Shrimp Farming and the Environment - Library

has also been converted to agricultural land. It is precisely because these resources are often on publicaccessproperty that settlers have been able to do this. However, much of the wood extraction andfishery activity has been unsustainable—and, coupled with the conversion activities, this has led tosignificant degradation of mangrove and estuarine resources in some countries. In other words,controlling shrimp farm development alone will not save mangrove, and may not even slow the rate ofdestruction in some areas. A much broader policy, planning, and regulatory framework will be requiredto stem the degradation of coastal resources.Suitability of mangrove areas for shrimp farmingMangrove forests are not considered to be the best sites for semi-intensive or intensive shrimp farms.Boyd (1997) lists the following problems associated with shrimp farming in the intertidal zone:• Soils are often highly acidic and contain large amounts of organic matter;• Water exchange is incomplete, so pond effluents may not be washed completely away; and• Crabs and other possible carriers of shrimp diseases are abundant.In addition to these reasons for not establishing shrimp farms in primary or secondary mangrove areas,Boyd notes that it is in the farmers’ interest to preserve the mangroves, since these forests are capableof efficiently removing solids and nutrients from shrimp farm effluents (Robertson & Phillips 1993). Inaddition to cleaning discharges from shrimp ponds, mangrove can stimulate the productivity of coastalareas, thereby improving coastal fisheries, minimizing pollution of the coastal environment, andproviding higher water quality for shrimp farming. However, these positive effects can be overriddenby discharging amounts of nutrients greater than the carrying capacity of the local ecosystem, bydischarging concentrated pulses of nutrients and organic matter, and by introducing chemicals andantibiotics (used to treat diseases and improve pond quality), all of which can be locally harmful.Despite this general warning about avoiding shrimp culture farms in mangrove forests, the severity ofthe constraint should not be overemphasized. A NACA/ADB survey (ADB/NACA 1995) of shrimpfarms in 12 countries in Asia showed that while an average 31% (range 0–88%) of intensive farmswere sited in what was previously mangrove, soil acidity was reported as a significant problem in only5% of them overall (range 0–6%). While these low figures may in part reflect ignorance on the part ofthe farmers as to the causes of water quality problems, they do suggest that some mangrove soils areindeed suitable—or at least acceptable—for shrimp production. Furthermore, most mangrove soils arepotential acid sulphate soils that may not become acidic if disturbance is limited and the soil is notdried out, as is often the case with more extensive systems.Mitigation of threats to habitatThere are three possibilities for minimizing the conversion of natural habitat to shrimp farms. First,shrimp farms can be constructed away from mangrove areas altogether. The (large-scale) shrimpfarming industry organizations, several NGOs, other international organizations, and most governmentsin the largest producer countries of farmed shrimp now agree that shrimp farms should not beestablished in mangrove forests. Unfortunately, such avoidance may result in destruction of othernatural habitats (such as other wetlands, forests, salt marshes, mud flats, salt flats) with their ownnatural functions and biodiversity value, or conversion from other uses, such as rice farming, coconutplantation, or other forestry/agriculture. Whether such changes are desirable will depend on localcircumstances and priorities. It should be noted, however, that in some countries (for example,Vietnam), the only land available to poor, displaced migrant and minority groups is in fact mangrove.Given the high population density of such areas and the limited sustainable productivity of naturalmangrove (Hambrey 1993), such settlements are inevitably resulting in overexploitation or conversionto agriculture and/or aquaculture. In these circumstances, carefully planned and limited conversion toaquaculture may be the best option, perhaps reducing the overall development pressure on mangroveand other valuable natural habitat.Second, shrimp farms can be constructed on the landward fringe of mangrove. In some circumstances,this may be an attractive option, since such land is often partially saline and of low value for alternativeuses. If mangrove is present, it may be highly degraded as a result of human pressure or in naturaldecline as part of the mangrove cycle of colonization, accretion, and stabilization. Ponds may beconstructed so that a belt of mangrove forest is maintained along the coast, with the ponds locatedimmediately behind the mangrove belt (Barg 199b). There are operating examples of such an22