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

BOX 2

Flow-through requirements for ten 5 m3 larval rearing tanks

INA FLOW-THROUGH system, the salinity of the seawater or brine available controls the amount of freshwater necessary to produce the 12 ppt brackishwater needed for larval rearing (Table 4).
The daily consumption of 12 ppt water for a single 5 m3 rearing tank in a flow-through system exchanging approximately 50% of the water per day would be 2.5 m3 (2 500 L).
However, emergencies sometimes occur, when it is necessary to rapidly change all the water in a tank.
Pumping capacity must be sufficient to fill any tank with brackishwater within one hour in order to make the daily water exchange as rapid as possible.
Thus, in this example, the pumping and pipe work capacity must be sufficient to supply a peak demand of 5 m3 within an hour (approximately 83 L/min) to each tank. For a complete larval cycle, allowing for some additional exchange to solve rearing water quality problems and assuming that the cycle lasts 35 days, a total of around 90 m3 of 12 ppt water would be consumed for every 50 000 PL produced.
This is equivalent to about 2.6 m3/day for each larval tank, or 25.7 m3 for ten tanks. Rounding up, and allowing an additional safety margin, a hatchery with ten tanks of this size would need about 30 m3of brackishwater per day.
Assuming a steady intake salinity of 30 ppt (and referring to Table 4), the requirement would be 30 ? 10 x 4 = 12 m3of seawater per day.
The need for the larval tanks would be 30 ? 10 x 6 = 18 m3 of freshwater per day.

In addition, sufficient freshwater to maintain holding tanks for PL must be provided. For a hatchery operating ten 5 m3 larval tanks, facilities for providing an average of 20 m3/day of additional freshwater (based on a PL stocking density of 5 000 PL/m2 and an average water exchange rate of 20%/day: 500 000 ? 5 000 x 20 ? 100) will be needed during the periods when postlarval holding tanks are being operated. [Note: much larger quantities of freshwater will be needed if the PL are held for more than one week, because stocking densities will have to be reduced]
The total water consumption for a hatchery operating ten 5 m3 tanks producing 500 000 PL in each larval cycle and selling the PL within one week after metamorphosis would therefore be 12 m3 of seawater and 18 + 20 = 38 m3 of freshwater per day.

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;

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

NOTE: INCOMING FRESHWATER IS ASSUMED TO BE ZERO SALINITY.

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.

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

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)

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 microbiologi-

FIGURE8a
Grow-out pond inlets need to be screened to exclude predators

SOURCE: EMANUELA D’ANTONI

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

SOURCE: OSCAR ORBEGOSO MONTALVA SOURCE: HASSANAI KONGKEO

cal 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

BOX 3 Grow-out water requirements

TO FILL A 0.2 ha pond with an average water depth of 0.9 m requires 10 000 x 0.2 x 0.9 = 1 800 m3 of water. Since it is usually desirable to be able to fill the pond within 12 hours, it follows that it must be possible to extract up to 1 800 ? 12 ? 60 = 2.5 m3 (2 500 L) per minute from the water source for this pond. Normally it is only necessary to completely fill a drained pond after a rearing cycle is completed and the pond has been drained and treated, that is, once every 6-11 months.
There will also be times when, because of poor pond water quality, you may find it necessary to flush the pond and replace a substantial proportion of the water while prawns are growing in it. However, it is very unlikely that it will be necessary for you to fill more than one pond at the same time, if you have a small farm.
Thus, for example, five 0.2 ha ponds would therefore not require a maximum water supply five times larger than one 0.2 ha pond.

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 6
Example of water requirements for ponds based on various assumptions

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 onsite, 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.

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

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.

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)

SOURCE: HASSANAI KONGKEO SOURCE: SPENCER MALECHA

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.

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!