For a number of reasons the coastal zones most attractive for brackishwater aquaculture development are reasonably coincident with the natural distribution of mangrove forests and their bordering provinces in tropical or subtropical countries. This is especially true for pond systems in which water exchange is accomplished by tidal currents rather than pumping, as is practised almost exclusively in Asia. Thus we can gain a first-order idea of the distribution of land which might be considered as having at least some potential for brackishwater aquaculture by considering the locations of mangrove forest areas. Clearly this does not imply that all or even a major fraction of these forests should be converted to brackishwater aquaculture since they have many essential functions in their present state (Odum and Heald, 1972; Ong, 1982; Hamilton and Snedaker, 1984).
Several comprehensive attempts have recently been made to summarize the current status of tropical forests (Lanly, 1982), including mangroves (Hamilton and Snedaker, 1984). By one estimate, tropical Asia and America both have total mangrove areas in the order of 6 million ha, while Africa has only about half as much (3 million ha), for a world total in the order of 15–16 million ha (Lanly, 1982). Total tropical forest area is about two orders of magnitude greater (~2 × 109 ha) (Table 10) and has probably decreased in total area over the past few decades at about 0.6 percent per year (Hadley and Lanly, 1983). It appears likely that the percentage rate of decrease for mangrove forest area has been significantly greater than for total tropical forests, since many of these areas are generally accessible and are situated in countries which have high mean population densities (Table 11) and low values of per capita GNP (Hamilton and Snedaker, 1984). Many of the countries which currently have extensive areas of brackishwater aquaculture (Indonesia, Thailand, Philippines, Ecuador) plus others which have begun major expansion of such activities (Panama, Brazil, Bangladesh, India and Malaysia) have large mangrove forest areas which appear likely to suffer considerable reduction in area. At present the total area of brackishwater aquaculture is probably in the order of 0.5 million ha, or about 3.5 percent of the total mangrove forest area and is largely concentrated in a few countries (Indonesia, Philippines, India, Ecuador, Thailand, Bangladesh, Burma and China).
The total land area in which acid sulfate soils have been mapped is approximately 10–15 million ha, or about 1 percent of the total area of cultivated agricultural land (Brinkman, 1982). Compared with the total area of other types of problem soils, such as peat soils (240 million ha) and saline, sodic soils (320 million ha), acid sulfate soils cover a relatively minor fraction of the earth, but they are generally found in high population density areas with good climatic conditions and thus would be extremely desirable to develop. About half of the total area of acid sulfate soils are in Asia (7 million ha), while Africa (4 million ha) and Latin America (2 million ha) also have appreciable amounts (Table 12). North America, Europe, Middle East and Australia have relatively small areas of acid sulfate soils, although the Netherlands encountered severe problems in newly drained polders in the 19th century (Brinkman, 1982). Thus the problem is mostly centered in low latitude coastal zones, especially in Asia (Figure 1, adapted from Kawalec, 1973). On a local scale the distribution of acid sulfate soils is far more complex, and includes large areas now situated 50 km or more away from the coast in freshwater drainage areas, such as the Southern Central Plain of Thailand (van Breeman, 1976), as well as strips of low-lying land in the immediate vicinity of the present coastline. The first order distribution of soils with abundant pyrite and mangrove forest sediments, have major areas of overlap with sites considered as having good potential for brackishwater aquaculture development. Thus a high proportion of tropical and subtropical coastal zones which meet many of the criteria for new brackishwater aquaculture sites contain sufficient pyrite in the soils and sediments to present a major barrier to successful culturing operations, especially for organisms such as shrimps which are sensitive to acidic impacts, when ponds are constructed using the traditional approach.
Without careful soil survey work prior to construction, and extensive preparation to minimize acidity impacts, brackishwater aquaculture development in acid sulfate soils is likely to encounter major long-term difficulties. This is also aggravated due to the fact that the inherent limitations of traditional pond design and management do not receive the attention required in most extension services. The corrective measures often recommended in manuals are clearly inadequate to cope with the problem. Substantial departures from traditional Southeast Asia practices have been advocated by FAO for a long time and have been put in practice only recently by private groups, as in the case of the Malaysian farm discussed earlier (Yunker and Scura, 1984). The new findings and altered management practices certainly deserve a wider diffusion at all levels.
