J. Kövári
Food and Agriculture Organization of
the United Nations
Rome, Italy
1. INTRODUCTION
2. FACTORS TO BE CONSIDERED IN SITE SELECTION
3. REFERENCES
The success of an aquaculture project depends to a large extent on the proper selection of the site to be developed into a fish farm or hatchery. A substantial number of papers have been published in the past on the subject of site selection for both freshwater and brackishwater farms. In compiling this paper, the author has made an attempt to summarize all the factors to be considered and the investigations to be carried out for the selection of sites for aquaculture projects.
2.1 Ecological factors
2.2 Biological and operational factors
2.3 Economic and social factors
In order to select a suitable site for aquaculture, the following factors have to be considered on the site (Pillay, 1977).
2.1.1 Water supply
2.1.2 Water quality
2.1.3 Climate
2.1.4 Hydrological characteristics
2.1.5 Soil characteristics
2.1.6 Land
An assured water supply of sufficient quantity and adequate quality is the most important factor to be considered when deciding on the suitability of a fish pond site. Therefore, the investigations for a proper water source should be most thoroughly conducted in site selection. The statement of the Irrigation Division or other Water Authority for the required amount of water to the ponds or the hatchery, and any restriction on water supply to the fish farm (for example, periods of maintenance in an irrigation channel), must be acquired.
The source of water may be an irrigation canal, river, creek, reservoir, lake, spring, rainfall runoff and dug or deep wells. Water can be supplied via feeder channel, storage tank or pipeline by gravity or by pumping to the ponds. The most economical method is by gravity. For a pond built in suitable soil, the minimum supply of water should be 5 l/sec/ ha of pond throughout the year.
If rainfall runoff is to be used, and stored in a reservoir to supply the ponds, a ratio of 10 to 15 ha of catchment area to 1 ha of pond is required if the catchment area is pasture; a slightly higher ratio is needed for woodland, and less for land under cultivation (Hora, 1962).
The drainage possibility of the ponds should be carefully investigated during the site selection. Gravity drainage of the ponds is preferable. For draining a pond by gravity, its bottom should be at a level higher than that which the maximum water table reaches during the harvesting periods in a normal year.
Quality of water is one of the most significant factors to be considered in site selection. It should be investigated by taking a number of water samples from the proposed water source for laboratory analyses of physical, chemical, biological and micro-biological properties, including health hazards. Water test procedures should be in accordance with the relevant Standard Classification in the country on water quality. From a production point of view, emphasis should be placed on the following:
(i) physical properties - temperature, colour, odour, turbidity, transparency, suspended solids.(ii) chemical properties - pH, dissolved oxygen, biochemical oxygen demand, free carbon dioxide, alkalinity, salinity, dissolved solids, ammonia, all as regards both useful and toxic qualifies; also whether pollutants of agricultural or industrial origin are present, and if so, to what extent.
(iii) biological properties - quality and density of plankton. (iv) micro-biological properties - species and quantity of parasites.
Important climatological factors to be obtained from the meteorological station nearest to the site are as follows:
- mean monthly temperature
- mean monthly rainfall
- mean monthly evaporation
- mean monthly humidity
- mean monthly sunshine
- mean monthly wind speed and direction
Obviously, the longer the period of record, the better the data will be.
Information on the pattern of precipitation (maximum in any 24 hours) and incidence of high winds, heavy storms or cyclones, should be considered. The incidence and amount of damage caused by storms or earthquakes in the project area should also be noted.
The most important data needed for site selection can be gathered from such sources as Irrigation Departments or other Water Authorities. The following are needed:
data for discharge, yield, floods and water elevations of existing water sources (rivers, irrigation channels, reservoirs, springs, etc.).
Field investigations to determine surface and sub-surface soil conditions at the site should be made as early as possible. Often money can be saved if proper soil explorations are made before the site is procured. They may reveal soil conditions undesirable for pond construction, in which case another site may have to be found. Investigations should be carried out in order to ascertain the suitability of soil both for construction and operation of ponds.
For engineering purposes, the techniques used for soil investigations vary from relatively simple visual inspection to detailed sub-surface exploration and laboratory tests. Visual inspection of the site is an essential preliminary step. In order to provide data on sub-surface soils, a test pit measuring 0.80×1.50 m with a depth of 1.50 to 2.0 m, depending on the land form and the elevation of the water table, should be dug in each hectare of the site. Digging of a test pit permits visual examination of soil and also makes it possible to obtain disturbed and undisturbed samples of soils encountered in the different layers below ground level.
Soils have characteristics that can easily be determined by sight and feel. Visual examinations are employed in place of precise laboratory tests to define the basic soil properties. A sandy clay to clayey loam is the best type of soil both for pond construction and growing natural foods at the pond bottom. Areas with a layer of organic soil over 0.60 m in thickness are unsuitable for any kind of fish pond, because it would be difficult to maintain water levels in the ponds due to high seepage; also, it would be necessary to transport suitable soils for dike construction to the site, and this will be costly. Big surface stones or rock outcrops may make an area unsuitable for anything except lined ponds or concrete raceways.
In general, a site will be suitable for construction of fish ponds if soils below the proposed pond bottoms have a grain-size curve which can be plotted to the left side of the grain-size curve A in Figure 1, together with a coefficient of permeability less than k = 5 × 10-6 m/sec. Dikes without any impervious clay core may be generally built from soils having grain-size curves plotted between the grain-size curves A and B in Figure 1 or having a coefficient of permeability between k = 5 × 10-6 and 1 × 10-4 m/sec (MI., 1972).
Figure 1. Semilogarithmic grain-size curves of soils
Clay used for impervious core in the dike should have a liquid limit < 80 percent, a plastic limit < 20 percent, and a plasticity index > 30 percent. On the other hand, using the boundary classification, the following soil groups are suitable for dike construction as follows (IS, 1970).
|
|
Stability of dike |
Permeability cm/sec |
|
GM |
Reasonably stable; may be used for impervious cores or blankets |
10-3 to 10-6 |
|
GC |
Fairly stable; may be used for impervious cores |
10-6 to 10-8 |
|
SM |
Fairly stable; may be used for impervious cores or dikes |
10-3 to 10-6 |
|
SC |
Fairly stable; use for impervious cores |
10-6 to 10-8 |
|
ML, MI |
Poor stability; may be used for dikes with proper control |
10-3 to 10-6 |
|
CL, CI |
Stable; impervious cores and blankets |
10-6 to 10-8 |
|
CH |
Fair stability with flat slopes; used for cores, blankets and dike sections |
10-6 to 10-8 |
For production purposes, a chemical analysis of the soil should be conducted by using representative samples from the different layers found in the test pits. In general, the pH, available nutrients such as phosphorus, potassium, organic carbon and nitrate, etc., are determined by chemical analysis of soil.
It should be confirmed that the proposed land area is suitable. The general conformation of the land should be with slopes not steeper than 2 percent. If wasteland, unsuitable for agriculture or other direct use, is selected for a project, the cost of the land will be low. Land elevation and flood level are important factors in determining the suitability of the area for the construction of a fish farm or hatchery. The land should be free from deep flooding; the maximum flood level for the past 10 years should not be higher than the top of the dikes. Observation of the marks left by flood waters on bridges or other structures at the site, or questioning of local people, may give information about the expected heights of floods. The shape and size of available land should be considered: land with a regular shape and extensive enough for future expansion is ideally suitable for a fish farm. It is very important to know the development plans for the area as it would be unwise to select a site for a project in a region where future industrial activity may cause air and water pollution. Similarly, if a site is adjacent to a heavily populated area, the risk of pollution should be borne in mind. However, some industrial and agricultural wastes may be utilized in fish farming. In such cases, special investigations should be conducted on their utilization or required treatment.
Underground utilities crossing the site (oil pipelines, etc.) may render otherwise good sites unsuitable for a project. Generally, high electric power poles, radio masts and the like are not allowed in the pond area.
The type and density of vegetation depend partly on the land elevation. Vegetation is also an indicator of soil types and of the elevation of the water table. The type and density of vegetation, its size and the root systems of trees largely determine the method of clearing the site and, therefore, the construction time and cost.
Grassland, abandoned paddy fields, open woodland or land covered with low shrubs and bushes allow cheaper construction than land with very thick jungle or swampy areas with high trees. However, in the cyclone belt or in areas where strong winds are frequent, it is very important to have a wide and high windbreak of thick vegetation against the direction of the prevailing wind.
Before a site can be selected for a project, the following should be ascertained:
- species to be cultured
- resources and availability of stocking materials (spawners, fry or fingerlings)
- type of project(i) small-scale rural project
(ii) large-scale rural project- system of culture adopted
(i) extensive
(ii) semi-intensive
(iii) intensive- operational method
(i) monoculture
(ii) polyculture
(iii) integrated- production target
- estimated size of area required.
The most important economic and social factors are as follows:
- development plans for the project area- ownership, availability of land and land values, land regulations and rights, as well as any legal restrictions relating to land
- proximity to all-weather road connections
- availability of electricity, telephone or radio connections, as well as unit power cost
- availability of equipment, services and supplies needed for running the project
- availability of construction materials
- location of markets for the produce and determination of demand
- availability of organic and artificial fertilizers, drugs and chemical materials
- availability of supplementary feeds
- costs of equipment, materials, feeds, etc. needed for running the project
- availability of suitable transport facilities
- availability of ice for marketing
- availability of staff with adequate experience of pond management
- availability of skilled and semi-skilled labourers
- reasonable amenities for permanent staff, for example, schools, shopping facilities, hospital, etc.
- information on the local financing methods or credits
- political realities
Hora, S.L. and T.V.R. Pillay, 1962. Handbook on fish culture in the Indo-Pacific region. FAO Fish. Tech. Pap.. (14):204 p.
Indian Standard, 1970. Classification and identification of soils for general engineering purposes. New Delhi, Indian Standards Institution; (IS:1498-1970):24 p.
Pillay, T.V.R., 1977. Planning of aquaculture development - an introductory guide. Farnham, Surrey, Fishing News Books Ltd., for FAO, 72 p.
Szabványügyi Hivatal, 1972. Müszaki irányelvek (MI:15218-53) Vizépités - Foldgátak tervezési irányelvei. (Hungarian Standard (MI:15218-53) Planning Standard of earthfill dams for hydraulic engineering). Budapest, Szabványügyi Hivatal; 5 p.
