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3 CRITERIA FOR SELECTION OF A SITE FOR BFRS

3.1 Introduction

(09) The appointment of the Soil specialist, responsible for the actual site selection of BFRS, could not be completed as early as scheduled, and therefore the present research plan could not be based on a selected site. However, the criteria for selection of a suitable site (from the point of view of research needs) were discussed with the Soil specialist, and projected on a preliminary identified site. These various criteria are detailed hereunder.

3.2 Topography

(10) The topography (the surface features) of an area proposed for fish farm construction is the most important factor in designing fish farms, whether they are for production purposes or for research. A suitable site for a fish pond has a topography that can be converted into a pond economically. The cost of construction can be greatly reduced if the surface features of the land are used to advantage.

Flat coastal swamplands, whose elevations are within the range of tidal pond bottom elevation, are prefered for brackishwater pond culture. In such areas excavation or filling are at a minimum, if any. Sites with ralling topography or those with elevations from low to high must be avoided as these will be too costly to level.

(11) Coastal zones are sometimes divided into four categories with respect to their suitability for fish pond development. In Bangladesh such a zonation appears to be applicable too. The zones are defined as follows:

While selecting a site, it might be worthwile to indicate such zonations, either through contour maps or by lined diagrams.

3.3 Soil type

(12) Many soil characteristics, especially those related to texture, determine its suitability for fish pond purposes. Soil texture refers to the relative proportion of sand, silt and clay in the soil contents. According to the International Society for Soil Science (ISSS) the soil particles are classified as:

- clay- under 0.002 mm;
- silt- 0.02 to 0.002 mm;
- sand (fine)- 0.2 to 0.02 mm;
- sand (coarse)- 2 to 0.2 mm;
- gravel- above 2 mm.

According to the percentage combination of sand, silt and clay in a soil, it is classified as sand, sandy loam, loam, silty loam, sandy clay loam, clay loam, silty clay loam, sandy clay, clay, or silty clay.

(13) Areas suitable for fish production must possess properties which allow for the economical construction of dikes, efficient growth of fish food, extended water holding and load carrying capacities, and favourable chemical properties. Soils belonging to the following textural classification are desirable for fish pond development:

Clayey soils are preferable, because they consist of superior material for diking and holding water. They have good compaction characteristics and low permeability. A very simple general rule can be followed: “as the clay content of the soil decreases, its suitability for fish pond construction also decreases”.

Loamy soils are also recommended. They have good organic matter content which favours the culture and growth of natural fish food.

(14) As standard practice, several samples should be obtained from pre-determined and scattered locations within a given site through borings. The best system is to have a number of borings in well-planned locations to cover the whole site. Soil samples should at least be drawn from 10 random locations per hectare.

The determination of the following factors from the soil are of greatest importance:

- For physical properties - textural class, load bearing capacity, and permeability;
- For chemical properties - soil pH, and presence of acid-forming substances such as pyrites and sulfides.

(15) As said before, the most important physical property of the soil is the texture. Much can already be said about the characteristics of the soil knowing its texture, e.g. seepage, attainable compaction, and load bearing capacity.

Load bearing capacity informs and guides the designers in determining the type of foundation for structures (gates, dikes, and buildings). Structures should be of light-weight construction, because brackishwater pond sites usually have low bearing capacity. From experience it has been found that structures (other than buildings) should not exceed contact pressure of more than 1.5 tons/m2.

(16) An immediate and initial characteristic of the chemical nature of the site is provided by the soil pH, which has direct relationship to fish production. The pH scale varies from 1 to 14 units and the desirable range of soil as well as water for aquaculture is 6.5 to 8.5. Lower than 6 is too acidic for most fish species and other aquatic animals. Likewise, greater values than 9 approach polluted conditions and are infavourable for growth. It is recommended that extensive determination of pH is done down to l m deep, at 0.25 m intervals, especially in potential acid sulfate soils.

(17) The salinity status of the semi-saline soils in Bangladesh widely varies from region to region and also exhibits seasonal variation within a region, depending on the quantum of fresh water received. A generalized account of the nature of soil salinity between different months may be stated as high during the summer and lowest during monsoon. Moreover, salt content along the soil profile also undergoes seasonal variation. During the dry summer months, due to capillary rise of saline ground water, salt accumulates on the surface of the soil. In contrast, due to continuous submergence during monsoon months, soil salinity is then lowered at all depths upto the sub-soil water table.

3.4 Tidal condition

(18) Knowledge of tidal characteristics of a site in a brackishwater area is very important for determining its suitability for fish ponds. The height of the tide and its range determine the sufficiency of water, the elevation of the pond bottom and the water gates, the height of the dikes, the size of the gate openings, the costs of construction, etc. In Bangladesh the critical spring tide period is between December, January and early February.

(19) Sites near factories, food processing plants, oil sources, and densely populated areas should be avoided as they are sources of pollution of the water supply.

3.5 Source, adequacy and quality of brackish/fresh water

(20) Primarily, a constant supply of brackishwater must be available to the ponds throughout the year. This usually acts as a buffer to all propensities in high density culture systems and also serves to replenish water lost through seepage and evaporation, and to exchange water in ponds. Conditions prevailing during all the seasons of the year must be known and investigations of the water sources should be conducted with regards to its flow patterns and directions, the rise and fall in tides, the silt carrying capacity, etc. The distance from the farm site to the water source is an economic consideration, determining the capital outly on pumps, pipes and channels.

(21) Attention must be directed to the salinity of the pond water in tropical areas where heavy evaporation takes place. If the salinity increases more than the desired level, fresh water from a ground source or a river is required to be added to the pond if exchange is not possible. In a research station a source of fresh water will also be needed for laboratory and domestic purposes.

3.6 Type and density of vegetation

(22) Coastal fish ponds in the tropical and sub-tropical zones are constructed on tidal lands, river estuaries, bays and sheltered coasts. The vegetation present in these areas is diversified, depending on the land elevation and soil type. Vegetation provides several beneficial uses not only in shrimp culture but also for other purposes, like checking erosion on the dikes, etc.

The type and density of vegetation is an important criterion in fishpond site selection. Knowledge of the composition and abundance can provide information for an easy and practical evaluation of the physical and chemical properties of the soil.

(23) There is a direct relationship between vegetation and the cost of fishpond construction work. Areas with dense rooting systems are more difficult to excavate. At the same time, densely vegetated areas take longer time to clear of stumps. As a result, cost of fishpond construction is higher.

3.7 Climatic condition

(24) Wind and rainfall are the climatic factors largely affecting site selection. The direction of prevailing wind is reckoned with in designing the layout of fishponds, as it generates erosive wave action against the dikes. Natural water circulation and aeration in the pond is also caused by the wind energy. For rainfall information, maximum intensity, duration, frequency and annual distribution within the watershed are important as these are associated with flooding that would affect the site. Rain also interferes with salinity.

(25) Information on the pattern and recurrence of maximum height of flood water in the site area must be determined from the rainfall or flood records of the appropriate agency. However, it is common that shrimp sites are located in remote areas where such records may not be available. In such a situation rough information on flood may be obtained from knowledgeable residents who have seen the recurrence of floods for years in the area.

(26) In addition to this it will be necessary to study the cyclonic and flood hazards and the wave and wind actions in the area, to properly evaluate a site. Particularly during inclement weather, these factors will determine the possible storm surge level and help in fixing the height of the peripheral dike to protect the farm/station from flood.

3.8 Accessibility, transportation, marketing facilities, and availability of construction materials/labour

(27) The selected site should have road connections to the nearest marketing centre. An outlay of roads to the nearest transportation network would be required. The materials and labour required for construction should be locally available. The materials required for maintenance and operation of the farm should also be available readily, along with a marketing facility for easy disposal of the products.

3.9 Ecology and environment

(28) It is important to assess the pattern of land and water use in the area to determine the impact of this on the project. The following activities must be included in the overall assessment: navigation, fishing, industries, public utilities, recreation, and nursery areas. It is best that a complementory rather than competitive relationship between these various uses and the project be established.

3.10 Availability of shrimp seeds

(29) Adequate quantity/quality shrimp seeds should be available near the area of the site. A survey may be required around the area to assess the assured supply of seed of culturable species. For polyculture the availability of culturable species of fin fish seed should also be explored.

3.11 Socio-economic impact

(30) A large portion of the village population relies, to a considerable extent, on brackishwater areas and its resources, both socially and economically. With a rapidly expanding population, the pressure for the utilization of these resources is now greatly felt. It is therefore necessary that the possible local socio-economic impact of the proposed research station and its expected spin-off be properly examined, and that the project is so oriented that it should ultimately cater to welfare of the local population.

3.12 Evaluation of a surveyed site

(31) The most commonly method of deciding the suitability of sites from among a number of prospective sites surveyed is the “point and ranking” system. For example, in evaluating the accessibility:

Similarly scores can be assigned to all other criteria as enumerated above. To repeat, the salient consideration should be for:

Based on this scoring the sites can be evaluated and ranked as per the percentage chart given below:

Range of scores (%)Evaluation
80 – 100Excellent site for development
60 –   79Very good
40 –   59Good
Below 40Not worth considering


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