Sources of pollution and user conflicts
As indicated earlier, it is essential to investigate any existing or potential sources of pollution and the nature of pollutants that are likely to affect the water supply to the proposed farm. Thorough local enquiries will be needed, as the situation at the time of the site selection studies may not represent conditions at other times of the year. Therefore data for previous years should also be examined as far as possible. Certain types of organic and harmless wastes can be used to increase the productivity of aquaculture farms. The use of waste heat in temperate and cold climates has already been referred to. Sewage effluents and properly treated animal wastes can be used successfully to fertilize aquaculture farms in order to increase the growth of food organisms. However, it will be necessary to incorporate such uses at the design stage of waste disposal (in order to render the wastes readily usable for aquaculture purposes) as well as at the aqua-farm (to provide for controlled use of the waste material in the appropriate form and doses to enable its safe use). The likelihood of discharges from facilities used for intensive aquaculture polluting public water bodies and spreading communicable diseases from farmed stocks to wild stocks should also be considered. Though these can be prevented in well-designed and managed farms, there is still the possibility of such arguments being used by neighbouring communities who are not very appreciative of the use of the selected site.
In open-water aquaculture, particularly cage and pen culture of fish and stick and raft culture of molluscs in lagoons, estuaries, bays, fjords, etc., there is a likelihood of the organic load from metabolic wastes of cultured organisms and unused feeds accumulating, sometimes giving rise to a high biological oxygen demand and accumulation of toxic gases. The pattern of water flow may also be altered. It will therefore be necessary to consider ways of preventing this and avoiding conflicts with other uses of the area, such as navigation, recreation and fishing.
Some of the major considerations in reclaiming mangrove swamps for aquaculture have been discussed earlier. From what is known of mangrove ecology and the effect of reclamation, it would appear that, if properly planned,
clearing of mangroves retaining a belt of at least 50 m along the coast ensures that their ecological functioning is unimpaired. It has been suggested that clearing of mangroves should be done without changing the general morphology of the area, leaving for every hectare of mangrove cleared at least 3 hectares untouched, for conservation purposes.
Conflicts may arise with agriculture, for example rice farming, in areas where for economic reasons rice fields may be converted into fish ponds. However, if national priorities require that they be used for rice cultivation, the possibility of integrated rice field aquaculture could be considered. In areas where crop/livestock/fish integrated farming is possible, conflicts with agriculture communities can be minimized by adopting such practices that will add to the income of the farmers.
With the expansion of aquaculture, many governments have brought in systems of licensing to regulate the enterprise. Where no such unified regulations exist, very often the prospective farmer has to obtain permits and clearances for his project from a number of agencies. Naturally these legal and administrative matters will also be major considerations in the selection of sites for aquaculture.
The majority of present-day aquaculture harvests come from inland or coastal pond farms, located in intertidal and foreshore open waters. While fertile areas are suitable pond farm sites, these are not always available for aquaculture, and farms have to be situated on areas that are described as wastelands or wetlands. As ideal sites are those that have access to surface and underground sources of water, the choice often falls on wetlands with a high water table or on floodland areas, which have an important role in the maintenance of the ecosystem that contributes to aquatic production and wildlife preservation. Such wetlands are known to be most productive natural systems, and are sources of nutrients for aquatic organisms. Coastal marshes are generally nursery and feeding grounds of the young stages of a number of commercial species of fish and shellfish. Because of this the establishment of aquaculture farms in coastal areas may be considered to have only negative consequences.
If domestic and farm wastes are utilized for fertilizing or feeding, not only would this constitute an inexpensive means of waste disposal, but it would also be an effective system of recycling to produce food and fodder.
Competing use of surface water is largely restricted to irrigation systems and drinking water supplies. Though use of water in aquaculture is often considered as ‘non-consumptive’, there are significant losses due to seepage and evaporation. If coastal pond farms are situated near agriculture and the ponds are not constructed to be seepage-proof, the soil of agriculture lands may be salinized. Well water may become undrinkable. Similarly if the subsoil water has to be pumped out in large quantities to adjust the salinity in nearby pond farms, it may reduce in the long run the under-ground water sources and cause subsidence of the ground. When farms are constructed along the water front near rivers, open estuaries and the sea, clearing of vegetation without leaving adequate protective barriers can result in soil erosion.
Sedimentation and obstruction of coastal flows are important consequences of aquaculture farms, particularly in the case of shellfish farming and cage culture. Sediments may consist of fine particles of organic detritus or coarse particles of sand derived from soil erosion. Tropical mangroves, which are reclaimed for coastal aquaculture farms, have other direct uses to man. Mangrove ecosystems are described as constituting a reservoir, refuge, feeding ground and nursery for several commercial aquatic species of fish and crustaceans. Saenger et al. (1983) compare the scales of impact of the various uses of mangrove ecosystems. Even though conversion of mangroves for aquaculture is minimal when compared with 22 other major uses identified, one has to consider the socio-economic importance of the conflict with their use by small-scale fishermen for their major source of income. One more important impact from the point of view of aquaculture is the time taken for the leaching out of acidity and the cost of acid sulphate reclamation of mangrove soils, plus the abundance of reclaimed mangroves being abandoned because of the lack of productivity of aquaculture farms built on these sites.