NATURE OF ENVIRONMENTAL IMPACTS NATURE OF ENVIRONMENTAL IMPACTS

NATURE OF ENVIRONMENTAL IMPACTS ผลกระทบทมตอธรรมชาต

NATURE OF ENVIRONMENTAL IMPACTS 1. Conflicts with other uses The nature and extent of environmental consequences of aquaculture depend largely on the location and type of farm, as well as the production technologies employed. The majority of present-day aquaculture production comes from inland coastal pond farms and intertidal and foreshore open waters. While productive agriculture land is and ideal site for pond farms, such land is not easily available for aquaculture, and farms have often to be located on land that is described as wasteland or wild land.

As the sitting of the farms has to be based primarily on access to surface or underground sources of water, the choice often falls on wetlands with a high water-table, or on flood land areas. These areas are not truly wastelands as they have significant uses. Even when they are not used directly by man, they may have an important role in the maintenance of the ecosystem of the area and contribute to aquatic productivity and wildlife preservation.

Coastal wetlands are considered to be among the most productive natural system known, and are sources of nutrients for organisms living in marshy areas as well as the water bodies into which they drain. The importance of coastal marshes nursery and feeding grounds of the young stages of a number of commercial special of fish and shellfish has been well described in fisheries literature.

A good proportion of the existing land- based aquaculture farms are located on converted wetlands and though quantified data are not available it is reasonable to assume that these farms have brought about ecological changes and effected other uses of such areas. Conflicts with other uses and possible adverse consequences of converting wetlands into aquaculture farms are therefore major consideration to be addressed.

It has to be recognized that not all environmental consequences of aquaculture are negative. Indeed, many of them are highly beneficial to effective environmental management and the socio-economic wee-being of human population, when the land use is regenerative rather than merely exploitative. For example, dumping of domestic and industrial diseases such as mosquitoes and flies that thrive in marshy areas, at least reduced, it not eliminated. If domestic and farm wastes are used for fertilizing or feeding, not only would this constitute an inexpensive means of wastes disposal, but it would be effective recycling to produce food and folder.

Another major impact of land-based closed-type aquaculture system is the abstraction, retention and drainage of water. Many farms depend on surface-water from streams, rivers, lakes, estuaries, and coastal areas, but some depend entirely on spring and underground sources. Competing use of surface-water is largely restricted to irrigation systems and drinking water supplies. Though use of water in a aquaculture is often considered as ‘non-consumptive’, there are significant losses due to seepage and evaporation, depending on soil properties and climatic conditions.

Aquaculture can often utilize water that is unsuited for drinking and irrigation such as saline waters occurring in semi-arid areas. In small-scale homestead type fish farming fish ponds often serve also as sources for crop irrigation watering of livestock and for household uses. Integrated rice-field fish culture is another example of noncompetitive use of water resources.

On the other hand when sub-soil water has to be pumped out in large quantities the water table of the area can in the long run become low affecting adversely the underground water resources. When brackish or salt water is used to irrigate the farm salt penetration of the soil in areas surrounding the farms can render them unsuitable for several types of vegetation crops and trees. When farms are constructed along waterfronts near rivers open estuaries and the sea clearing of mangroves or other vegetation without leaving adequate protective barriers can result in soil erosion.

2. Sedimentation and obstruction of water flows In coastal culture of shellfish and cage farming, sedimentation and obstruction of coastal flow are important consequences to be considered. The sediment may consist of fine particles of organic detritus, or coarse sand particles derived from the processes of erosion. Mollusk aquaculture in inter-tidal areas often faces severe problem of active sedimentation, which may result in the abandonment of beds or the transfer of beds towards the sea.

In traditional forms of bottom culture , horizontal beds of deposits are formed and they seldom affect the pattern of water flow. But in the rack culture of oysters, where bags of oysters are placed on rank arranged in lines hundreds of meters long, parallel to the tidal current, sediments accumulate beneath and between culture racks. In the ‘bouchot’ type of mussel farming on pole driven into the seabed, as practiced in France (Pillay , 1990), the line of posts act as walls against the circulation of water when the mussel grow to adult size and block the spaces between the poles.

There is heavy sedimentation and part of the sediments may be redeposited. Eventual submersion of the poles to as much as about one-half of their length has been observed (Ottman & Sornin, 1985). Studies made in Sweden that the sedimentation rate in mussel farms can be three times that found in areas away from the farm (Dahlback & Gnnarsson, 1981)

In addition to the deposition of detritus that the pattern of water flows; the production of biodeposits by filter-feeding bivalves under culture augment the rates of sedimentation. The biodeposits utilize considerable quantities of oxygen in oxidizing the organic matter contained in them; and eventually create a reducing environment and the production of hydrogen sulphide (H 2 O). in addition, shear velocity and viscosity of mud is increased. Biodeposits increase the quantity of mud; and since organic mud is resistant to erosion; sedimentation rates are enhanced; resulting in the elevation of the seabed to the extent of 30 -50 cm per year.

Bouchot

Cage-farming

mollusk aquaculture

beds in aquaculture