When expansion of an activity such as brackishwater aquaculture is considered in terms of the overall development strategy for a country, (and in some of them it may be a new activity), it is important to have realistic ideas about the types and extent of problems to be encountered, as well as about likely yields and the time scale to reach those yields. The problem of acid sulfate soils in brackishwater aquaculture has been recognized for more than a decade (Potter, 1976; Pedini, 1981; Ti, 1981; Brinkman and Singh, 1982) in several countries, including the Philippines and Malaysia, and given knowledge of the problems generated by the utilization of acid sulfate soils, some measures can be recommended for their development into shrimp and fish ponds. These criteria have been discussed in Cook, 1976; Singh, 1980, 1982; Pedini, 1981; Brinkman and Singh, 1982. Research to verify the hypotheses was carried out by the UNDP/FAO “Coastal Aquaculture Demonstration and Training Project” (Cook, Pongsuwana and Wechasitt, 1984) and the application of these concepts has already shown promising results in Malaysia, where production levels comparable to those of normal farms have been achieved with few undesirable side effects. A new farm with a total surface area of 250 ha was constructed using soil from the mounds of mud lobsters to cover the embankment of pond dikes and parts of the pond bottom, thus limiting problems of acid runoff from dikes (Yunker and Scura, 1984).
The recommendations can be divided into three broad categories reflecting the sequence of events required to establish a new facility for brackishwater aquaculture: site selection, design concepts and management criteria.
For site selection, all authors recommended a careful survey of the area to be developed into brackishwater ponds to ascertain the existence of acid sulfate soils prior to initiating construction and to quantify the extent and gravity of the problem. Cook, Pongsuwana and Wechasitt (1984) point out that in many cases the upper 20–30 cm of soil tend to be suitable for dyke construction even if deeper layers are very acid. After this initial analysis of the soils it can be decided what type of farm design is more suitable. In the case of good soils, several design and management alternatives exist, depending upon physical characteristics of the site, cost of construction, cost of operations, and interest to be paid on loans. These alternatives can be found in several other publications, such as de la Cruz (1983). Should the initial analysis indicate the presence of large amounts of pyrite in the soils, then greater care is necessary for selection of the design and type of construction. Two general approaches have been proposed in the literature.1
The first accepts the traditional Southeast Asian pond design and then suggests special preparation of the ponds to shorten the time required to reduce acidity problems which are likely to arise; the second school advocates a different design concept to minimize acidity problems from the very start, but requiring acceptance of a new (for Southeast Asia) approach in management too. The first approach can be applied to existing ponds, as it provides tools for their reclamation. The second school (Cook, 1976; Pedini, 1981; Cook, Pongsuwana and Wechasitt, 1984) recommends construction of new ponds with minimum excavation, and proposes the use of pumps for filling ponds instead of tidal exchanges employed in the traditional practices.
This second approach advocates minimal excavation in order not to expose deep soil layers which are usually more problematic in terms of acid and toxic ion release, and to avoid building massive dykes with the excavated soil which would then oxidize and create serious acidity leaching problems. Additional advantages of having small dykes is that the percentage of usable water surface increases and that the amount of runoff from dykes diminishes, lowering the amounts of acid and toxic metals that may leach into the ponds. In addition, top soil layers with lower acidity should be used to cover the surface of the dykes (Cook, Pongsuwana and Wechasitt, 1984), an approach which has been verified recently in Malaysia (Yunker and Scura, 1984).
The Gelang Patah group has also proposed the use of pumps to fill ponds. Using pumps is not new in Asia being commonly employed in both Thailand and Taiwan, province of China; their use is widespread in Latin American countries, where the vast majority are filled by pumping. Their use allows construction of ponds in areas which are at higher ground elevation than traditional tidal ponds in or near mangrove forest. This reduces considerably the cost of clearing land and permits utilization of elevated marshes not suitable for agricultural use because of the salt content of the soils, and limits the damage to mangrove forest. In Southeast Asia use of pumps is viewed with some suspicion because of extra operating costs and because management schemes using tidal exchange have been considered as preferable by many planners, extensionists and farmers. However, recent studies comparing new pond designs requiring the use of pumps, and traditional pond construction (Gedney, Shang and Cook, 1983) show clearly the economic advantage of using pumps (Table 9) in some situations. The use of pumps also has advantages for management purposes which will be briefly discussed later.
As flow-through ponds are usually recommended, two types of pond design have been suggested:
the first one with separate intake and outlet (at opposite ends of the ponds) requiring two different canals but ensuring good flushing of the pond and an homogenous distribution of the shrimp in the pond; and
a U-shaped pond, in which water circulation is maintained with mechanical agitation, but a single canal is used for water exchanges.
Regarding management measures, one of the earliest recipes for ponds having acidity problems was a long period of repeated flushing and extensive use of lime, a type of management which can require years before substantial improvement in the condition of the ponds occurs. Low productivity of fish ponds sometimes can be substantially improved with large phosphate additions. However, heavy fertilization may prove to be uneconomical in terms of increased production. The total capacity to fix phosphate in some pond muds in Malaysia is in the order of 8 t of triple super phosphate/ha, which is far in excess of practical fertilization rates (Watts, 1969). Thus, if phosphate fertilization is to be effective, some alternatives to broadcast applications are needed and ideally mud acidity levels should be reduced substantially (Watts, 1969). Fertilizer application rates of 40–120 kg/ha of triple super phosphate have produced favourable fish yield response in many freshwater ponds (Hickling, 1962; Boyd, 1979) and are regularly recommended by many extension services. Such applications will usually have little positive effect in brackishwater ponds in acid sulfate soils because the phosphate will become rapidly immobilized in the sediments.
Measures to reduce the period necessary for reclaiming ponds with acid sulfate soil problems and to increase yields from reclaimed ponds were first proposed by Singh (1980), Brinkman and Singh (1982) and Singh (1982). In the first case, reclamation, attention was directed to pond bottoms and to dykes using methods that can be summarized as follows:
For pond bottoms, a repetitive sequence of filling the pond with sea-water, draining and drying, followed by cultivation with tooth harrow and broadcasting of small amounts of lime without incorporating it into the soil was recommended. Concurrently small ditches should be built on the top of the dykes and clean (non-acidic) seawater pumped into them. The whole process requires from 2 to 3 months and should be started at the onset of a dry season. The amount of lime used for pond bottoms and dykes was in the order of 1 ton/ha. Water should be drained from the pond after a constant pH of 5 is measured in the water, and harrowing should be repeated if a pH value of 5 is measured after first filling of the pond.
To reduce phosphate fixation by aluminium and iron, broadcasting of silica rich ash of rice hulls was recommended.
For the management of ponds after reclamation, the following points were recommended (Singh, 1982):
After the first flooding, use 500 kg/ha of agricultural lime to speed up soil acidity reduction, then use organic manure (chicken manure preferably) at a dosage between 2 and 5 t/ha, depending on the soil organic matter content.
If after one week no algal growth is observed, inoculate the pond with algae.
To maintain sufficient dissolved nutrients small doses of phosphate should be added on a weekly basis.
It is recommended to stock older fish fingerlings (5 g instead of the more usual 1–2 g) after the algal mat has developed. During the first year of operation it is preferable to stock the pond with fish rather than with shrimps, which are more sensitive to water quality deterioriation.
Monitor the pH regularly and should it reach as low as pH 5, especially during rainy periods, scatter agricultural lime on the slopes of the dykes.
During harvest take out only standing water and dry only the surface of the pond bottom. Do not allow digging of drains and formation of deep cracks on the pond surface.
During the early 1980's, the Gelang Patah group had to cope with a station that was severely affected by problems derived from excavation of ponds in acid sulfate soil layers and which had enormous dykes built with the excavated soil. The farm had very low production and frequent fish and shrimp kills. After quantifying the extent of the problems, it was quite obvious that traditional remedial measures to reclaim these ponds would be of little avail and would require too long a time to be economically practical. The experience at Gelang Patah led to a series of managerial recommendations, some of which imply a clear departure from previous practices. To control acid and toxic metal release from dykes due to fluctuation of water levels and to runoff from rain, the following were suggested:
Use pumps to eliminate or reduce to a minimum variations of water levels in the ponds which add interstitial pore water from dykes (high in acid and toxic metals) to pond water, especially when volume is minimal in tidal ponds, thus stocked animals are most vulnerable.
Plant vegetation on dykes, to reduce soil erosion by rain (thus reducing acid runoff into the pond) and to favour iron uptake by plants.
Other recommendations related to pond bottom preparation and water management, are:
To remove iron as well as acid during pond preparation, after drying till the bottom and fill the pond with only a little water which will then become very acid. When the pH reaches values below pH 4, flush the water and repeat the process. Do not let the water reach a pH of 5 or higher since iron will not be soluble much above pH 4 and will precipitate and be retained in the sediments.
Do not add organic fertilizers during pond preparation as the bacteria involved in the oxidation of sulfides use them as a source of carbon to produce acid. Avoid overfeeding of culture organisms and do not use the peripheral canal to broadcoast feed into the ponds. Over-feeding can lead to anaerobic bottom waters and hydrogen sulfide formation due to the relatively low capacity of these ponds for digestion of organic matter.
Continuous water exchange is recommended to help maintain alkaline reserve in the pond water, to wash out harmful substances, and to maintain a water depth between 80 cm and 1 m for shrimp ponds. Maximum volume ensures maximum dilution of acid and toxic metals. As continuous water exchange may stress young fry, nursing of fry probably should occur in tanks rather than in earthen ponds. These suggested practices imply the use of pumps to accomplish water exchange.
Due to the relatively deep water in these ponds (80–100 cm) they have a tendency to stratify, and thus mechanical aeration to move the water and to add more oxygen to the bottom water layers may be necessary. Airlift type aerators have been tested successfully.
The main differences in these management practices compared with earlier recommendations are:
Use of agricultural lime is not recommended as a critical step to reduce acidity.
Inorganic fertilizers to boost primary production are preferred over organic fertilizers to reduce as much as possible the sources of free organic carbon which could favour the build up of sulfur oxidizing bacterial populations.
Pond water should be flushed when pH levels are below 4 instead of at 5, in order to maintain iron in solution and thus remove as much iron as possible from the pond sediments.
Continuous exchange of water in the ponds and retention of a water depth of 80–100 cm for shrimp ponds is preferred over use of tidal fluctuations to exchange pond water.
As can be seen, management procedures proposed by the Gelang Patah group represent a drastic departure from traditional management of brackishwater ponds in Southeast Asia. This departure is especially great for areas where the tidal ranges are more than 2.5 m, thus requiring construction of rather massive dykes, in the traditional way, and which for this reason will encounter more severe problems if pyrite is present in the soils and sediments.
While earlier suggestions to improve culture organism productivity are based on accelerating the leaching of acid with extra inputs of time, work, lime and fertilizers, to build up progressively the production, the second school approach attempts to minimize any leaching into the pond to obtain a level of production similar to normal ponds operated in a semi-intensive way from the very beginning. This second proposal reduces costs in the initial excavation, and increases operational costs to run the pumps. Both of the two approaches can bring substantial improvements to production in ponds affected by acid sulfate soils problems, and should be considered as complementary rather than alternative solutions.
Summing up, we would recommend the following for three different situations:
In all cases careful soil and sediment examination should clearly outline the presence, distribution and quantities of pyrite mineralization, preferably before new capital commitments are made.
For areas to be developed for the first time, design criteria of ponds should emphasize location inshore of the mangrove, using pumps for water exchange and with minimal earth movement.
For areas in which old ponds have to be renovated, minimal soil disturbance with minimal excavation should occur. Use pumps as necessary to elevate pond water levels, and definitely obtain soil survey information on the distribution of pyrite before beginning renovation.
For areas with new or relatively new ponds, in which problems due to acid sulfate soils are experienced, consider suggestions proposed by Singh (1982), modified to include flushing of water from the pond at pH 4 instead of 5, and use of pumps to maintain relatively constant water levels and little or no liming.
