Investigations of Key Environmental Factors Determining the Sustainability and Productivity of Shrimp Aquaculture in Tropical Coastal Regions
Chapter One ofInvestigations of Key Environmental Factors Determining the Sustainability and Productivity of Shrimp Aquaculture in Tropical Coastal Regions
INTRODUCTION
BACKGROUND TO THE STUDY
For centuries, the coastline has been the most important human habitat, and, as a result, has been subject to a wide range of development pressures (Holland, 1998). Shrimp farming represents additional pressure on these areas, at least potentially. While shrimp farming per se does not necessarily have a significant adverse impact on the coastal environment, inappropriate practices and unplanned development have led to a number of problems. According to Chua (1992), the main environmental impacts associated with shrimp aquaculture, and ways in which specific impacts can be reduced or mitigated are being researched into in this study.
The actual or potential environmental impacts of shrimp farming fall into the following categories: • Destruction of natural habitat (through direct conversion);
• Abstraction/contamination and salinization of groundwater;
• Organic matter and nutrient pollution;
• Chemicals;
• Disease;
• Harvest of brood stock and wild post-larvae (PL);
• Introduction of exotic species;
• Abandonment; and
• Use of fish meal in feeds.
Measures have already been adopted widely by the industry; in others, the suggestions are based more on theory than on practical application. In either case, more research needs to be undertaken to document the effectiveness of the proposed mitigation measures in addressing the impacts. From the outset, however, it should be emphasized that many of the impacts from shrimp aquaculture are not unique to that industry. Rather, they are typical of agricultural practices in general, especially where land is in short supply. It should also be noted that shrimp farms suffer a great deal from pollution caused by other activities, including agriculture and industry. Indeed, while many other activities are relatively careless of their environment, experienced shrimp farmers realize that long-term benefits result from maintaining environmental quality. When establishing new farms, water quality is one of the most important factors to consider; sites where industry, agriculture, or other activities are polluting the water should be avoided (Fegan, 1996).
Extensive shrimp farming takes place in the intertidal zone, commonly in or adjacent to estuarine systems. Semi-intensive and intensive shrimp farming usually takes place in the upper intertidal or just behind/above the intertidal zone, often in adjacent wetlands. Some shrimp farming now takes place in inland areas. Most tropical estuarine systems are dominated by mangrove, an intertidal ecosystem of tree and shrub species specially adapted to saline habitats, that support a wide range of other organisms. Shrimp farms can be constructed away from mangrove areas altogether. The (large-scale) shrimp farming industry organizations, several NGOs, other international organizations, and most governments in the largest producer countries of farmed shrimp now agree that shrimp farms should not be established in mangrove forests. Unfortunately, such avoidance may result in destruction of other natural habitats (such as other wetlands, forests, salt marshes, mud flats, salt flats) with their own natural functions and biodiversity value, or conversion from other uses, such as rice farming, coconut plantation, or other forestry/agriculture. Whether such changes are desirable will depend on local circumstances and priorities. It should be noted, however, that in some countries 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 natural mangrove (Hambrey 1993), such settlements are inevitably resulting in overexploitation or conversion to agriculture and/or aquaculture. In these circumstances, carefully planned and limited conversion to aquaculture may be the best option, perhaps reducing the overall development pressure on mangrove and other valuable natural habitat. 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 alternative uses. If mangrove is present, it may be highly degraded as a result of human pressure or in natural decline as part of the mangrove cycle of colonization, accretion, and stabilization. Ponds may be constructed so that a belt of mangrove forest is maintained along the coast, with the ponds located immediately behind the mangrove belt (Barg 1999).