CHAPTER 2
SITE SELECTION

A STUDY OF THE POTENTIAL market for the product and careful selection of suitable sites for prawn culture, whether it be for the larval (hatchery) or grow-out phases, is an essential prerequisite for successful farming. Failure to realize this before any project is commenced is likely to cause the ultimate downfall of the enterprise, which not only has unfortunate consequences for the farmer and investor(s) involved but may also cause serious damage to the image of prawn farming, both nationally and even internationally. Marketing is covered later in this manual.

The current section of the manual contains a brief description of the essential characteristics of good sites for freshwater prawn farming. More detailed information is available in a review by Muir and Lombardi (2000). You are also strongly recommended to obtain and study the FAO manuals on topography (FAO 1988, 1989b), soils (FAO 1985), and water (FAO 1981), as well as the section on site selection in FAO (1995)¹.

2.1 Hatcheries and indoor nurseries

The site requirements for hatcheries and indoor nurseries, which are normally associated with each other, are similar. In this section of the manual, reference to hatcheries therefore includes indoor nurseries.

NEEDS FOR GOOD QUALITY WATER

Although the larval stages of freshwater prawns require brackishwater for growth and survival, hatcheries do not have to be located on coastal sites. Prawn hatcheries can be sited on inland sites. There, the necessary brackishwater can be obtained by mixing locally available freshwater with seawater or brine (and sometimes artificial seawater) which has been transported to the site. Two decades ago, when the original FAO manual was written, most hatcheries operated on flow-through systems. Many still do so but the establishment of inland hatcheries, the costs of obtaining and transporting seawater or brine, and increasing concerns about the discharge of saline water in inland areas have encouraged some operators to minimize water consumption through partial or full recirculation systems. Inland hatcheries have the advantage that they can be sited wherever suitable freshwater is available and their market (namely outdoor nurseries and grow-out facilities) is close by. Where to site a hatchery is therefore not only a technical but also an economic consideration. This involves balancing the costs of transporting seawater and brine, or using recirculation, against the advantages of an inland site. Prawn hatcheries, regardless of type, require an abundant source of freshwater as well as seawater or brine. The quality of intake water, whether it be saline or fresh, is of paramount importance for efficient hatchery operation. Water quality is thus a critical factor in site selection. Hatchery sites should preferably be far from cities, harbours and industrial centres, or other activities which may pollute the water supply.

Due to the extra problems and dangers involved, it is generally recommended that freshwater prawn hatcheries should not be sited where the only source of water is surface water. However, this guidance has not always been observed. The minimum requirement during site evaluation should be to carry out watershed surveys and water analyses, especially for pesticides and oil spill residues. In coastal areas, it may be possible to draw good quality water from sub-surface layers, usually with freshwater overlying more saline water. The ideal site, where wells sunk to different depths provide both freshwater and seawater, is rare, although it is sometimes possible to make good use of groundwater sources, which are usually cleaner and less liable to become contaminated. The quality of water depends on the soil materials. In coastal areas with underlying coral rock, hatcheries can often get good quality seawater, free of pollution or harmful protozoa and bacteria. If sites with borehole seawater are not available, direct access to a sandy beach with mixed sand particle size can be selected. On this type of site a shallow beach filter of the type described in Annex 2 can be utilized. Muddy areas are not so suitable, but a larger filter may be used, provided it can be cleaned out periodically.

Many freshwater prawn hatcheries utilize surface supplies for both freshwater and seawater. Often, seawater can be drawn from areas where the salinity is 30 to 35 ppt, usually through a rigid pier off-take in the sea or a flexible buoyed system. Crude screening can be used to prevent the entry of the larger flora and fauna but this alone is not sufficient to protect the larvae from disease and parasitical problems. The use of unfiltered water will almost certainly result in disaster, so additional filtration is essential. Brine, sometimes used instead of seawater for inland hatcheries to minimize transport costs, can be obtained from salt evaporation pans. The brine, which is often between 80-100 ppt salinity but can be as high as 180 ppt, can be diluted with freshwater to form brackishwater (in theory, the higher the salinity of the brine used, the better; this is because the sudden osmotic shock which occurs when brine and freshwater are mixed together may reduce the numbers of bacteria and parasites present in the original supplies). Some hatcheries obtain freshwater pumped or fed by gravity from surface supplies such as rivers or irrigation canals. This practice exposes the hatchery to severe variations in water quality and particularly to water contamination from agricultural chemicals.

In all cases, water supplies need careful analysis during site selection, to determine their physical, chemical, and biological characteristics, and the extent to which these may vary daily, seasonally, or through other cycles. Special care is needed where hatcheries are situated in or near areas where the use of pesticides, herbicides, and fertilizers is intensive. Ideally, freshwater should be obtained from underground sources, though some of these may be unsuitable because of high levels of iron and manganese, which are lethal to prawn larvae. Methods of reducing the levels of these ions are provided later in this section of the manual. City tap water is also normally suitable, provided it is vigorously aerated for 24-48 hours before use to remove residual chlorine, but may be too expensive to use. Well water should also be aerated, by cascading for example, to bring its dissolved oxygen level up to, or near to saturation point.

The brackishwater derived from the mixture of seawater, brine or artificial sea salts with freshwater for use in M. rosenbergii hatcheries should be 12-16 ppt, have a pH of 7.0 to 8.5, and contain a minimum dissolved oxygen level of 5 ppm. Water of various levels of salinity is also required for hatching Artemia as a larval food (Annex 4); the ideal hatching salinity depends on the source of cysts. The use of estuarine water, which would theoretically limit the need to balance freshwater and seawater to obtain the optimum salinity, is possible. However, the salinity of estuarine water varies, both diurnally and seasonally, making management difficult. In addition, although estuarine water can be utilized if its salinity is above the hatchery operating salinity, its use is not recommended because the levels of micro-organisms and potential pollution may be high.

Both freshwater and seawater must be free from heavy metals (from industrial sources), marine pollution, and herbicide and insecticide residues (from agricultural sources), as well as biological contamination (e.g. as indicated by the presence of faecal coliforms, which can be common in residential and agricultural areas). The analyses of water found suitable for use in freshwater prawn hatcheries are given in Table 2. Not much is known about the tolerance of larvae to toxic materials but it can be assumed that larvae are at least as (probably more) susceptible to pollution and toxicity as juveniles. As safe and lethal levels of specific substances are not yet fully understood, it is inappropriate to provide a summary of current research in this manual. Those who wish to know more about this topic are recommended to consult Boyd and Zimmermann (2000), Correia, Suwannatous and New (2000) and Daniels, Cavalli and Smullen (2000).

If seawater or freshwater is drawn from surface supplies, some form of treatment is essential, as discussed later in this manual. Both freshwater and seawater used for hatchery purposes should have a pH and a temperature as close as possible to the optimum range. Hydrogen sulphide and chlorine (e.g. from tap water) must be absent. High levels of nitrite and nitrate nitrogen must be avoided. Seawater should have as little diurnal or seasonal variation as possible. Very hard (reported as CaCO3 level) freshwater should be avoided. The levels of iron (Fe) and manganese (Mn) should be low; copper (Cu) toxicity may also be a problem, especially after larval stage VI. However, some iron and manganese can be precipitated from well water by aeration; the resultant floc can be removed by sand filtration, or by biofiltration and settling (Box 1).

High levels of heavy metals, such as mercury (Hg), lead (Pb) and zinc (Zn), should also be avoided - these are most likely to be caused by industrial pollution. In general, especially where surface water is used, hatcheries should not be sited where their water supplies are endangered by pollution from tanker discharge, oil refineries, tanning, agricultural pesticides and herbicides, or chemical factories. In practice, an ‘ideal’ water supply might be difficult to define, but a summary of the characteristics of water found suitable for use in freshwater prawn hatcheries is provided in Table 2.

Artificial seawater has been used in some recirculation systems, especially in research. The stimulus for such work is that its use may reduce the problems caused by water pollution, parasites, and the presence of prawn competitors and predators in larval rearing tanks. Many formulations for artificial seawater exist and commercial preparations are sold in the aquarium trade. However, not all have been found suitable for freshwater prawns and many are complex and expensive. The exact and specific ionic composition that is optimum for freshwater prawns is not yet known. The formula for a simple preparation which has been used in Macrobrachium rosenbergii hatcheries is given in
Table 3. This contains the essential ions sodium, potassium, chloride, bromide, carbonate and sulphate, together with the correct ratio of calcium and magnesium. This preparation may not be complete, and there is some evidence that its use increases oxygen consumption after larval stage V, but it (and variations of the formula) have been used in research and a few commercial cycles in Brazil. The unit cost, even for such a simple formula, is high (e.g. US$ 75/m3 in Brazil in 2000). However, not much is required because evaporative losses can be made up with freshwater alone and, if properly handled and processed, the same brackishwater can be used for two consecutive larval cycles without affecting production. The productivity of systems using artificial seawater is reported to be as high as 40 PL/L but the larval cycle may take about 10% longer than when natural seawater is used. Due to its cost and the uncertainty about its effectiveness, the use of artificial seawater is not recommended in this manual. Whenever possible, the use of natural seawater or brine is recommended.

DECIDING HOW MUCH WATER IS NEEDED

The quantity of freshwater and seawater required for a freshwater prawn hatchery depends not only on the proposed scale of operation but also on the type of management utilized (flow-through, recirculation, use of brine). Flow-through systems obviously require the maximum quantities of water. All other systems will either require less seawater or, in the case of those which utilize brine or artificial seawater, none. It is therefore not possible in this manual to define the exact quantities of water needed, as these are scale, site and management system dependent. An example of the water requirements for a flow-through system using seawater that includes ten 5 m3 larval tanks, each capable of producing 50 000 postlarval prawns (total 500 000 per larval cycle) within a maximum of 35 days, is provided in Box 2.

OTHER REQUIREMENTS FOR HATCHERY SITES

In addition to having sufficient supplies of good quality water, a good hatchery site should also:

• have a secure power supply which is not subject to lengthy power failures. An on-site emergency generator is essential for any hatchery - this should be sized so that it has the output necessary to ensure that the most critical components of the hatchery (e.g. aeration, water flow), can continue to function;
• have good all-weather road access for incoming materials and outgoing PL;
• be on a plot of land with an area appropriate to the scale of the hatchery, that has access to the quantity of seawater and freshwater supplies required without excessive pumping. The cost of pumping water to a site elevated high above sea level, for example, may be an important factor in the economics of the project;
• not be close to cities, harbours, mines and industrial centres, or to other activities that may pollute the water supply;
• be situated in a climate which will maintain water in the optimum range of 28-31°C, without costly environmental manipulation;
• have access to food supplies for larvae;
• employ a high level of technical and managerial skills;
• have access to professional biological assistance from government or other sources;
• have its own indoor/outdoor nursery facilities, or be close to other nursery facilities; and
• be as close as possible to the market for its PL. In the extreme case, it should not more than 16 hours total transport time from the furthest farm it will be supplying.

TABLE 2
CHARACTERISTICS OF WATER SUITABLE FOR FRESHWATER PRAWN HATCHERIES

NOTE: THE SIGN ‘-’ MEANS ‘NOT KNOWN’ OR ‘NO SPECIFIC RECOMMENDATION’.

SOURCE: DERIVED FROM NEW AND SINGHOLKA (1982), CORREIA, SUWANNATOUS AND NEW (2000) AND VALENTI AND DANIELS (2000)

TABLE 3
Artificial brackishwater (12 ppt) for M. rosenbergii hatcheries

NOTE: WEIGH AND DILUTE THE SALTS INDIVIDUALLY WITH PREVIOUSLY FILTERED FRESHWATER. ADD THE RESULTING SOLUTIONS TO A TANK IN THE ORDER SHOWN ABOVE, AND MIX THOROUGHLY USING A PVC STIRRER. THEN ADD FRESHWATER UNTIL THE SALINITY IS REDUCED TO 12 PPT. MAINTAIN THE FINAL SOLUTION UNDER STRONG AERATION FOR 24 HOURS AND ADJUST THE SALINITY AGAIN TO 12 PPT, IF NECESSARY, BEFORE TRANSFER TO THE RECIRCULATION SYSTEM.

SOURCE: VALENTI AND DANIELS (2000)

2.2 Outdoor nurseries and grow-out facilities

The success of any nursery facility or grow-out farm depends on its access to good markets for its output. Its products may be sold to other farms (in the case of nurseries), directly to the public, to local markets and catering facilities, or to processors or exporters. The needs and potential of each type of market need to be considered. For example, more income may result if you can sell your market-sized prawns alive. The scale, nature and locality of the market is the first topic that you should consider and the results of your evaluation will determine whether the site is satisfactory and, if so, the way in which the farm should be designed and operated. Despite the obvious importance of the market, it is surprising how often that this topic is the last criterion to be investigated. It is considered in more detail later in this manual.

It also important to consider other factors to ensure success, including the:

• suitability of the climatic conditions;
• suitability of the topography;
• availability of adequate supplies of good quality water;
• availability of suitable soil for pond construction;
• maximum protection from agricultural and industrial pollution;
• availability of adequate physical access to the site for the provision of supplies and the movement of harvested animals;
• availability of supplies of other necessary inputs, including postlarval and/or juvenile prawns, equipment, aquafeeds or feed ingredients, and power supplies;
• availability of good skilled (managerial) and unskilled labour;
• presence of favourable legislation; and
• availability of adequate investment.

These topics have been discussed in detail in many FAO and other publications, including FAO (1981, 1988, 1989b 1995) and Muir and Lombardi (2000). This section of the manual concentrates on those factors which are particularly important or specific to freshwater prawn farming.

CHOOSING YOUR SITE: TOPOGRAPHY AND ACCESS

Farms must be close to their market so the road access must be good. Large farms will need to have local access for heavy trucks be able to reach the farm easily, for the delivery of supplies and the efficient collection of harvested prawns.

A survey is necessary, to assess the suitability of a site from a topographical point of view. This will include transects, to evaluate slope and to determine the most economic ways of constructing ponds and moving earth. It is important to minimize the quantities of earth to be shifted during pond construction. Flat or slightly sloping lands are the most satisfactory. The ideal site, which slopes close to 2% (2 m in 100 m), allows good savings on earth movement. In addition, ponds constructed on this type of site can be gravity filled (either naturally or by the creation of a dam) and gravity drained. Where potential farm sites are steeper, or if gradients are irregular, care should be taken to ensure that pond sizes and alignments allow efficient construction, and at the same time permit good access and effective water supply and drainage.

The ideal site is rarely available, however. Many successful farms exist where the only feasible method to fill and drain the ponds is by pumping. Some sites, where ponds are excavated in flat, often seasonally flooded areas, may require higher pond banks for flood protection. Prawn farming may be practised in rain-fed ponds but their productivity may be low. The level of productivity in grow-out ponds is governed by complex management factors, which are dealt with later in this manual. The cost of filling and draining ponds, which depend on the characteristics of the site, must be carefully assessed before the site is chosen.

TABLE 4
Diluting seawater and brine to make brackishwater for larval freshwater prawn culture

NOTE: INCOMING FRESHWATER IS ASSUMED TO BE ZERO SALINITY.

CHOOSING YOUR SITE: CLIMATE

This is another fundamentally important issue. You should study the meteorological records to determine temperature, the amount and seasonality of rainfall, evaporation, sunlight, wind speed and direction, and relative humidity. Avoid highly unstable meteorological regions. Strong storms and winds increase the risks of flood and erosion damage, and may lead to problems with transport access and power supply. As far as possible, do not site the farm in an area which is subjected to severe periodic natural catastrophes, such as floods, typhoons, landslips, etc. If you decide to site your farm in an area subject to floods, you will need to make sure that the banks of individual ponds are higher than the highest known water level at that site, or you will need to protect the whole farm with a peripheral bank.

Temperature is a key factor. Seasonal production is possible in semi-tropical zones where the monthly average air temperature remains above 20°C for at least seven months of the year. This occurs, for example, in China and some southern States of continental USA. For successful year-round farming, sites with large diurnal and seasonal fluctuations should be avoided. The optimum temperature range for year-round production is between 25 and 31°C, with the best results achievable if the water temperature is between 28 and 31°C. The temperature of the rearing water is governed not only by the air and ground temperature but by solar warming and the cooling effects of wind and evaporation. The rate by which pond water is exchanged and the temperature of the incoming water are also important considerations.

Rainfall, evaporation rates, relative air humidity and wind speed and direction also need to be investigated. Ideally, evaporation losses should be equal to or slightly lower than rainfall input, to maintain an approximate water balance. However, in some locations this balance changes seasonally. There may be cooler high-rainfall periods during which water can be stored in deeper ponds, and hotter high-evaporation periods in which water supplies decrease. In these areas, it is still possible for you to produce one or more crops by adjusting production plans. Mild winds are useful to promote gas exchange (oxygenation) between water and the atmosphere. However, strong winds can increase water losses by evaporation and may also generate wave action, causing erosion of the pond banks. Avoid areas where it is constantly cloudy because this makes it hard to maintain a steady water temperature, as it interferes with solar penetration. Periods of cloud cover of several days’ duration may also cause algal blooms to crash, which in turn lead to oxygen depletion.

Apart from the dangers of water-supply contamination, you should not site your farm in an area where the ponds themselves are likely to be affected by aerial drift of agricultural sprays; prevailing wind direction should therefore be taken into account. Constructing ponds adjacent to areas where aerial application of herbicides or pesticides is practised is also undesirable. Freshwater prawns, like other crustaceans, are especially susceptible to insecticides.

CHOOSING YOUR SITE: WATER QUALITY AND SUPPLY

Freshwater is normally used for rearing freshwater prawns from postlarvae to market size. Prawns will tolerate partially saline water (reports indicate that they have been experimentally cultured at up to 10 ppt; however, they do not grow so well at this salinity). You could rear Macrobrachium rosenbergii in water which may be too saline to be drinkable or useful for irrigation. Water of 3-4 ppt salinity may be acceptable for the culture of M. rosenbergii, but do not expect to achieve results as good as those obtainable in freshwater.

The reliability of the quality and quantity of the water available at the site is a critical factor in site choice. However, as in the case of hatchery water supplies, the absolute ‘ideal’ for rearing sites may be difficult to define; a range of water qualities may be generally suitable. As for hatchery water, the level of calcium in the freshwater seems to be important. Growth rate has been reported to be lower in hard than in soft water. It is recommended that freshwater prawn farming should not be attempted where the water supply has a total hardness of more than 150 mg/L (CaCO3). Table 5 provides some criteria for water supplies for freshwater prawn nursery and grow-out facilities. The water supply must be free from pollution, particularly agricultural chemicals. Prawn performance is likely to be adversely affected long before lethal levels are reached. However, the exact lethality of various chemicals is still being researched and it is not appropriate to list safe levels in this manual. Those who wish to examine the status of this research may wish to consult Boyd and Zimmermann (2000), Correia, Suwannatous and New (2000) and Daniels, Cavalli and Smullen (2000).

As with hatcheries, the water must also be as predator-free as possible, though standards need not be quite so high. This may be achieved by screening (Figures 8a, 8b and 8c) or by the use of well water. Underground water, because of its chemical and microbiological quality and its lack of predators, is undoubtedly the preferred water source. In practice, sites that only have access to surface water supplies (rivers, lakes, reservoirs, irrigation canals, etc.) are the most commonly used. However, you must be aware of the extra risk that their use brings. Screening the water supply helps to reduce the initial entry of predators but cannot clean up chemically polluted water or water containing disease organisms. You should consider the location of other existing or planned freshwater prawn farms. You can then make an assessment of the risk that the water supplies of the new farm may be contaminated by the effluent from other farms. If you are going to use surface water, constructing your farms close to a waterfall bringing water from a remote and unpolluted watershed or below the dam of a reservoir (though such water, if drawn from the epilimnion, may initially be high in hydrogen sulphide) would be ideal.

The minimum farm size for economic viability depends on several other factors but the quantity and continuity of the available water supply sets an absolute technical limit on the pond area of your farm, and on its potential productivity. Water is required for four major purposes, namely filling ponds, compensating losses from seepage and evaporation, water exchange, and emergency flushing. When determining the amount of water available on a specific site for freshwater prawn farming you should take the rainfall pattern into account. This may be sufficient to replace or exceed evaporative and seepage losses, at least at some time during the year. An example of grow-out water requirements is provided in Box 3.

In addition to having enough water to fill the ponds it is, at the very minimum, necessary to have enough water available throughout the growing period to replace evaporative and seepage losses. Evaporative losses depend on solar radiation and wind and relative humidity and are therefore governed by the climatic features of the site. Seepage losses depend on the soil characteristics of the farm area, mainly its permeability. Seepage losses may be small where the water table is high or where the water level of the pond is the same as in adjoining fields (e.g. in a paddy field area). However, in other cases, particularly where pond construction is poor, seepage losses can be very great. The quantity of water necessary for this purpose must be assessed locally and the cost of providing it is an important economic factor. As ponds mature, ponds tend to ‘seal’ themselves, through the accumulation of detritus and algal growth, thus limiting seepage losses. Seepage losses can also be minimized by a number of techniques, including sealing the ponds with organic matter, puddling, compaction, laying out a ‘soil blanket’, applying bentonite, or lining them with polyethylene, PVC, or butyl rubber sheeting. Details of these procedures are provided in another FAO publication (FAO 1996).

There is no substitute for the site-specific determination of the water requirements for your farm but an example of water consumption needs for different sized farms, using a number of assumptions is given in Table 6. Techniques for measuring water resources are given in books on hydrology and agricultural water assessment such as ILACO (1981). Methods for estimating seepage and evaporation losses and calculating water requirements are given in FAO (1981). Large-scale farms may wish to consult specialist contractors.

TABLE 5
Water quality requirements for freshwater prawn nursery and grow-out facilities

NOTE: THE SIGN ‘-’ MEANS ‘NOT KNOWN’ OR ‘NO SPECIFIC RECOMMENDATION’.

SOURCE: MODIFIED FROM BOYD AND ZIMMERMANN (2000)

A supply of drinking water and waste disposal facilities are an added advantage to a freshwater prawn farm site but are not absolutely essential. Provision can be made on-site, for example by obtaining batch supplies of drinking water, sinking a borehole, or collecting and filtering rainwater. However, if ice is going to be made, or prawns are to be processed and packed on site, a supply of high quality water, normally the equivalent of drinking (potable) water, is essential. Aqueous waste disposal from such activities can be routed to a septic tank, a waste lagoon, or a simple soak-away.

FIGURE 8b
Screened inlets being used in this freshwater prawn grow-out pond (Peru)

FIGURE 8c
This type of inlet screen is used in Thailand, especially when ponds are filled by long-tail pump

TABLE 6
Example of water requirements for ponds based on various assumptions

CHOOSING YOUR SITE: SOIL CHARACTERISTICS

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.

Figure 9 Pumps can be powered by old diesel bus engines (Thailand)

Figure 10
More expensive pumps are used in some countries; this one is being used to harvest freshwater prawns (Hawaii)

CHOOSING YOUR SITE: SYMPATHETIC AUTHORITIES
AND TECHNICAL ASSISTANCE

You should consider many other factors in selecting your farm site. These include the local and national government regulations concerning water usage and discharge, land use, movement of live animals, import of non-indigenous stocks (where M. rosenbergii is not already present), disease monitoring, taxation, etc. In most countries where freshwater prawn farming is technically and economically viable, these regulations are less restrictive than those, for example, applying to the culture of temperate aquatic species in Europe and the USA; the governments concerned are keen to encourage freshwater prawn farming. You should ask the advice of your local inland fisheries department, whose officers should be helpful and anxious to participate in your project. In some countries there may be NGOs that can provide the assistance that you need. The ease of access to assistance and advice when the farm is in operation is an important factor in site selection. No matter how competent you are, there will come a time when you need help, such as water analysis, disease diagnosis, and technical advice. These types of assistance can be obtained from government, university and private sources. Do not site your farm too far from someone who can heed your cries of “help!”. Speedy access to qualified personnel and to well-equipped laboratories is invaluable. You should always keep in touch with local fisheries officers but do not expect them to know all the answers. No one does!

¹ These manuals are not specific to freshwater prawns. They are relevant to many forms of fish and crustacean farming and are designed for advanced extension workers.

² Assumes an average water depth of 0.9 m

³ For filling ponds at the beginning and on future occasions. Assumes that the unit pond size is 0.2 ha and that the pond can be filled within 12 hours. Also assumes that it will never be necessary to fill more than one pond (or 10% of the pond surface area, whichever is the greater) at the same time. Local experience will tell if this allowance is either not enough or too generous.

⁴ Assumes average seepage losses of 10 mm/day, which is typical for a clayey loam which has not been puddled (FAO, 1981), 500 mm/yr evaporation (this is extremely site-specific) and 2% water exchange per day. This is equivalent to 100 m3/ha/day (approximately 0.07 m3/ha/min) for seepage, approximately 13.7 m3/ha/day (0.01 m3/ha/min) for evaporation, and 180 m3/ha/day (0.125 m3/ha/min) for water exchange in ponds with an average depth of 0.9 m. Total maintenance requirements are therefore 0.205 m3/ha/min.

⁵ This combines the maintenance rate with the quantity necessary to fill all ponds twice per year, averaged out to a volume per minute consumption basis.