Brackishwater aquaculture in the tropics: the problem of acid sulfate soils. 32p. 256645-E
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Brackishwater aquaculture is presently responsible for a relatively minor component of the total fish landings for the world, but is frequently mentioned as offering excellent potential for major expansion. Under the stimulus of the very high international market value of penaeid shrimp, the investment community, both private and public, has become substantially involved in brackishwater shrimp aquaculture projects in tropical countries, especially during the past decade. Experience from field r esearch and private development of shrimp aquaculture in the tropics has been mixed, with some notable failures, especially in the early stages, as well as successes. One major cause of low shrimp and fish productivity in a number of coastal aquaculture projects is the presence of pyrite, a very common iron sulfide mineral in tropical coastal soils and sediments, often associated with mangrove ecosystems. When exposed to air, pyrite in moist soils is oxidized by bacteria to produce iron oxides and sulfuric acid. Sulfuric acid leaches aluminium as well as iron from soil minerals, and the combination of acidic conditions and high concentrations of reactive, toxic metals is destructive to most plants, including rice, as well as fish, and especialy shrimp. The result is chronically low yields of fish and shrimp, and, in the worst cases, extensive kills of the culture organisms immediately following the runoff of acidic water from pond dikes after rainfalls. Some of the acid impacts can b e reduced by extensive soil leaching programmes prior to culturing, and applications of large amounts of lime during growing periods. Eventually the soils will become relatively free of pyrite given sufficient oxidation and leaching. However, the time-scale for effective removal of the pyrite using traditional construction and management schemes can often be decades. From the viewpoint of development programmes, and the repayment of loans acquired on the expection of high yields of valuable prod ucts such as penaeid shrimp, a long period of poor production can easily lead to economic failure of a project. In addition, large-scale conversion of mangrove forests to low productivity aquaculture systems can substantially undermine the ecological basis for shrimp and other capture fisheries in coastal environments. Furthermore, many development schemes involve the rehabilitation of old coastal ponds which, in many cases, imply new design of canals, extensive earth movements and excavations, and hence re-exposure of acid sulfate soil layers, creating a serious problem in areas where soil acidity had long ago ceased to present difficulties. Thus the net result of a major capital investment in aquaculture can be a reduction in the total capture fishery and aquaculture yield of the area concerned.