as a supply canal for all procedures while a second serves as a discharge canal.
The use of dry pellet feeds containing fishmeal would avoid most of the problems associated with the present wasteful feeding using ground wet feed/feeds. Pellet feeds can be formulated to supply the nutritional requirements of fish; hence the pellet is almost entirely absorbed during digestion and fecal volume is decreased.
Artificial aeration would significantly improve water quality in catfish ponds. Electric pond aerators spray a thin sheet of water into the air and air stripping of toxic compounds such as ammonia and hydrogen sulfide takes place. The oxygen content of the water is increased and some disease-causing organisms are killed through exposure to sunlight. Occasional but not constant aeration would probably be necessary and this can be determined by adequate water quality monitoring and experimental tests on selected ponds.
Under the auspices of the newly established National Inland Fisheries Institute at Bangkhen, a monitoring programme should be set up to include 8–10 representative selected Clarias ponds, covering different magnitudes of catfish mortalities (10–70 per cent).
The following parameters are recommended to be measured:
dissolved oxygen
total ammonia
nitrate
nitrite
soluble reactive phosphorus
chlorophyll “a”
pH
hydrogen sulfide (free and total)
specific conductance
dissolved organic nitrogen
dissolved organic phosphorus
dissolved organic carbon
dissolved inorganic carbon (or alkalinity)
BOD
Redox potential
total organic matter (oxidability or loss by ignition)
Secchi disc transparency
primary productivity (oxygen method)
Sampling frequency: weekly intervals, primary productivity in 2–3 week intervals. Sampling sites: centre of each pond, surface and the near-bottom layer.
Since NIFI is going to be equipped with the essential laboratory and field instrumentation supplied through bilateral assistance by an institute it is recommended that the standard methods of that Institute be used (Stainton et al, 1974). Experts from the assisting agency will probably be available to introduce these methods. As the ponds are within a one-hour drive from NIFI, it will be possible to process samples at the central NIFI laboratory.
In addition to the above-mentioned parameters, information on major ion chemistry and some trace metals should be gathered at least twice a year, namely during the dry and rainy season (sulphate, chloride, bicarbonate-carbonate, Na, K, Ca, Mg, Fe, Mn).
Monitoring of water quality parameters will make possible improved pond management as previously recommended (Whitaker, 1974) and help to maintain adequate or optimum water quality in the ponds by indicating early stages of deterioration.
The pond culture of Clarias in Thailand has the potential for economic success provided the problems of water quality and diseases can be adequately contained. This situation should be rectified as soon as possible in order to prevent severe economic hardship among producers. Regular monitoring of water quality parameters will make possible an evaluation of current pond management practices and will draw attention to deteriorating water quality and the need for corrective measures.
The consultant visited the Bung Borapet reservoir, which is the oldest of Thai reservoirs (built in 1930). An important feature of this reservoir, which is used primarily for fish production and flood control, is fluctuation of its water levels. The fluctuations vary annually by 2–4 m, and can be manipulated. As the terrain is flat, these fluctuations expose considerable amounts of inundation area, which becomes overgrown rapidly (within 3 weeks) by rooted plants, namely Leersia luxandra, Hymenachne psendoitterum, Isachne globosa, Imperata cylindrica and others. The original intention of the Nakhon Sawan Fisheries Station on Bung Borapet reservoir was to utilize these exposed areas for extension of spawning grounds and increasing productivity of major natural fish species namely snakehead and giant snakehead.
The consultant suggested that the green plant material be utilized as natural fertilizer by raising the water level in the reservoir and letting the plants partially decompose under water, releasing nutrients from their cells into water for uptake by algae. This will increase both plankton and benthic production for fish food.
A laboratory experiment with aquaria was recommended, which will give general information on the nutrient content of the plants, its availability, and time required for release into the water column. The research team of the Station is interested in the suggestion and will incorporate the experiment in the research programme. Details of the experiment, parameters to measure, and reporting the results were discussed and worked out on site.
The consultant also assisted in revising the Thailand pollution proposal after discussions with Thai authorities, CIDA team and UNDP representatives (in conjunction with R.F. Johnson's mission for SCSP).
It was arranged with Canadian and Thai representatives that the bilaterally-funded National Inland Fisheries Institute (NIFI) at Bangkhen will provide laboratory space for pollution monitoring equipment and experts for the proposed project (gas chromatograph, atomic absorption spectrophotometer plus two experts).
