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Evaluation of Land Suitability for Crab Culture: A Methodological Study using GIS
Construction of Sub-models Analysis was first made of essential criteria for crab farming potential, and the spatial data obtained for these criteria are described. The primary data were then grouped into series of sub-models. Water sources: When evaluating a site for aquaculture potential, the total land and water resources of the site are the prime determining factors. However, growing demands for water from an expanding aquaculture industry competes with other water users such as irrigation, industry, navigation, drinking, and for meeting environmental needs day by day for this limited resources (Muir and Beveridge, 1987; Patricia, 1999; Phillips et al.1991). For this study, an indication of water availability for crab culture in the coastal zone was obtained by constructing a layer based on of perennial streams, tidal canals and river channels. Available Land: All land in various categories, which would not be available for development was removed from consideration, such as rivers, roads, mangrove forest and urban development. The land, which was considered most suitable for this study, was that which is near the coastline and not intensively used for agriculture, in order to minimise the conflict with other activities. Support: Supports services were designated as the extension services and other help which farmers can obtain from Non Governmental Organisations (NGO), Government offices, Research stations, Universities and by agglomeration with other farms. Using tabular data, a layer was compiled for support from NGO, Government offices, Research stations, Universities and agglomeration with other farms for crab farming in the region. Soil chemistry: Soil quality is also an important land criterion. From the soil reconnaissance map of the Soil Resources Development Institute (SRDI) during the period of 1972 to 1975, a soil texture and pH layer were created by digitising the map and converted into raster format. Soils were classified according to their texture (Figure 3, Coche,1985). Data on salinity and pH collected during a 1998 field visit were also used to develop the soil sub-model. Risk assessment sub-model: To compile the risk sub-model, different layer as winter rainfall, pollution from the industries and town centre, drought and elevation were added together from maps and tabular data. Sources of crab seed: Crab seed have to be available for successful culture practice. If hatchery reared seeds are not available then the juvenile crab must be collected from the wild. Thus, there is an advantage in knowing where crab seed are most abundant so that they can be collected. In Khulna region, most of the seeds come from the environment, especially from the Sundarban mangrove forest. Water chemistry: The water chemistry sub-model was prepared from different sources of thematic layer and tabular data. Water temperature was calculated from air temperature using regression equation (Kapetsky,1994) and likewise, dissolved oxygen was derived from temperature and salinity relationship describe by (Colt, 1984). Moreover, water salinity was taken from Surface Water Modelling Centre (SWMC) report (1996) and data on water pH collected during field visit (1998), were also used to develop the water sub-model.  Figure 2: Comparison of the best soils for pond construction with the soil texture classes (after Coche, 1985). |