Chapter 12
RECOMMENDATIONS FOR CONSTRUCTION AND MANAGEMENT OF BRACKISHWATER AQUACULTURE PONDS IN AREAS WITH ACID SULFATE SOIL

by

H.L. Cook, Umpol Pongsuwana and Somnuk Wechasitt

1. INTRODUCTION AND BACKGROUND INFORMATION

Efforts to develop pond culture at the Brackishwater Aquaculture Research Centre, Gelang Patah, Johor Bahru, Malaysia have encountered problems. Mortality of the species cultured is high and growth is frequently slow due to a number of complex conditions associated with acid sulphate soils. The major problem is entry of acidic water containing high levels of dissolved iron into the ponds. There are three principal ways by which the acidic water enters a pond. The most important is runoff of rainwater which becomes acidic after reacting with oxidized pyrite in the soil (Potter, 1976). Another problem is entry of acidic dike pore water during draining and filling the ponds with tidal water (see Chapter 2 of this report). There is also some addition by seepage through the dikes. The pond bottom is not a continuing source of acid once it is leached well and kept submerged.

Little is known of the mechanism through which mortalities occur. There is probably no single causative factor, different factors probably play a dominant role at different times or coexist. The most obvious are reduction of pH and low levels of dissolved oxygen (DO) in bottom water as a result of stratification. Associated changes in water chemistry are perhaps more important to the cultured species than the effect of low pH itself. Wickins (1976) pointed out the importance of distinguishing between an absolute value and events associated with the change of pH toward acidity and Singh (1980) stressed that while occasional mass mortalities may be observed, the chronic sublethal effects which adversely affect pond biota in general are perhaps most detrimental on a long-term basis.

With reduction in pH there is a rapid loss of alkalinity with a concurrent rise in CO₂ to possibly toxic levels. Wickins (1976) reported that, at pH 6.4 in the presence of inorganic carbon, Penaeus monodon had reduced growth, but that survival was normal. Heavy mortalities did occur when there was a loss or rapid reduction of inorganic carbon when the pH fell. To illustrate the importance of inorganic carbon to Crustacea Wickins reviewed the work of several researchers. Probably most applicable to the conditions in these ponds was that of Adelung (1971) who showed that in Carcinus calcification was inhibited and 80% mortality occurred at ecdysis after culture for 7–8 days in seawater which had the pH reduced to 6.4.

At this station P. monodon cultured in ponds with low pH frequently suffer from a soft shell condition. Examination showed that except for poor calcification of the shell the shrimp were normal. The problem of soft shell is probably related to the low pH values of the pond water (D. Lightner, personal communication).

High levels of carbon dioxide can be toxic to fish, both directly and indirectly by decreasing their ability to extract oxygen from water (Spotte, 1979). Truchot (1983) reported that an increase of CO₂ in the external water causes the blood pH to drop in crayfish (Astacus leptodactylus) and crab (Carcinus maenas) and that at the lower pH the blood can carry less oxygen. A loss of ionic regulation capability for both fish and invertebrates in water of low pH was reported by France (1982).

In addition to the effects of low pH on algae precipitation of phosphorus from the pond water by iron and aluminium limits phytoplankton production. Also, heterocoagulation of colloidal iron particles on algae and particulate organic matter removes them from the water by sedimentation. Due to the low natural productivity, supplemental feed must be supplied. The feed residues add organic matter to the bottom which increases the problem of anaerobsis of the pond bottom.

In brackishwater ponds the difference in water chemistry associated with acidic run-in reinforce stratification of the pond water. This contributes to a reduction of oxygen in the bottom which results in a decrease in redox potential. When the reduced zone rises to the surface most bottom-dwelling organisms die and an important source of food is lost. As a result of increased sulphate reduction sulphide is produced and diffuses to the surface (Ellaway, et al., 1980). Un-ionized H₂S is toxic to fish and at low pH the percent of un-ionized H₂S increases. At 32°C only 7.6 percent of the H₂S is un-ionized, but at pH 5.0 it rises to 98.9 percent (Boyd, 1979).

The gills of shrimp (and other crustaceans in the ponds) become coated with iron, and the shrimp die - presumably by suffocation. This occurs frequently after heavy rains, but the shrimp also suffer from red gills during intervals when there is no rain to add iron to the pond water. Iron bacteria are present in the ponds (see Chapter 2 of this report) and may play a role in producing the form of iron which causes mortality. At times the iron bacteria are abundant, especially along the sides of the ponds where ironrich water seeps out of the dike during tidal exchange. Oxidation of iron by bacteria might result in significant consumption of alkalinity at times.

Problems connected with brackishwater ponds constructed on acid sulphate mangrove soil are not unique to this station. The problem is widespread. Singh (1980) has reviewed the situation in some detail. Most plans for construction of brackishwater fishpond envisage utilization of mangrove swamps. It is not known how much of the mangrove soil in Malaysia is acidic, or potentially acidic, but evidence shows that it is extensive. Chow and Ng (1973) state that the area of acid sulphate soils in West Malaysia is 80 000 ha, mainly in Kedah, Perlis, Malaka and Southwest Johor. The area they refer to includes agricultural land, but there is an estimated 34 700 ha of mangrove in these states. Allbrook (1972) reports acid sulphate mangrove in Kedah, and Andriesse et al. (1972) report acid sulphate soils in mangrove and deltaic estuarine areas along the west coast of Sarawak.

On a regional and world basis it is also important. According to Kawalac (1973) in the tropics there are several million hectares of acid sulphate soils and potential acid sediments cover at least 7.5 million ha. Chow and Ng (1973) report that there are nearly 5 million ha of acid or potentially acid sulphate soils in Southeast Asia and over 15 million in the tropics. Ruddle (1982) states that in the young marine deposits in the coastal areas of Sumatra, Kalimantan and Irian Jaya, acid sulphate soils cover 2 million ha. That attempts to use coastal mangrove for aquaculture will present problems can be seen from Tang's (1979) estimate that out of a total of 170 000 ha of coastal fish farms in the Philippines approximately 60 percent, or 100 000 ha were affected by soil acidity.

2. DESIGN CONCEPTS

In order to make use of coastal mangrove the problems associated with acid sulphate soils must be attacked on two fronts: elimination or reduction of the amount of acid entering the ponds and minimizing the effect of the acid once it has entered a pond. The best way to reduce the amount of acid entering ponds is by appropriate design and engineering. The condition or conditions causing problems must be understood and then features must be incorporated in the design to eliminate or reduce the effect. In many cases this will require deviation from traditional concepts of pond design developed in other countries.

2.1 USE OF SOIL WITH THE LEAST ACIDITY

The intensity of acidification in any area is patchy and in general the deeper the soil is excavated the more acid the soil. Also, the content of iron, sulphur and aluminium increases with depth. Frequently, the top 20–30 cm of soil is good. An example of this can be seen in a study of soil distribution in the delta of the Sarawak River (Andriesse et al., 1972). In the Sarawak River Delta there is only a small area of Pendam series soil which has acid and potentially acid sulphate soil (pH 3.9) in the surface 15 cm. Most of the area is composed of Rajang series soil which is characterized by top soil with a near to neutral pH and subsoil (60–75 cm depth) which is potentially acidic. If brackishwater ponds were built in the Pendam series area they would experience problems no matter how they were constructed. In the Rajang series area severe problems would be encountered if it was decided to excavate to expose the potentially acid sulphate soil which occurs at depth. It is preferable for ponds to be constructed with only minimal excavation. The land should be levelled and only enough of the neutral top soil to build the dikes should be used.

At this point it should be remembered that pH is a logarithmic function. That is, pH 4 is ten times more acidic than pH 5, and pH 3 is ten times more acidic than pH 4 and 100 times more acidic than pH 5. Thus tremendous leverage is obtained when soil with a higher pH is used for the dikes. It may not be necessary to have the dike soils near to neutrality to obtain significant benefit. As pointed out in this report many of the problems in shrimp culture involve iron. Iron is not soluble at pH 4 and above. There are few places where the surface layer is lower than pH 4.

2.2 THE DIKES SHOULD BE AS SMALL AS POSSIBLE

In most places it is necessary to excavate in order to locate pond bottoms at a level where tidal fluctuation can be used to fill the ponds with water. That was the case at the Gelang Patah Research Centre. Natural ground elevation at the site rose to 1.8 m above mean sea level (MSL). Excavation was undertaken to situate the pond bottoms at 30 and 45 cm above MSL. As the excavated soil must be disposed of, in this case it was incorporated into dikes which resulted as massive. The dikes around one 0.25 ha pond adjacent to the main dike were measured as an example. The height of the dikes varied from 1.1 to 2.8 m above the water line. Distance from the mid-point of the dike to the water varied from 4.5 to 16 m. The surface area of the dike amounted to about 36 percent of the total area and 55 percent of the pond water surface. The great soil surface area ensures that a large amount of runoff will enter the pond when it rains, and the fact that it was excavated from depth ensures that the runoff will be very acidic, with pH under 3.5.

If the pond had been built by levelling the land and using only top soil to construct a small dike it is assumed that the height of the dike would be 0.5 m above water and the average distance from the mid-point of the dike to the water would be only 2.2 m. This would reduce the surface area of the dikes to only 18 percent of the total area and 15 percent of the water area. Rainwater runoff would be drastically reduced, as in addition to the reduction in the surface area of the dike, the volume of water in the pond had been increased. In the existing pond a 10 cm rain would result in 137 m³ of rainwater runoff into 2 400 m³ of pond water. In a non-excavated pond only 60.5 m³ of rainwater would run off into 3 312 m³ of pond water. The amount of run-off would be reduced from 5.5 to 1.8 percent of the volume of water in the pond. Even if the runoff was acidic the effect on the pond would be drastically reduced, but as seen earlier the top soil would probably be less acidic.

An experiment was carried out to assess the effect of adding acid water to the ponds. Thoroughly dried dike soil was pulverized and different amounts were added to two beakers of tap water to make them acidic. One solution was made with a pH of 3.44 and another with pH 2.88. Each of these solutions was added sequentially to brackishwater from the ponds which had a pH of 6.9. The resulting pH values were then plotted as a percentage of total water volume. This showed that adding 5.5% of pH 2.88 water reduces the pH of the pond water 19.5% and adding the same amount of pH 3.44 solution reduces pH by 9%. Adding only 1.8% of the pond volume at the same pH would reduce the pond water pH by 8.8 and 5.2% respectively. Thus by reducing the amount of runoff from 5.5 to 1.8 percent of the pond volume the reduction of pond water pH would decrease from 19.5 to 8.8%.

This also points to the importance of reducing pH of the runoff water. A reduction of only 0.56 pH units would exert about the same effect as reducing the amount of runoff by 67 percent. The change in pH would be reduced from 19.5 to 9 percent.

2.3 INCREASING THE VOLUME OF WATER IN A POND

The volume of water in a pond relative to the surface area of dike subject to runoff can be increased in two ways; increasing depth and increasing pond size. Both of these options have serious limitations if a tidal water supply is planned. The primary problem revolves around the elevation of the pond bottom. In order to ensure an adequate supply of tidal water the land must be excavated to construct the pond bottom at the proper elevation in relation to the tide. This usually requires some compromise. If the bottom is situated low enough to maintain a high level of water in the pond it is usually not possible to exchange water on neap tides. If water can be exchanged only on spring tides, management is restricted. If the bottom is too low, it is not possible to drain the ponds for harvesting and certain pond preparation procedures. Also, as already pointed out, excavation into the deeper zones exposes soils with a lower pH, and the soil removed must be incorporated into dikes or removed. Removal is very expensive. If the land has to be excavated, the cost of building very large ponds becomes prohibitive because the earth from the centre of the pond must be moved a long distance. In most locations ponds larger than 1 ha probably would not be economic if they have to be excavated.

2.4 ELIMINATING FLUCTUATIONS IN WATER LEVEL

It would seem that if water in the pond is of poor quality it should be changed. This itself has some bad effects. Draining and refilling a pond during water exchange causes movement of water in and out of the dike. The pH of interstitial dike water can be as low as 3 and iron content ranges from 100 to 200 ppm. Iron content of rainwater runoff is only of the order of 20–25 ppm. So even if less water is involved in the exchange of pore water its higher iron content increases its relative importance (see Chapter 2 of this report). This is an important factor even during dry periods and may account for a great deal of the iron-related problems. The only way to prevent this is to maintain a constant level of water in the pond.

2.5 INCREASING pH AND DECREASING IRON OF RUN-IN WATER

Several methods have been suggested for reducing the amount of acid and iron entering the pond. One of the most important is to establish vegetation on the dike and at the water's edge. Vegetation on a dike stabilizes the surface and prevents erosion so that the soil has time to weather before it is washed away. On acid sulphate dikes with a high clay content the surface cracks with only a small amount of drying. During the next rain many of the small dry particles are washed into the pond. As the soil particles are fully oxidized they contribute a maximum amount of acidity and iron to the pond water. Perhaps an equally important factor is that the fairly large pieces of soil form a porous zone right at the water's edge. This acts as a sink for small floating organic particles which are washed up to the shore by wind. Often the organic material which originates on the pond bottom (benthic bluegreen algae) is coated with iron. Also films of oxidized iron which form on the pond surface are blown on the pond edge by wind. Consequently, high levels of iron and organic matter accumulate in the soft intertidal zone. Due to the high level of organic matter the soil becomes anaerobic and in the absence of oxygen the iron assumes the soluble Fe⁺⁺ state. During drawdown for tidal water exchange the pore water high in iron seeps back into the pond and the cycle starts again. Planting grasses such as buffalo grass (Paspalum conjugatum) and digit grass (Digitaria decumbens) prevents this, because the grasses trap the soil particles before they enter the water. Furthermore, these grasses take up iron from the soil thus removing them from the cycle (Cook et al., 1984).

The main point brought out in the preceding sections is that in areas with acid sulphate, or potentially acid sulphate soil, ponds should be built with a minimum of excavation. Such ponds would naturally have a high bottom elevation and it is unlikely that tidal water exchange would be effective. Water would have to be pumped into such ponds. Pumps are frequently used to fill ponds in Thailand and South America, but many people still question the use of pumps as an economically viable alternative to “free” tidal water.

A case study was made by Gedney et al., (1983) to compare cost factors involved between constructing and operating tidal water exchange aquaculture farm systems and a pumped water system. A comparative assessment was made of three different systems:

• A tidal farm excavated to a depth of +0.3 m above mean sea level (MSL) from a natural ground elevation which varied from +0.9 m to +1.2 m.

• A tidal pond system excavated to a depth of +0.3 m above MSL from natural ground elevation which varied from +1.5 to 1.8 m.

• A pumped water pond system at a natural ground elevation varying from +1.8 to +2.7 m above MSL.

The study showed that a pump system offers advantages in terms of lower costs. A summary of construction and operating costs for the different pond system is given in Table 1. As the study was done at the Brackishwater Aquaculture Research Centre, costs and benefits reflect Malaysian conditions.

Table 1

ESTIMATED CAPITAL COSTS AND ANNUAL OPERATING COSTS
AND BREAK-EVEN PRODUCTION FOR TIDAL AND PUMP-OPERATED
SYSTEMS PER 1 ha OF WATER AREA

It can be seen that capital costs for construction of the pump-operated system are much lower than for the tidal system, and that the higher the ground elevation the greater the cost of construction of tidal ponds. Pumping causes additional costs, but this is more than compensated for by the cost of money for capital expenses. Annual interest and principal payment on a per hectare basis were estimated at US$ 9 067 for the deeply excavated pond, US$ 6 405 for the shallowly excavated ones and only US$ 4 787 for the pumped system. The annual cost for fuel and pump maintenance came to only US$ 1 101 per ha.

In terms of production, a deeply excavated pond would have to produce 559 kg/ha/year more shrimp just to pay for cost of additional capital. This is extremely important when considering how low production is in excavated tidal ponds built on acid sulphate soil. At this station the average production of P. monodon has been of the order of 270 kg/ha/ 90–100 days (Chuah, personal communication). If three crops were grown in one year, production would only amount to 710 kg/ha.

In ponds with a pumped water supply system and very small dikes the acid-sulphate related problems would be minimal and it is expected that production would be higher. A pumped water supply system would have additional benefits which should normally be expected to result in improved management and, consequently, increased production. For example, water could be exchanged when needed without waiting for the proper tidal cycle.

Fluctuations in water level could be eliminated. In tidal ponds it is often necessary to exchange water during the heat of the day. This mandates that a high level of water be maintained in the pond. If the full level of a pond is 60 cm, and a half of the water volume is exchanged, the drawdown level must be 30 cm. This is too shallow for effective temperature control and on sunny days it will become too hot. In addition, when water is maintained at shallow levels while waiting for the tide to turn, sunlight on the bottom is strong and production of benthic blue-green algae is high. It will float to the surface and cause problems by clogging screens during water drawdown. The bluegreen algae often accumulate at the edge of a pond where they decompose and contribute to poor water quality. If it rains hard during the period while the water is low, effects of the acid runoff would be accentuated. Many types of aeration and water movement devices do not operate effectively at such shallow depths. In ponds with a pumped water supply, water can be kept at a constant depth and water flowed through the ponds.

Lower full level of water can be maintained. When there is no necessity for drawdown during water exchange the pond water level can be kept at 60–80 cm. This is deep enough for temperature control, but shallow enough for sunlight penetration so that photosynthesis can supply oxygen at lower levels.

Water can be exchanged when needed. One of the greatest problems experienced is that bad conditions develop in ponds during periods of neap tide when it is not possible to renew the pond water. Another serious problem occurs when it rains just after the ponds are filled. With a well designed pump system the water can be freshened at any time.

Scheduling of work. With a tidal water exchange system many activities must be scheduled according to the tide. This often creates difficulties, both for scheduling the work of the labour force and for carrying out the activity itself. Harvesting is a big problem. The pond must be drained on low tide and on some farms it must even be on a low tide during neap tide. If there is an emergency such as an impending storm or an outbreak of disease, the pond cannot be drained. In tidal systems the water must be let in and out at specific times. It is often a problem for workers to do this at odd hours and if the job is not done as scheduled it must wait for the next tide. For good water management, water in the canals must be kept at certain levels in order for water to flow evenly into the ponds. In tidal systems there is a tendency for workers to open the gates more than is necessary when water in the canal is still low instead of waiting until the water in the canal rises. Then as water in the canal rises the rate of flow into the pond through the wide opening results in increased pressure which can pop screen materials out of their frames. With a pumped water supply system the labour force can work a set schedule which usually makes them happier and easier to supervise. Activities can be scheduled for the most efficient work, i.e., harvesting can be done at the time best suited for marketing. Flow of water can be systemized so that decision-making by workers is minimized. This last point is extremely important in areas without a trained labour force.

Greater effective area. The effective area reflects the percentage of the total land area which can be used for culture. It is essentially the ratio of water area to gross area. When ponds are excavated the soil is put into dikes which take up space. The more land used for dikes the less the effective area. Deeply excavated ponds have much less effective area than would be possible with a pumped system. Gedney et al., (1983) estimated that a farm with ponds excavated 1.5 m would have an effective area of only 59 percent and one excavated 0.9 m would have 67 percent. That of a pumped water supply with no excavation was 75 percent. This can be an important point if land costs are high, especially for smallholders.

Use of currently unused land. With a pumped water supply the pond bottom can be situated at any elevation desired. This will permit ponds to be constructed in areas where they normally would not be considered, such as those with low tides. It offers an alternative to the use of environmentally-sensitive nearshore mangrove areas for ponds. The higher areas which are less important as nursery areas to the natural fishery could be used. Cost of construction would be lower in these areas as it is easier to operate heavy equipment, and costs for an access road for example would be reduced.

3. MANAGEMENT CONSIDERATIONS

In planning any type of aquaculture facility consideration must be given to the type of management which must be practised. Ponds constructed on acid sulphate soil require types of management which are unique or of special importance due to the nature of the soil. Some of these management practices will require special consideration in the design.

3.1 ORGANIC MATTER

Shrimp farms in India achieve good yields by supplying organic and inorganic fertilizers to increase natural productivity of ponds (The Marine Products Export Development Authority, Cochin, 1980). It is a common practice to add organic fertilizers to the bottom during pond preparation to encourage the growth of benthic blue-green algae (Chen, 1972; Djajadiredja, 1972).

Experiments were conducted in which triple superphosphate, urea and chicken manure were added to ponds at rates of 4 kg/ha, 16 kg/ha and 40 kg/ha respectively every two weeks. These experiments were unsuccessful as oxidized iron developed in the ponds and the shrimp developed red gills and died. It was decided that fertilizer-type management where water was exchanged infrequently was not suitable for the acid sulphate ponds and the programme was changed to one of supplemental feeding where one half of the pond water was exchanged daily. The increased water exchange helped to some extent, but oxidized iron was still evident in the ponds and the shrimp developed red gills, sometimes after rainless periods. If the water exchange scheduled is maintained the reddish deposit usually dimishes a short time after culture ceases.

The heavy reddish granular deposits around the pond edge were found to contain filamentous forms which were assumed to be iron-oxidizing bacteria. Experiments in which soil leachate was added to beakers containing only water and others containing water and pond sediment, showed that oxidized iron sediments and surface mineral slicks developed only on the beakers containing soil. It was concluded that the sediment water interface is a zone of complex pyrite oxidation which contributes to water quality deterioration (see Chapter 2 of this report).

Similar experiments were subsequently carried out in which different materials were added to beakers of pond water. The substances were: (1) clay pond bottom soil with no appreciable iron content which had its surface coated with oxidized iron; (2) a rust coloured mat from the pond bottom composed of algae, zooplankton, organic material and bacteria which had been dried and powdered; (3) clay pond soil (1) mixed with shrimp feed; (4) material (2) with shrimp feed and; (5) material (1) and material (2) with shrimp feed. The initial pH of the pond water was 7.25 and the salinity was 22 ppt.

Results of the experiment are given in Table 1. The most important aspect is that no forms of oxidized iron developed in beakers with soil alone. This indicates a source of organic carbon is required for bacterial nutrition. Second, oxidized forms of iron occurred in beakers with organic matter and water pH of 6.48 and 3.81 indicating that their production is not pH-dependent.

The preceding experiment and observations have important implications for management. Any excess organic matter creates more problems in acid sulphate ponds than in normal ponds. Organic fertilizers should not be added to the pond bottom during pond preparation. If fertilizers are used to induce a bloom of phytoplankton, only inorganic fertilizers should be used. It is important that feed ingredients be well ground and that pelleted feeds be well bound so that shrimp cannot feed selectively and leave unwanted residues in the pond. It is also important not to overfeed. This is difficult in acid sulphate ponds because of the high mortality which occurs in them. To prevent overfeeding feed inspection trays were used (Liu and Mancebo, 1983). Feed is spread evenly around the pond with some falling on the trays. The trays are lifted after one hour and if no feed remains on the tray the level of feed is increased. If feed remains after one hour the trays are inspected again after a further hour. If feed remains on the tray at this time the shrimp are being fed too much and the amount of feed is reduced at the next feeding.

No matter how carefully a pond is managed organic matter accumulates in the bottom soil and causes problems. In ponds with interior canals, the canal serves as a collector for organic matter and silt.

Ponds for shrimp culture are frequently made with a peripheral canal around the pond. The ponds at this station are made that way. Feed is broadcast from the dike and it is inevitable that most of it finishes in the peripheral canal which is already loaded with organic matter, and consequently anaerobic conditions develop there readily. To prevent this it is necessary to clean them periodically. This is a difficult task but Wechasitt (1983) has described a method which combines the use of a high pressure water jet to stir up the sediments and water flowing through the pond to carry away the disturbed debris. Ponds should be cleaned at least once a year. The cleaning process would be much easier in ponds with a pumped water supply. In tidal ponds a portable pump must be rigged up to flow water through the pond on low tide.

3.2 IRON

Acidity of the pond bottom soil is not really a problem. After a short period of flushing and submergence the top layer becomes neutralized by carbonate in seawater and deeper layers become anaerobic. In either case the pH rises. As pointed out earlier iron is harmful. It accumulates in the pond, and once in the pond it acts independently of acidity. It recycles between a reduced state in the pond sediments and an oxidized state depending on bacterial action and the quantity of oxygen in the soil. Bacteria oxidize the iron and the bottom flora becomes coated with oxidized iron. During the day the benthic algae, coated with oxidized iron, float to the surface where wind action carries them to the side of the pond. This material accumulates at the pond edge and becomes incorporated in the loose unconsolidated sediments carried into the pond from the dike by rainwater. Gradually as one layer forms on top of another the bottom layers become anaerobic and the iron reverts to the soluble ionic state. Then it is leached back into the pond in dikepore water during drawdown for tidal exchange.

It is advisable to remove as much iron as possible from the pond. Addition of lime would just bind more iron. One way to remove the iron is to make the iron change to a soluble form and then flush it from the pond. The iron is soluble below pH 4. A good way to remove iron would be to fill the pond with acid to dissolve the iron. This can be accomplished by drying the pond thoroughly and tilling it to aerate and dry the soil at deeper levels.

After the soil is completely dry, just enough water is let in to cover the bottom. This water will become very acidic and the iron will be dissolved. The water is then drained from the pond and the flushing process repeated until the water pH rises above 4. If too much water is let in during the initial filling, pH will rise above 4 and most of the iron will precipitate back out of solution. Furthermore, moist soil should not be flushed. The pond bottom must be thoroughly dried to oxidize the iron. After the pH of the water no longer drops below 4, flushing should be continued with the water level as high as conditions permit. The entire system inside the perimeter dike should be flooded if possible. This will leach acidity from the dikes.

Planting grass, especially at the water edge is recommended (Cook et al., MS.a). Grass at the water's edge prevents entry of small oxidized soil particles which are washed into the pond by rainwater and contribute iron to the pond. Prevention of the accumulation of these loose soil particles at the water's edge reduces the amount of soil pore water which enters the pond during drawdown. The plants also take iron up from the soil, and its removal from the recycling process is beneficial.

It must be mentioned that maintaining water level at a constant depth would help reduce the recyling of iron.

Table 1

RESULTS OF SOIL INCUBATION EXPERIMENT

3.3 WATER DEPTH

It is recommended that water depth be kept at 80–100 cm. This gives sufficient water to dilute acidic rainwater runoff and minimize its effects. If there is some turbidity it is also sufficiently high to protect against high water temperature and to limit light penetration to the bottom sufficiently to retard growth of benthic algae. Small ponds with this depth of water have problems due to poor water circulation by wind. Studies of productivity in this station's ponds showed that the bottom water of ponds maintained at one metre depth had low levels of oxygen and high respiration. This could be caused by many factors. One would be consumption of oxygen by the sediment through both biological activity and sulphide oxidation. Light in the bottom layers was found to be limiting and thus respiration by phytoplankton, zooplankton and bacteria would account for much oxygen uptake. Measurements in one pond showed that 30 percent of the respiration was due to bacteria alone.

It was concluded that aeration is essential in ponds where turbidity is high (and thus photosynthesis would not supply much oxygen) and/or organic matter loading (feed, etc.) is high, which encourages bacterial growth (Moriarty, 1983).

3.4 WATER STRATIFICATION

If the amount of rainfall is sufficient pond water becomes stratified. Comparative values of pH, salinity, temperature and DO between surface and bottom layers show that in these ponds stratification occurs frequently. Variations as great as 5–10°C, 1–3 pH units, 5–10 ppt salinity and 1–3 ppm DO have been observed (Rosly, personal communication). On one occasion the following conditions were observed after a rain of 55 cm. In the surface layer salinity was 15 ppt, pH 3.75 and DO 8.0 ppm. At that pH the calculated alkalinity would be zero and that of CO₂ would be over 80 ppm. At the bottom salinity was 20 ppt, pH 6.45 and DO 2.5 ppm. As the shrimp became stressed by low levels of DO at the bottom they swim to the surface. In this case the surface was a completely hostile environment and P. monodon 7–10 cm in length were actually observed jumping out of the water trying to escape, and considerable mortality occurred.

Some means must be provided to break up stratification in ponds. The action of wind across large ponds has a pronounced effect on water circulation. It is, however, not economical to excavate large ponds, for the larger the pond the further the excavated soil must be moved and, consequently, the higher the cost. For small ponds, mechanical means of water circulation are recommended. A method which utilizes a divider placed down the centre of a long pond to make a modified raceway and water circulated with a series of air-lift pumps has been found effective in breaking up water stratification (Cook et al., MS.b). Not only was stratification eliminated, but movement of oxygenated water over the pond bottom resulted in movement of the anaerobic zone downward. Biological degradation of organic matter is much faster in an aerobic environment and the bottom-dwelling organisms which serve as food for cultured species can live. Ponds in which the air-lifts operated showed much less evidence of oxidized iron. This could be due to increased oxygen levels in the bottom soil. When the soil lacks oxygen, iron in the soil changes to the ionic form which can be utilized by bacteria.

3.5 WATER EXCHANGE

No matter what the innovations in pond construction are introduced designed to reduce the impact of acid sulphate soils it will probably be necessary to exchange large fractions of pond water on a regular basis. To accomplish this, flow rates through the water control gates must be high. This causes problems for the operation of nurseries. In ponds with normal soil it is common practice to place fry in a nursery and add fertilizers to encourage growth of natural food. Water is not exchanged for several weeks. It is not possible to do this in acid sulphate as water must be exchanged regularly. When the rate of flow through a gate is high it is difficult to keep small fry from being pressed against fine mesh screens which injures or kills them. In addition, eggs and larvae of predators are forced through the screens. For this reason it is recommended that earthen nursery ponds are not used in areas with acid sulphate soils. It would be more practical to have aboveground tanks in which the fry are cultured until they are large enough to swim away from the screens, that is, until they attain a length of about 3 cm. An above-ground nursery with a hard bottom would be useful for P. monodon in any case since they are difficult to harvest from earth nurseries. In a tank nursery it is no problem to flush them out.

For effective tidal water exchange there must be a sufficient number of gates of the right size. From the experience here with culvert type gates it is recommended that gates for a 0.25 ha pond have a cross sectional area of at least 0.8 m². A 0.5 ha pond requires gates with a cross sectional area of 1.8 m² and a 1 ha pond 3.6 m². Ponds with a pumped water supply require less gate capacity because water can be flowed in at a slower rate over a longer period of time. A single 30 cm diameter inlet pipe and one gate with a cross sectional area of 0.8 m should be adequate for a 0.5 ha pond.

Water at the far end of the pond is not effectively changed in a traditional pond with a single water gate and frequently water quality there is hard to maintain. Maintenance of pond water quality is more effective if water is moved through the pond, entering at one end and draining from the other. For this reason it has been recommended that shrimp ponds be constructed with gates at each end (Cook and Rabanal, 1978).

A point to be considered is that this type of pond costs considerably more to build because of the extra gates and water canals required. Sivalingam (1983) suggested that ponds be constructed in a more or less U shape with an earthen divider down the middle and two gates at one end opening into a single canal. This would eliminate the requirement for an extra canal for drainage water. One of the authors (Wechasitt) had experienced that with this type of pond the shrimp tend to stay only on the side where the water enters. Experiments at this station have shown that ponds with a single gate are adequate if water is circulated mechanically (Cook et al., MS.b). The induced circulation not only breaks up stratification, but ensures that the water is thoroughly mixed. There is, therefore, no need for extra canals or gates.

4. DESIGN AND CONSTRUCTION

For reasons discussed in detail earlier in the paper it is suggested that ponds be built at higher elevation with a minimum of excavation. Water should be supplied by pumps. Modified raceway ponds with a system for mechanical aeration and water circulation are recommended. An above-ground nursery should be built separately from the ponds. With these guidelines, details of design and method of construction are still important to the success of the project.

Before any other work is started a detailed topographic and soil survey should be carried out. The soil survey should include measurement of acidity and potential acidity to a depth of 0.5 m below the intended pond bottom. After the land elevations and soil pH are plotted a layout can be made. If the soil is acidic or potentially acidic the land must be excavated as little as possible. If there are large difference in elevation the land should be terraced. This will prevent cutting too deeply into the soil and exposing the more acidic subsoil. It will also decrease the expense of levelling. If care is taken when the land is levelled and terraced, much of the good top soil skimmed off the higher elevations can be used to form the dike for the lower adjacent pond.

The varying pond bottom elevations can be accommodated by pumping water to the highest level and using simple drop structures in the water supply canal to lower water level systematically to lower elevations. The system is operated like an agricultural irrigation project with a water supply to the highest elevation and gravity flow to successive lower levels (Gedney et al., 1983). Water should be let into the ponds through a pipe. Drainage should be through a sluice gate at the opposite end of the pond.

The perimeter dike will normally have to be large in order to provide protection against flood. The effects of acid rainwater runoff from the large dike can be minimized by isolating the dike from the rest of the system. This can be done by placing the drainage canals adjacent to them. In places where no drainage canal is needed, a culvert drain should be built into the side of the dike to catch the runoff and direct it to the drainage canals (Figs. 1 and 2). The tops of the perimeter dike should be sloped to the outside to facilitate runoff away from the pond system. For added protection, a small dike can be placed along the interior edge of the main dike.

The pond dikes must be made as small as possible. A height of 1.5 m with a crown of 1 m is suggested. A slope of 1:1 is acceptable if grass (Zoysia matrella) is sodded just above the waterline. This grass grows at the back of mangroves at elevations which are only flooded on high spring tides. If interior canals are built within the pond, it should be possible to construct small pond dikes in a manner such that the lower acidity top soil is used to cover the surface. An interior canal about 0.5 m deep and 4 m wide at the base is located 5 m from the base of the dike. A backhoe then moves down the dike a little and scoops off the top soil in the adjoining section of the canal. This soil is placed on top of the base section that was already laid in place. The process continues around the pond until the last portion of dike base is in place. This is covered with the pile of top soil which was set aside in the beginning. For dikes with an internal canal on each side it is necessary for two backhoes to work in concert, one on each side of the dike.

It is the opinion of the authors that placing the interior canals 5 m away from the dikes has advantages for management in ponds receiving supplemental feed. The feed can be broadcast from the pond edge without it all ending up in the canal. A canal helps in draining the pond and makes drying easier. Most of the organic debris in a pond will collect in the canal where it can be cleaned out more easily.

REFERENCES

Allbrook, R.F. 1973 The identification of acid sulfate soils in northwest Malaysia. In Proc. Bangkok Symp. on Acid Sulfate Soils, edited by H. Dost, Wageningen, The Netherlands, ILRI Publication 18:131–40

Andriesse, J.P., N. Van Breeman and W.A. Blokhuis. 1973 The influence of mud lobsters (Thalassina anomala) on the development of acid sulfate soils in mangrove swamps in Sarawak (East-Malaysia). In Proc. Bangkok Symp. on Acid Sulfate Soils, edited by H. Dost, Wageningen, The Netherlands, ILRI Publication 18:131–40

Boyd, C.E. 1979 Water quality in warmwater Fish Ponds. Auburn University, Agricultural Experiment Station, Auburn, Alabama. 359 p.

Camacho, A.S. 1977 Implications of acid sulfate soils in tropical fish culture. In Joint SCSP/SEAFDEC Workshop on Aquaculture Engineering, edited by R.H. Gedney, SCSP, Manila, Vol. 2:97–102

Chen, T.P. 1972 Fertilization and feeding in coastal ish arms in Taiwan. In Coastal aquaculture in the Indo-Pacific Region, edited by T.V.R. Pillay. Fishing News (Books) Ltd., London, p. 410–6

Chow, W.T. and S.K. Ng. 1969 A preliminary study of acid sulfate soils in West Malaysia. Malaysia Agricultural Journal, 47(2): 253–67

Colt, J.E. and G. Tchobanoglous. 1981 Design of aeration systems for aquaculture. In Proc. of the Bio-Engineering Symposium for Fish, edited by L.J. Allan and E.C. Kinney, Fish Culture Section of the American Fisheries Society. FCS Publication 1:138–48

Cook, H.L. and H.R. Rabanal (eds.). 1978 Manual on pond culture of penaeid shrimp. ASEAN National Coordinating Agency of the Philippines, Ministry of Foreign Affairs, Manila, Philippines. 132 p.

Cook, H.L., U. Pongsuwana, L.D. Rajamanickam and R.B. Khamis. Experiences in establishing vegetation on acid sulfate pond dikes. (MS.a)

Cook, H.L., S. Wechasitt, T.T. Chuah and E. Salvam. Aeration and water circulation in brackishwater aquaculture ponds. (MS.b)

Djajadiredja, R. and A. Poernomo. 1972 Requirements to successful fertilization to increase milkfish production. In Coastal aquaculture in the Indo-Pacific Region, edited by T.V.R. Pillay. Fishing News (Books) Ltd., London, p. 398–409

Ellaway, M., R. Beckett and B.T. Hart. 1980 Behavior of iron and manganese in the Yarra Estuary. Australian Journal of Marine and Freshwater Research, 16(2): 589–96

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Gedney, R.H., Y.C. Shang and H.L. Cook. 1983 Comparative study of tidal and pumped water supply for brackishwater aquaculture ponds in Malaysia. Manila, South China Sea Fisheries Development and Coordinating Programme, SCS/83/WP/117, 40p.

Kawalac, A. 1973 World distribution of acid sulfate soils. In Proc. Bangkok Symp. on acid sulfate soils, edited by H. Dost, Wageningen, The Netherlands, ILRI publication 18:292–5

Liu, M.S. and V.J. Mancebo. 1983 Pond culture of (Penaeus monodon) in the Philippines: Survival growth and yield using commercially formulated feed. Paper presented at the 1983 Annual Meeting of the World Mariculture Society

Marine Products Export Development Authority, Cochin. 1980 Proceedings of the first National Symposium on Shrimp Farming, Bombay, 16–18 August 1978. Cochin, India, The Marine Products Export Authority (Government of India, Ministry of Commerce), 269 p.

Potter, T. 1976 The problems to fish culture associated with acid sulfate soils and methods for their improvement. Report of the ASEAN Seminar/Workshop on Shrimp Culture, 15–23 November 1976, Iloilo City, Philippines

Ruddle, K. 1982 Brackishwater aquaculture in south-east Asia. Mazingira, 6(4): 58–67

Singh, V.P. 1980 Management of fish ponds with acid sulfate soils. Asian Aquaculture, 4:4–6

Sivalingam, S. Improved designs for double and single gate tidal aquaculture ponds. (MS.)

Spotte, S. 1979 Fish and invertebrate culture, water management in closed systems, 2nd ed. New York, John Wiley and Sons, 179 p.

Tang, Y.A. 1979 Physical Problems in Fish Farm Construction. In Advances in Aquaculture, edited by T.V.R. Pillay and W.A. Dill. Fishing News (Books) Ltd., Farnham, England. 653 p.

Truchot, J.P. 1983 Regulation of acid balance. In The biology of Crustacea. 5. Internal Anatomy and Physiological Regulation, edited by L.A. Mantel. New York, Academic Press, 457 p.

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FIGURE 1. POND LAYOUT

- MAIN DIKE

- POND DIKE

→ AIR-LIFT PUMP

→ DIRECTION OF WATER FLOW

-→ DRAIN CULVERT

FIGURE 2. POND SECTIONS