FISHPOND ENGINEERING: A TECHNICAL MANUAL FOR SMALL-AND MEDIUM-SCALE COASTAL FISH FARMS IN SOUTHEAST ASIA
C.R. dela Cruz1
Coastal pond aquaculture is best developed in southeast Asia and has existed in this region for a long time. It is an important source of food and has provided income and employment to thousands of people in the region. Based on a 1978 regional compilation2, it is estimated that there are over 400 000 hectares of developed coastal fishponds in the region, providing close to 300 000 metric tons of fish and crustaceans. Valued at equivalent to over US$350 million (Table 1.1). It is also estimated that there are over 70 000 coastal fish farm units in the region involving the employment of close to 30 000 workers (Fig. 1.1).
Some 413 484 hectares of coastal fishponds are located mainly in the Philippines. Indonesia, Thailand, Vietman, Malaysia, Taiwan (China). Hong Kong and Singapore. They are used for raising finfish, mainly milkfish such as in Indonesia, Philippines and Taiwan or penaid shrimps such as in Thailand, Singapore and Malaysia. At present, polyculture of milkfish and penacid shrimps and stocking of sea-bass are fast developing.
Fig.1.1 Areas in Southeast Asia where coastal fish farming is practiced
1 Consultant (Aquaculture Engineering). South China Sea Fisheries Development and Coordinating Programme. Manila, Philippines.
2 SEAFDEC. Fisheres Statistical Bulletin for the South China Sea Area. 1978 (1980).
Status and potential of brackishwater (coastal) pond aquaculture in Southeast Asia1
|Area (ha)||Production (mt)||Value (1 000 US$)||Potential areas for development (1 000 ha)1|
|China (Taiwan)||18 665||50 317||75 981||-|
|Hong Kong||1 975||1 385||4 527||-|
|Indonesia||171 544||87 995||109 592||3 618|
|Philippines||176 231||118 682||130 062||220|
|Thailand||24 962||10 049||18 392||312|
|Vietman3||20 000||10 000||12 000||600|
|TOTAL||413 484||278 462||350 595||5 455|
1 Based mainly on data from Fishery Statistical Bulletin for the South China Sea Area. 1978. SEAFDEC (1980).
2 Calculated from SS2.45 kg.
3 Data on Kampuchea and Vietnam are based on information from government obtained by SCSP staff during travel to these countries.
4 Based on total area of unexploited mangroves in Peninsular Malaysia and in Sabah and Sarawak states.
5 Based on SCS 80 WP 94 (Revised).
Development of this industry at the beginning can be attributed primarily on private sector initiative. Based on very crude facilities and simple rural experience, the production has been generally low, at present about 670 kg/ha/year on the average in the whole region, although ranges from as low as 300 kg/ha to as high as 2 000 kg/ha (e.g., Taiwan) occur. The value of production from this industry is evaluated at over US$350 million per year, which can be a big boost to the economy of the region (Table 1). Prices has been relatively low for milkfish (average US$1/kg) but rather high for penaeid shrimps (US$3-8 kg) and also good for seabass (US$3-5/kg).
The potentials for the further development of this industry in the region is high. There are still large acreage of mangrove swamps and tidal mudflats that can be suitable sites for development into fishponds, estimated at about 5½ million hectares in the Southeast Asian countries (Table 1). Of course, it has now been realized that for ecological balance and for rational conservation of the fishery resources, some part of these mangrove areas should be reserved. But if some 10 to 30 percent of the existing swamps can be developed, such a ratio being deemed feasible and affords proper ecological considerations, the area that can be developed in the region can be about 0.5 to 1.6 million hectares.
Also the average production of about 670 kg/ha can easily be increased to double this amount with better engineering and the use of improved technology of management in the existing areas. This can increase the production capacity to over 550 000 metric tons per year. The additional pond areas to be developed can likewise contribute about 600 000 to 1.9 million metric tons per year or an overall production of 1 to 2 million metric tons per year in this region. Likewise, the value of production can considerably be increased.
Poor or inadequate engineering of coastal fishponds is one of the major causes of low production and/or failure. It can be said that 30 to 50 percent are inadequately constructed while close to 100 percent can take further engineering improvements. Such engineering deficiencies can be classified into three categories, viz. (i) problems brought about by climatic and hydrological factors: (ii) problems due to environmental factors; and (iii) engineering specific problems.
The type of rainfall, occurrence of typhoons, and prevailing tidal charcteristics in the fishpond location can influence the nature of construction of fishponds in such area. Where rains are strong and severe and where typhoons are frequent, the fishpond structures need to be bigger and more firm. Likewise, areas with high tidal ranges (average daily range of 3 m or more) will require bigger dikes and sturdy water control structures, whereas areas where the tidal fluctuation is small (one meter or less daily range), the dikes can be smaller and water gates need not be massive. Areas prone to earthquakes and tidal waves should likewise make some extra provision for these occurrences.
The engineering of coastal fishponds can be affected by various environmental influences. These include such factors as the nature of the soil, vegetation, elevation of site, topographic characteristics, availability of freshwater supply and occurrence of pollution. If the site has porous type soil (sandy or peaty), bigger dikes need to be provided. In some cases, better clayey soil for diking may have to be brought from outside. Well vegetated areas especially with big-size trees will require bigger construction effort. Elevation of the site based on the tidal datum will determine whether excavation or filling will be required, while those with uneven topography will need more work in levelling the area.
It is better to have some source of freshwater supply for coastal fishponds so that the brackishwater salinity which is usually more suitable for growing food organisms as well as the cultured species can be maintained. If however this is not available, the fishpond should be so engineered so that the periodic occurrence of freshwater such as from rains can be taken advantage of.
Freshwater supply from the tidal river or stream is usually the cheapest source of freshwater as this can be taken in by gravity. However, this may not be available so that other sources have to be determined and tapped. The seasonal rains can be another source, although this can be seasonal and not very reliable. Underground water is another source of this, if available. Sometimes the pond bottom is low enough in relation to the water table so that underground water can seep in naturally to the ponds. Piped water through wells of varying depths is good, if this is available. All the above sources of freshwater will need engineering structures so that the required water can be put into use. Pumps, either to draw in or drain out excess water, may be found necessary and helpful.
Occurrence of pollution is a difficult problem in coastal fishpond areas and should be avoided if this was noted before the farm is established. However, if this condition should happen after the fishpond has been constructed, additional structures may need to be installed to minimize the effects of this adverse factor.
These are the site specific problems that are encountered during actual construction or after the construction of the fishpond. For instance, after the fishpond has been constructed, there is a need to shift the kind of management from the traditional extensive method to the modular progression method or to the stock manipulation method: this will require a renovation of the layout of the fishpond system. Again, if the fishpond is to shift from milkfish monoculture to milkfish shrimp polyculture or to shrimp monoculture, some definite pond modifications have to be made for such a shift.
During the construction, it sometimes occur that there is excess soil that needs to be disposed of properly, or there may be lack of soil that can be adequate for the needed diking or filling work. These have to be solved through engineering means.
Many engineering problems occur with regard to the water control structures. These have to be properly designed and well constructed and located in appropriate places in relation to the entire fishpond system. These structures are usually expensive to put up and once made they are very difficult to change. It is noted however, that some progress have been attaind in better designs and in the method of constructing these water control structures. More lasting materials like fiberglass, ferrocement, etc., especially if these can be prefabricated may lessen the inherent costs encountered with these fishpond structures.
Correcting water leakages and seepages in finished fishponds often present many problems. Even if these have to be dealt with on a case to case basis, there is need for aquaculture engineers to develop and improve the technology involving these very frequent problems in coastal fishponds.