Shrimp Farming and the Environment - Library

the cropping cycle. In recent years shrimp farming has become a significant industry in predominantlyfreshwater rice-growing areas in Thailand, and has resulted in some social conflict which has recentlyled to a ban on shrimp farming in some areas (Flaherty & Vandergeest 1998).Seepage through pond bottoms, discharge of pond water into freshwater areas, and seepage of salt fromsediment disposal sites can salinize freshwater reservoirs (Boyd 1997), canals, and adjacent ricepaddies (Funge-Smith & Stewart 1996). The severity and significance of salt’s impact is littleresearched and is highly variable, depending on local hydrology and salinity regimes, as well as pondsoils and management practices. Clearly, this topic warrants further research, but generalizationsshould be avoided. In some slightly brackish and even freshwater areas, shrimp farming may representthe best long-term economic option, but potential social, environmental, and land-capability impactsmust be thoroughly researched and assessed, and adequately planned for, if implementation is to besustainable.Mitigation of water contaminationBoyd (1997) claims that if ponds are built on sites with soils of adequate clay content, seepage will notbe a factor, and he suggests that the practice of discharging pond water into freshwater bodies shouldbe prohibited.Research on the nature of the local aquifer, hydrology, and soils, along with knowledge of watermanagement practices, is required if this issue is to be adequately addressed. In addition, the relativebenefits and costs to the various stakeholders resulting from the introduction of shrimp farming needsto be assessed. Results from such research might then contribute to a land-use planning or zoningscheme to minimize conflict between users as well as long-term impact on the environment, and tomaximize social benefits.In effect, a precondition for mitigation of this kind of impact is effective research capacity linked to anatural resource planning system. In the absence of such a system, a precautionary approach should betaken. Shrimp farming should not be allowed to operate in ways or in locations where it may disruptthe aquifer and salinity regime.Organic matter and nutrient pollutionThe water in shrimp ponds is high in nutrients and organic matter, especially towards the end of theproduction cycle. These nutrients are derived mainly from waste food and metabolic products, as wellas from the small quantities of fertilizer added at the start of the cycle to stimulate plankton blooms(Institute of Aquaculture 1996). Poor feeding practices, particularly over-use of feed, allows feed tosink to the bottom of the pond. This pollutes the pond and significantly increases the cost of operation,since feed comprises approximately 40 to 60% of operational costs (Lin 1995). A survey in Thailandshowed that larger operations achieved higher feed conversion ratios than smaller operations, onaverage, suggesting greater commitment and more effective monitoring of feed consumption in familyrunand -operated farms (Asian Shrimp Culture Council 1994; Lin 1995).When pond water containing high concentrations of nutrients and organic matter from a large numberof shrimp farms is discharged into coastal waters, the effects can be negative, depending on theecosystem’s capacity to receive the discharges. Potential negative effects include (Clay 1996; Dierberg& Kiattisimkul 1996; Lin 1995):• Unusual rates of sedimentation;• Eutrophication, with increased risk of harmful algal blooms;• Change in the nutrient cycle;• Oxygen depletion;• Toxicity from sulfide compounds and ammonia following degradation of organic matter; and• Increased incidence of disease, stemming from poor water quality and stress on marine life.These impacts may be detrimental to the farm itself, to neighboring farms, and to the widerenvironment. It should be noted, however, that increased levels of nutrients and organic matter may bedesirable for some coastal ecosystems. Indeed one valuable function of mangroves is their capacity toabsorb and use the detritus and nutrients that arrive in estuaries and coastal waters. So long as carrying24