There must be enough soil available for pond construction, whether the ponds are to be excavated or pond banks are to be erected above ground.
Unless good information about the soil characteristics is already available, site assessments should include taking a suitable number of soil cores up to 1 m deeper than the expected pond depth. These must be analysed for their soil classification and chemistry.
If rocks, boulders and tree stumps are present, you must consider the cost of their removal (to make the pond bottoms flat and for constructing impervious pond banks) while you are assessing the economic feasibility of the farm.
Flooded and saturated areas are difficult to construct ponds in, and the expenses of doing so must be taken into consideration.
Construction of concrete pond structures (e.g. pond outlets) is difficult in soils with a high salt content. Preferably, the site should have a shape which allows you to construct regular-shaped ponds.
Irregular-shaped ponds are difficult to manage; rectangular ponds are more efficient to operate.
Although supplemental food is given to freshwater prawns reared in earthen ponds, a considerable amount of their food intake is from natural sources. It is therefore preferable to site the farm where the soil is fertile, as this will reduce the need and costs of fertilisation.
Since a water pH of 7.0-8.5 is required for successful freshwater prawn culture, it is preferable not to build the farm on potentially acid sulphate soils. These soils have pH values of 4.5 or less, together with high concentrations of soluble iron, manganese and aluminium.
Most people associate the occurrence of acid sulphate soils with mangrove areas but they also occur far away from such areas. Aquaculture ponds are frequently constructed on such soils, despite their poor suitability. However, their production levels are often too low, or the costs of liming and fertilisation are too high, for them to be financially viable.
Freshwater prawn ponds should be constructed on soil which has good water retention characteristics or where suitable materials can be economically brought onto the site to improve water retention.
The water retention characteristics of soil are highly site-specific and prospective farmers must seek the professional advice of soil engineers and fishery officials from local government departments, such as the Ministry of Agriculture and the Public Works Department. If there are other fish farms or irrigation reservoirs in the area, you should ask the neighbouring farmers for advice, based on their specific local experience.
Pervious soils, which are very sandy or consist of a mixture of gravel and sand, are unsuitable unless the water table is high and surrounding areas are always waterlogged. Soils which consist of silt or clay, or a mixture of these with a small proportion of sand, normally have good water retention characteristics. Peaty soils are not suitable.
The clay content should not exceed 60%; higher clay content soils swell when moist and crack during the dry season, thus making repairs necessary. Methods for the preliminary assessment of particle sizes, permeability and plasticity (how well soils will compact to their optimum strength and permeability) are given in FAO (1985).
Choosing Your Site: Power Supplies
A source of electricity is desirable but not essential. A variety of power sources may be used for supplying the energy necessary for water movement on the farm including:
- water power itself (gravity and current flow);
- wind;
- electricity;
- petrol and diesel fuel; and
- wood.
Electricity is desirable, although it need not be the sole source of energy, for powering lights, wells and feed-making equipment. The most suitable power source to use is entirely site-specific and depends upon such factors as equipment availability, unit power costs and the characteristics of the site and its water supply.
Generating electricity on the farm may be cheaper than running a new supply from the nearest point on the national power grid. Where a power failure would quickly result in severe losses, for example in farms operating highly intensive systems dependent on aeration, a back-up power source (usually a diesel generator) is essential.
The ideal would be for you to be able to move water within your site by gravity but this depends on the nature of the site. In practice, most farms use electric or fuel-driven pumps for supplying water to the ponds (Figure 9) and some also use them for draining the ponds during harvesting (Figure 10). Some small farms prepare cooked feed using wood as a fuel source, while others utilize the time-old methods of wind and water power for transporting water. Windmills and water-wheels can also be used to pump water for filling ponds, or to generate a farm supply of electricity.
Choosing Your Site: Fry And Consumables
There is no fundamental technical difficulty in transporting postlarval freshwater prawns long distances by road, rail or even air. However, you need provide vehicle access close to the pond site. It is not satisfactory to bring PL long distances to your grow-out site if there are going to be further delays due to poor local access.
In selecting the site of your farm, it is important to assess the cost of obtaining PL. Transport costs can add enormously to basic stocking costs. Also, PL prices themselves tend to rise as the distance between the farm and the nearest hatchery increases (because there is less competition between hatchery operators).
Also, you need to consider the availability and cost of getting feeds to your potential farm site. A large farm (say 40 ha) which achieves an average output of 2 500 kg/ha/yr, for example, would require an average of about 5 mt of dry feed per week.
Supposing that this feed is delivered to the site monthly, it would arrive in 20 mt batches; this means you need good vehicle access to the site. You would also need to provide clean, dry, cool, and secure feed storage facilities on the site. Similar factors apply to the supply of other consumables, such as fertilizers and equipment.
Smaller farms, of course, do not have such sophisticated requirements. However, these factors are still important, especially the availability of good storage facilities.
Choosing Your Site: Labour
Small freshwater prawn farms can be successfully maintained by unskilled labour but outside assistance from community (e.g. cooperative groups of farmers) and commercial sources (hatchery operators, feed suppliers, etc.), is necessary at times of stocking or harvesting. Larger farms require a competent, on-site manager. The amount of labour utilized on freshwater prawn farms varies considerably.
For example, it is estimated that a 40 ha farm needs two senior staff and six labourers. At the other extreme, one person should be able to take care of normal maintenance, including feeding but excluding harvesting, of a 1-2 ha freshwater prawn farm. Often this type of farm is family owned and operated.
Author:
Michael B. New
