The Coastal Aquaculture Demonstration and Training Project at Gelang Patah is a major investment of resources and personnel to provide guidance for brackishwater aquaculture in Malaysia. As such, one of its crucial roles is to define the most important problems in brackishwater aquaculture development and establish workable solutions to these problems. The staff and facilities at Gelang Patah seem very well suited to address these tasks. It is clear that one of the major problems confronted at the project is acid production from the dike soils and other associated chemical and biological phenomena. This problem is quite a general one in Southeast Asia, and can be expected to occur in many locations which would otherwise be very desirable for brackishwater aquaculture in Malaysia. Thus it is probably good in the long run that the research facility at Gelang Patah has to face this problem early in the management of aquaculture. It is not known what aspects of acid soils are most detrimental to shrimp and fish aquaculture but there are a number of likely candidates, including (a) low pH (perhaps due to interference in Ca transport and storage); (b) high dissolved iron; (c) high dissolved aluminum; (d) iron particles clogging gill surfaces and (e) low primary productivity due to stripping of dissolved phosphate by cycling of iron through oxidation-reduction reactions. It is important to collect good, systematic data on chemical properties in the water, soil and sediments, in conjunction with documentation of shrimp and fish yields, to help define what factors associated with acid soil are most important in limiting aquaculture yields.
With this in mind, some of the steps which would benefit collection of such data are:
Provide dependable 24-hour line electricity to the facility as soon as feasible. This would permit better operation of analytical equipment and improved storage of perishable chemicals and samples, as well as other benefits.
Establish air-conditioned space for crucial analytical equipment. Many electronic instruments are quite difficult to maintain and operate in high humidity environments. Even if only a small room with good insulation were available, the operation of electronic equipment would probably be significantly improved.
Acquire a better spectrophotometer for colorimetric measurements of parameters such as dissolved phosphate and iron.
Obtain several spare electrodes for the pH meter and microburets and more magnetic stirring equipment to simplify alkalinity titrations.
Improve the quality of distilled water, either by upgrading the feed water or employing a pre-deionization step before the still.
Obtain a small programmable calculator for computation of alkalinity data.
Collect pH and alkalinity data from a representative set of ponds on a regular basis (at least twice a week), with samples collected near the gate after the discharge flow from each pond to the canal has been well established, to obtain water which is as representative as possible of the whole pond volume. Data of this type should be obtained over at least the next several years, especially in conjunction with monitoring the effects of any major attempts at leaching of pyrite acidity from the dike soils.
The above recommendations related to the ability to collect data in support of the main activity of enhancing shrimp and fish productivity. How to effect significant improvement in that productivity appears to be quite a difficult problem. Without some quite major changes in direction, one would expect such improvement to be unlikely. Any of the suggestions which follow would require some major policy decision changes and obviously should not be approached without careful consideration.
The total amount of pyrite in the pond and dike network is so large that it will probably take many years for the acidity problems to dissipate by the leaching processes under present conditions. The most practical way to accelerate leaching of pyrite from the dikes appears to be flooding the entire system, or major portions of it, with brackish water, and draining away the resultant acidic water. This would entail some risk from erosion to water control structures. The consultants cannot judge how serious that risk would be, but it should be considered. Their calculations suggest it would be necessary to repeat such flooding many times, at intervals long enough to permit further pyrite oxidation in the dike soils between flooding episodes (1 week-1 month?). If such a course were chosen, it would be important to collect data on water chemistry before, during and after flooding to document how effective the leaching had been, with special attention to getting good alkalinity data, as the most reliable chemical indicator of the total amount of acid which has been produced.
The consultants do not think application of crushed carbonate rock or ash would provide much benefit on the scale of the entire pond network until the pyrite burden had been substantially reduced. It might be productive to try such applications on a small scale, but the consultants are not optimistic that they would be very effective. Some measurable chemical changes in the sediments and interstitial waters of Pond 29 remain approximately a year after limestone application but significant increase in shrimp yields has not occurred.
Another experiment which could be done on a small scale would be air bubbling. Although this could raise brackish water pH in the range of 6.5 to 7.5 by stripping CO2 from the water down to a value in equilibrium with the air, the consultants can see no way this would significantly improve the chemical conditions for the aquaculture organisms (if the chemical conditions are indeed the main problem).
The scientific findings (see sections 3 and 7 below) generally suggest two principal conclusions. First, the reduced sediments of the ponds do not contribute to low alkalinity and pH of the pond waters and may even keep the water buffered at intermediate alkalinity levels. Second, inwelling of water through the dikes during filling and draining operations may contribute as much iron and acid to the ponds as is contributed during rainstorm-related leaching of the dikes. Thus the consultants suggest increasing the amount of sediment surface area and volume in the dikes, while at the same time reducing the surface area and volume of oxidized, acid-containing dike soil, by raising pond water levels. This could have a beneficial effect on water quality and is unlikely to harm water quality. An experiment designed to test this hypothesis could be performed as follows: select a group of nine ponds (e.g. ponds 15–17, 20–22, 25–27); while maintaining normal filling/ draining schedules, raise the level of three ponds by 1.0 m, and raise three other ponds by 2.0 m; this would allow testing of alkalinity in a set of ponds maintained at three different water levels. The experiment could be continued over an entire shrimp growth season in order to test the effect of sediment volume dike volume on shrimp growth. In any event, it would be necessary to continue the experiment for 2–3 months, making biweekly alkalinity determinations on each pond.
If culturing of shrimp is to remain a major research activity at Gelang Patah, the consultants would recommend that serious considerations be given to construction of additional ponds which would be as free as possible of acid soil conditions. To accomplish this, a good survey of soil chemistry (for potential acidity and/or pyrite mineralization) prior to site selection and construction would be essential. This should include borings, which are analysed at different depths, as well as over all the potential pond areas. It should be feasible for much of the sampling and analysis to be guided and executed by the staff at Gelang Patah. The new ponds should have a little exposed dike soil as possible. This could be accomplished by building the dikes from sediment only up to the low-water level and then edging the ponds with other construction materials (such as bricks?) and then filling the space between pond edges with a very low dike sloping away from the pond water toward a trough in the center. The entire construction plan should be built around minimizing the amount of exposed soil to prevent oxidation of pyrite. Because shrimp are so sensitive to acid conditions, it may not be possible to achieve good yields in a practical length of time without going to such extremes in design of the ponds.
With the present pond system and no major acceleration of pyrite leaching, it is thought unlikely that a significant improvement in shrimp productivity could be achieved in the near future. Because of the area and height of the dike system, pyrite leaching following precipitation will continue to be a substantial problem, even if less were occurring due to a change in water exchange practices to minimize dike leaching during pond water renewal. Thus, a third possible major policy change would be to shift emphasis from crustacean culture to identify species of fish which are least sensitive to the water conditions typical of acid sulfate soil environments. Obviously, such a shift could greatly alter the overall strategy for development of aquaculture in Malaysia but, given the investment already made at Gelang Patah, it is a possible alternative to maximize the research benefit of the facility.
The consultants cannot choose between the above three suggestions, since that is essentially a matter of policy to be decided in Malaysia, but they think that unless major change is undertaken, the yield per hectare per year of culture organisms will remain quite low at Gelang Patah for a long time. The Thailand aquaculture facilities the consultants visited had more than an order of magnitude less pyrite than the lowest estimate of pyrite they found for Gelang Patah. Shrimp yields were approximately an order of magnitude greater per unit area than at Gelang Patah even without intensive stocking. The ratio of pond area to dike area was much greater in the private traditional aquaculture ponds in Thailand, the dike heights were much lower, often sloping to a central trough, and the abundance of natural organic materials in the water was apparently considerably greater, based on their limited observations of water turbidity and other properties. The consultants think it is likely the soils of the area they visited in Thailand originally had considerable pyrite, based on their XRF measurements of total sulphur and iron. If that were true, the removal of the original pyrite probably occurred over a period of many years during flooding for salt crystallization and from rain leaching before shrimp aquaculture was initiated.
