Shrimp Farming and the Environment - Library

arrangement in Thailand, for example the Kung Krabaen Bay Royal Development Project inChantaburi.The third approach is to reduce the total area of shrimp farming, and/or ensure that any future increasesin production come from the same or a lesser area of land, so that no further destruction of mangrove isallowed. This implies intensification of production. It is essential that appropriate skills, improvedwater management and technology, strategies for disease management, and appropriate infrastructureare developed in parallel with intensification, if this approach is implemented.Whatever approach is taken, the value of natural habitat and productive land must be assessed withinan appropriate natural resource planning framework, and some form of rational land use allocationand/or zoning must be introduced. If effectively implemented, such policies should minimize habitatdestruction by coastal aquaculture and other activities, and protect the most valuable remaining naturalhabitat. Approaches to resource assessment and land use planning in relation to aquaculture have beenrecently reviewed by GESAMP (1999). Where shrimp farms are developed on acid soils, pond wateracidity may be countered through regular flushing and treatment with lime, or in some cases withoxidation and flushing. Alternatively, ponds may be lined with concrete, laterite, or PVC, althoughthere may be some disadvantages in using PVC pond liners in terms of water quality (Wanuchsoontorn1997). However, all these practices will increase the cost of operation and the risk of failure, comparedto farms constructed in nonacidic or low-acid soils.Contamination and salinization of groundwaterContamination of groundwaterWater use in shrimp farming is extremely variable, ranging from little more than make-up water tocompensate for evaporation and seepage to very high rates of exchange. For example, for each metricton of shrimp produced, intensive farms require 50 to 60 million liters of water, according to Gujja andFinger-Stich (1995). Recent trends in Thailand, however, have intensive farms using minimal waterexchange, relying on intensive aeration and skilled management to maintain water quality.While extensive farms often rely on exchanging water by using tidal ebb and flow, some semiintensiveand intensive farms use large amounts of freshwater to mix with seawater, or to make up forevaporation in ponds, in order to maintain what is thought to be an optimum salinity of 15–20 ppt.Pumping freshwater from groundwater is reported to have significant environmental effects,contributing to problems such as:• Saltwater intrusion into groundwater reservoirs;• Land subsidence; and• Loss of water supplies for agricultural and domestic purposes.These effects are reported in a number of countries and regions, including Taiwan, Republic of China;Thailand; Indonesia; the Philippines; and Ecuador (Primavera 1994; Clay 1996; Dierberg &Kiattisimkul 1996). In Taiwan, Republic of China, such problems contributed to the shrimp crisis in thelate 1980s.Some experts claim that the practice of mixing water to obtain a salinity of approximately 15 ppt is notnecessary, and that this practice is rare even in Asia (Boyd 1997). New management practices that uselow water exchange rates have probably reduced this problem substantially worldwide.Shrimp farming in freshwaterPenaeus monodon is highly tolerant of low salinity and, if given time to acclimatize, can be grown inwater of close to zero salinity (Ullah 1995). This adaptability has allowed the spread of shrimp farminginto inland waters (Ponza 1999). Concentrated seawater is trucked inland and added to small ponds ornursery enclosures to provide an initial salinity of 7–10 ppt. Passive water exchange with a partiallyfilled grow-out pond results in a steady reduction in salinity. Finally the ponds are filled, and salinity isreduced to close to zero for the bulk of the grow-out cycle. Water management is usually based on thesemiclosed system, in which water is added to the system, but effluent is not discharged until the end of23