CHAPTER 6
GROW-OUT PHASE

FRESHWATER PRAWNS may be stocked into concrete and earthen reservoirs, ponds, irrigation ditches, cages, pens and natural waters. Cage and pen culture is experimental, while the production from irrigation ditches is low. Stocking natural waters and reservoirs is called fisheries enhancement. Freshwater prawns are obtained from rivers, or (less frequently) from nurseries, for stocking into open waters. Stocking PL is impractical because most would be lost through predation. Larger juveniles (2-3 g) are usually used for enhancement purposes. The topic of fisheries enhancement is only mentioned here but is described in more detail in New, Singholka and Kutty (2000). This section of the manual deals only with the management of freshwater prawns being reared in earthen ponds.

A freshwater prawn farm is very similar to a freshwater fish farm. A detailed farm design is not provided in this manual because every farm must be unique to its site characteristics. A photograph of a large freshwater prawn farm is given in Figure 43. This section of the manual briefly introduces some general principles of aquafarm development. In doing so, it draws upon some other FAO manuals on site surveying (FAO 1989b), the provision of water supplies (FAO 1981), and farm and pond construction (FAO 1992b, 1995). A simple manual on small-scale freshwater fish farming (FAO 1994) is also available. If you are going to build your own farm, it is highly recommended that you obtain these publications before you develop your farm.

6.1 Site requirements and construction

Site selection has been covered earlier in this manual. Having selected the site you will need to thoroughly survey it to determine the best layout for water intake, ponds, access roads, and effluent discharge. These topics are not specific to freshwater prawn farms, so there is no attempt here to duplicate the FAO manuals already available, which have been mentioned above. The development of sites for freshwater prawn farming is discussed in detail in Muir and Lombardi (2000).

DEFINING THE POND

Choosing its area and shape

If you are going to use seining for harvesting, which is often practised in freshwater prawn farming because of the necessity to cull out larger animals (and sometimes to separate females from males, when they have different values) before the final harvest, rectangular ponds are the most suitable shape. The maximum width for this type of management should not be wider than the space through which a seine can be conveniently drawn from one end of the pond to the other by manual labour. A convenient width is 30 m. In practice, of course, wider ponds can also be seined but not so efficiently as narrow ones. The length of the pond depends partly on the topography of the site and partly on the pond size and farm layout chosen. It is best to standardize the width of ponds; otherwise a range of different seine nets will be required for harvesting.

FIGURE 43
Macrobrachium rosenbergii farms can be large (this one was 70 ha) but need careful production, marketing and business management for sustained success (Brazil)

SOURCE: MICHAEL NEW

The most easily managed pond sizes range between 0.2 ha and 1.6 ha, with most farms having ponds around 0.2-0.6 ha. If kept to a 30 m width, a 0.6 ha pond will be 200 m long. Narrow ponds should be oriented so that the prevailing wind (which enhances the dissolved oxygen content of the water) blows down the long axis towards the drain end, to lessen the area of the pond bank subject to wave erosion.

Large ponds are normally wider than 30 m and often drained for harvesting. If the total harvest is going to be taken at one time (batch management), the size of the pond should be influenced by the maximum weight of prawns that the market will accept at one time without price deflation. For example, if a quantity greater than 300 kg of freshwater prawns would swamp your market and reduce prices it would be pointless to have a drainable pond greater than 0.15 ha in area (assuming a productivity of 2 mt/ha/crop).

Information on pond construction is introduced in Muir and Lombardi (2000); details of construction techniques are provided in FAO (1995).

Choosing its depth

The average depth of water in freshwater prawn ponds in tropical areas should be about 0.9 m, with a minimum of 0.75 m and a maximum of 1.2 m. Deeper ponds (an average of 1.2-1.4 m) are used in colder areas to maintain more stable water temperatures. However, deeper ponds are difficult to manage. Even if you have ponds of the recommended average depth you may have to drain or pump out part of the water to facilitate seining operations at the deep end. In the cool season, the temperature of the water at the bottom of deep ponds may become low enough the reduce feed consumption by the prawns. On the other hand, the water in shallower ponds may become too hot for the prawns in the hot season and the water may be quite clear, exposing the prawns to greater predation. Shallow ponds also tend to support the growth of rooted aquatic plants and are not recommended.

The bottom of the pond must be smooth (Figure 44). There must be no projecting rocks or tree stumps in it; these would prevent efficient seining and damage nets. The pond bottom must slope gradually and smoothly from the water intake end towards the drain end so that, when drained, pockets of undrainable water in which prawns become stranded and die do not occur. A slope of 1:500 (0.2%) is suggested for ponds 0.4 ha or more in area and 1:200 (0.5%) for smaller ponds towards the outlet, where drain harvesting occurs. This is equivalent to 2-5 cm per 10 m length. Thus (for example), in a pond which is 100 m long with an average water depth of 0.9 m (90 cm) and a slope of 0.5%, the water would be 65 cm deep at the intake end and 115 cm deep at the outlet end.

Constructing the pond banks

The banks of the ponds (sometimes referred to as embankments or bunds) must be high enough to provide a freeboard of 30-60 cm above the highest water level expected in the pond. Thus, in a pond with a water depth of 65 cm at the shallow end and 115 cm at the deep end, the total bank height must be a minimum of 0.95 m (inlet end) to 1.45 m (outlet) high. The pond banks must also be high enough to protect the pond from exterior flooding. Proper compaction must be employed, both in the construction of the pond banks and the treatment of the bottom of the ponds to maximize water retention. Where the retention characteristics of the soil on the site are not good, a core of impervious material brought from outside the site must be provided during pond bank construction. This core should extend below the level of the bottom of the pond (Figure 45).

For ease of management the internal slope of the pond bank should be 3:1 but it may need to be as high as 4:1 in sandy areas to minimize erosion (and the consequential need for repairs). In highly stable soils the inner slope should not be less than 2.5:1 (Figure 46). Very small ponds with almost vertical sides may be constructed for artisanal purposes in floodplains having very sticky and impermeable clay soils. Fruit trees or other crops may be planted on the pond banks. Sometimes attempts are made to protect the banks from excessive erosion by stakes (for example). Having vertical or near-vertical pond banks almost certainly leads to rapid erosion problems; Figure 47 illustrates the result. This means that a lot of maintenance will be required and they are certainly not recommended for larger commercial-scale farms.

The external pond bank slope should preferably be at least 2.5:1 and never less than 1.5:1, even for highly stable soils. Properly constructed pond banks are more expensive and use more land but failure to build them correctly may result in severe erosion (Figure 47). After construction, you should plant the banks with fast growing grass (e.g. Phyla nodifera), kudzu (a woody vine) or taro (dasheen), to help prevent erosion. Figure 48 illustrates pond banks overlaid with turf. Figure 49 shows banks planted with grass, banana and coconut trees. The planting of large trees or plants with extensive root systems on top of the pond banks may break up the pond bank and cause leakage, so caution is recommended. Plants such as banana, palm trees and papaya are acceptable and palms form wind breaks.

The tops of the pond banks between ponds should be a minimum of 1 m wide to allow workers to walk round the ponds carrying feed and harvesting gear. Narrow pond banks with almost sheer sides are sometimes staked to prevent collapse but they need constant maintenance particularly if the site is sloping and the water level in adjacent ponds is different. Make sure that you have a pond bank width of at least 2-3 m at one side of the pond (usually the drain end or where harvest nets are to be beached) so that trucks can be brought to the pond side for delivering PL and feed and picking up harvested prawns, especially live prawns. On larger farms, particularly where mechanical broadcasting of feed is employed, you must provide a wide pond bank top (usually 3.5-4.0 m) on one of the long sides of the pond as well as at one end.

Information on pond bank construction is introduced in Muir and Lombardi (2000) and construction details are provided in FAO (1995).

FIGURE 47
The bank of this freshwater prawn pond is being eroded because its slope is too steep (Hawaii) The banks of these ponds have had grass turfs laid on them (Brazil)

FIGURE 48
The banks of these ponds have had grass turfs laid on them (Brazil)

Figure 49
Pond bank planted with coconut, grass, and banana; besides stabilising the bank this is a form of integrated farming (Thailand)

SUPPLYING WATER TO THE PONDS

The characteristics of the water supply required for freshwater prawn farming have been discussed earlier in this manual. The topic of water supply is introduced in Muir and Lombardi (2000) and detailed in FAO (1981).

It is not normal to treat the water entering freshwater prawn ponds except to screen it to prevent entry of predators. Screening is not necessary where the water supply is piped from a well or a spring but is essential where surface water or open channel distribution is used. Well water requires aeration by cascading (Figure 50) or by supplying it above pond water level to re-establish gas equilibrium, as it is often initially very low in dissolved oxygen content. There are many alternative methods of screening. Crude screening excludes adults and fingerlings of unwanted species but not their eggs or larvae. Figure 51 shows a simple gravel filter which will exclude fish eggs and larvae as well. Water filtering devices are discussed in other FAO manuals (e.g. FAO 1992b, 1996).

The way in which water is distributed and supplied into freshwater prawn ponds is of great importance. Farms must be designed with a water distribution system that will allow the filling of one pond (or 10% of the pond surface area, whichever is the greater) at any time without starving the other ponds of replacement and flow-through water (Table 6).

There should not be any contact between incoming water and water drained from other ponds. Each pond should have its own individual supply from a central water distribution channel and should not receive the outflow from another pond (Figure 52). The transfer of water from one pond to another is not recommended, because it means poorer water quality conditions in the second (and subsequent) ponds and brings the risk of disease transfer. Ideally, water should be distributed in pipes or open channels by gravity if the topography of the site allows it (Figure 53). Similarly, inlet pipes or channels should be constructed above the water level in the ponds so that the incoming water falls onto the surface of the water (Figure 54). This may be achieved by pumping the water supply to an elevated channel, if this is economically feasible. The water inlet is normally placed at the shallow end of the pond, opposite the discharge point. The inlet channels (or pipes) and the outlet pipes must be correctly sized according to the water demand and draining needs of each pond. Table 12 gives the water discharge capacity of concrete pipes under various pressure heads. Detailed information on these topics is provided in other FAO manuals (FAO 1992b, 1995).

FIGURE 51
Simple gravel filters on the water intake system help to minimize the predators in freshwater prawn ponds (Peru)

SOURCE: OSCAR ORBEGOSO MONTALVA

The flow of water into each pond must be controlled by valves, weirs, stop-logs or plugs (Figure 55). Detailed instructions on building these structures are given in FAO (1992b). While gravity supply, elevated water inlets and lack of cross-contamination of water between adjacent ponds represent the ideal, many freshwater prawn farms exist which do not comply with these recommendations. Many farms (due to site, technical, or financial limitations) have water inlets below the pond water level and receive water from an inlet channel (or a low-lying area such as paddy fields) with the same water level as the pond. In some cases ponds are directly interconnected. These farms produce freshwater prawns, often profitably. However, the use of such water supplies increases risk substantially; a proper water distribution system is essential for reliable high production.

FIGURE 53
Where the topography of the site makes it feasible, supplying water by gravity keeps the dissolved oxygen level high (Brazil)

SOURCE: JULIO VICENTE LOMBARDI, REPRODUCED FROM NEW AND VALENTI (2000) WITH PERMISSION FROM BLACKWELL SCIENCE

FIGURE 54
Supplying water above the pond water level provides some oxygenation, while grass minimizes erosion of the bank (Brazil)

Methods of minimizing water losses through seepage, by sealing ponds with organic matter, puddling, compaction, laying out a ‘soil blanket’, bentonite, or lining them with polyethylene, PVC, or butyl rubber sheeting are described in another FAO publication (FAO 1996).

DISCHARGING WATER FROM THE PONDS

It is preferable to be able to drain ponds by gravity than to have to pump the water out and, where this is possible, you should construct a ‘monk’ or sluice gate outlet structure. These structures (Figure 56) will allow you easily to control water depth and drainage speed and can be screened to prevent the loss of stock. In flow-through water management, water is continually flowing through this structure. The monk allows you to totally drain your pond and, more importantly, enables you to control the water level during seine harvesting operations, flushing and water circulation.

TABLE 12
Water discharge capacity (in m3/hr) of concrete pipes under various pressure heads

NOTE: THE WATER DISCHARGE CAPACITY OF PIPES INCREASES WITH THE PRESSURE HEAD (VERTICAL DISTANCE BETWEEN WATER SURFACE ABOVE AND
CENTRE OF LINE OF PIPE BELOW).

SOURCE: DERIVED FROM FAO (1995)

Static (non-flow-through) ponds can have a simple screened and plugged outlet pipe or a turn-down drain, as previously shown in Figure 40. Outlet structures, whether they are pipes or monks, must be carefully sized so that the pond does not drain too slowly (to prevent poor water quality during harvesting operations), and they should be sited so that the pond can be totally drained (Figures 57 and 58). Table 13 gives the appropriate pipe sizes for ponds with monk outlets, for example, while Table 14 gives the time to drain a pond under various circumstances. Figure 59 is an illustration of a sluice gate structure. The top of the sluice gate should also be constructed at least 50 cm above the highest pond water level as a safety measure. Detailed information on these topics can be found in other FAO manuals (FAO 1992b, 1995).

Where the pond outlet is a pipe below water level, there should also be an overflow pipe inserted about 20-30 cm below the top of the pond bank but above the normal water level in the pond. This overflow pipe should be screened in the same way as the normal pond outlet, to prevent loss of stock. If the water level in the area to which the pond drains also rises, however, the overflow pipe will be ineffective.

Where drainage by gravity is not feasible because of the limitations of the site, the only way to empty the pond or control its water level is by pumping. A screened ‘long-tail’ pump is one method of emptying ponds on flat sites (Figures 60 and 61). These pumps are readily available because they are used for paddy-field irrigation.

When water is discharged, enriched water and waste solids should be treated to prevent adverse effects on receiving waters, or to permit some or all of the pond water to be reused at various stages, pumped back for the same ponds, or drained into other systems downstream. Solids removal is the main form of treatment, usually based on sedimentation in settling ponds. Aeration may be used to increase oxygen levels, and algal growth in the settling ponds may help to remove nutrients. Every effort should be made to minimize water exchange, in order to reduce effluent loading and to conserve the water supplies themselves; water is essential to many forms of human activity and its use needs to be responsible.

FIGURE 56
The outlet structure, sometimes known as a ‘monk’, can be used to control the level of the water as well as to screen the water to prevent the loss of freshwater prawns (Brazil)

SOURCE: JULIO VICENTE LOMBARDI, REPRODUCED FROM NEW AND VALENTI (2000) WITH PERMISSION FROM BLACKWELL SCIENCE

The topics of harvesting and harvest structures are dealt with later in this manual. More information on outlet structures can be found in Muir and Lombardi (2000) and details about their construction are contained in several other FAO manuals (e.g. FAO 1992b, 1994).

TABLE 13
Sizes of outlet pipes for ponds with monks

SOURCE: DERIVED FROM FAO (1992b)

AERATION

Most prawn farms use water exchange to keep dissolved oxygen levels high, as well as curing other water quality problems. When the original FAO manual on freshwater prawn farming was written in 1982 it was pointed out that the dissolved oxygen level of incoming water can be enhanced if ripples (Figure 50) are built into gravity inflow channels and water is injected into the ponds above water level (Figure 54). It was also noted that permanent aeration equipment was not normally provided for many freshwater prawn grow-out ponds but that equipment for emergency aeration was useful in times of oxygen depletion (perhaps caused by an algal crash). However, since that time, aeration has become more commonplace in freshwater prawn farming because the higher stocking densities that are used in some grow-out systems and nurseries increase oxygen demand.

Paddlewheels (Figure 62) are the most efficient method of increasing dissolved oxygen levels in pond water (Table 15). Recently, long shaft engine-run paddlewheel aerators have been developed, which can be operated in remote areas far from power supplies (Figures 63 and 64). Aerators are needed to ensure the water quality necessary for increased productivity (for which maximum growth and survival rates are required) and emergency use, especially after partial harvesting. According to Boyd and Zimmermann (2000), aeration is valuable not only to maintain dissolved oxygen levels sufficiently high at night time (when they are naturally low) but also in the daytime, when they can become low at the pond bottom, where most prawns dwell. These authors noted that the provision of supplementary aeration equivalent to 1 horsepower could increase the productivity of a pond by about 400-500 kg/ha (this observation is based on experience with fish and marine shrimp and has not yet been quantified for freshwater prawns).

The selection of aeration equipment is discussed in another FAO manual (FAO 1996).

TABLE 14
Time taken to drain ponds (in hours) with different drain pipe sizes

NOTE: THESE FIGURES ASSUME AN INITIAL WATER DEPTH OF 1 M, WITH PIPE VELOCITY LIMITED TO 1 M/SECOND; IF YOU HAVE TWO PIPES, THE TIME FOR DRAINING IN EACH CASE WOULD BE HALVED.

SOURCE: DERIVED FROM FAO (1995)

MISCELLANEOUS

In addition to its ponds and its water distribution systems a freshwater prawn farm has the following equipment and facility requirements:

• power;
• roads and access paths;
• accommodation: every farm should have accommodation for some of its workers to live on site;
• fencing: a perimeter fence and, on larger farms, lighting, to deter poachers (human predation);
• storage facilities: dry storage is needed for feeds (or ingredients), chemicals, nets, etc.;
• feed distribution and monitoring equipment;
• nets;
• water quality monitoring equipment;
• predator protection; and
• transport: larger farms will need trucks for prawn distribution and feed collection.

FIGURE 57
Most prawns will have been previously removed by seining; the rest are harvested not only at the drain but also by cast net (as shown in this photo from India) while draining proceeds.

NOTE: PIPES HAVE BEEN USED AS PRAWN SUBSTRATES (SHELTERS) IN THIS POND

SOURCE: STEPHEN SAMPATH KUMAR

FIGURE 58
This freshwater prawn pond has just been totally drained (Thailand)

6.2 Management of the grow-out phase

This section of the manual briefly introduces some general principles of aquaculture farm management. In doing so, it draws heavily upon some other FAO manuals on the management of ponds and water (FAO 1996) and fish (FAO 1998). A simple manual on small-scale freshwater fish farming (FAO 1994) is also available. You are strongly recommended to obtain and read these publications before commencing operations on your prawn farm. This section of this manual concentrates on matters which are specific to the management of freshwater prawn farming and is based on the original FAO manual on this topic, supplemented with material extracted from Boyd and Zimmermann (2000), D’Abramo and New (2000), Johnson and Bueno (2000), Karplus, Malecha and Sagi (2000), Tidwell and D’Abramo (2000), Valenti and New (2000) and Zimmermann and New (2000). Information is provided on the management of the monoculture of freshwater prawns (at various levels of intensity in different climatic zones), as well as their polyculture with other aquatic species and its integration with other types of farming. Harvesting is dealt with later, in a separate section.

SIZE VARIATION

The management of size variation is an extremely important aspect of growing freshwater prawns, because of the uneven growth rate of individual prawns, especially males, known as HIG. If you are going to be able to grow prawns in your ponds which have the maximum marketable value and the highest total production rate, it is essential that you understand this topic properly. For this reason a special annex to this manual has been prepared (Annex 8) and you are encouraged to read it carefully.

TABLE 15
Oxygen transfer efficiencies of basic types of aerator

SOURCE: BOYD (1990)

SEMI-INTENSIVE MONOCULTURE IN TROPICAL ZONES

Although this section concentrates on the management of prawn monoculture in tropical zones, it also contains information which is equally applicable to other types of freshwater prawn rearing. Freshwater prawn monoculture can be extensive, semi-intensive or intensive but the definition of these terms is rather vague (Valenti and New 2000). For the purpose of this manual, the definitions used are shown in Box 14.

Most of this section of the manual is targeted at the semi-intensive level of intensity (Box 14, Level 2). Semi-intensive freshwater prawn grow-out in ponds can be managed by a ‘continuous’ or ‘batch’ system, or a combination of the two, the ‘combined system’. A variant of the combined system is known as the ‘modified batch system’. These systems are described in Box 15. The system which the grow-out and harvesting sections of this manual is built around is System 3 (the combined system).

FIGURE 60
Long-tail pumps are easily available in Thailand

SOURCE: HASSANAI KONGKEO

FIGURE 61
Long-tail pump being used to lift water from a Thai irrigation canal into a supply channel for freshwater prawn ponds (this type of pump can also be used to drain ponds by pumping)

SOURCE: HASSANAI KONGKEO

Preparing your pond

Before you stock your pond you need to prepare it. After the final harvest of the last batch of prawns that you reared, the pond should be drained to remove all predators. Make any necessary repairs to the pond banks and the major structures at this time. Check all inlet and outlet screens. Completely dry the pond for 2-3 weeks (this may not be possible between every cycle, for example in the rainy season, but should be done at least once per year). It is not normally necessary to remove pond sediments from freshwater prawn ponds after every cycle. However, sediment build-up over several batch cycles, or during a long period of continuous management (Box 15, System 1), can be excessive (Figure 65). The sediment consists of particles contained in the incoming water, the effects of erosion, the remains of dead pond organisms, prawn faeces, remnants of feed, and exoskeletons cast during prawn moulting. One of the effects of a heavy sediment build-up is a decrease in the volume of water available for the stocked prawns to occupy. Scraping the bottom of the pond can be used to remove sediment but care must be taken not to place the excavated sediment where it will wash back into the pond or supply/discharge canals when it rains, or cause a local environmental problem. Site-specific means of sediment removal need to be developed. However, if there is no opportunity to place the sediment elsewhere, it can be spread in a thin layer over the pond bank surfaces and allowed to dry until it cracks.

You should till (harrow) the bottom of your ponds during the drying period to increase the oxygen content of the soil, especially if it has a heavy texture (clays and clay loams). A disc harrow (Figure 66) is the best equipment to use and tilling should take place while the soil is still wet but is dry enough to support the weight of the tractor. Where there has been a severe disease problem in the previous crop, you should spread 1 000 kg/ha of agricultural limestone (CaCO3) or 1 500 kg/ha of hydrated lime [sometimes called slaked lime – Ca(OH)2]. It is better if you use agricultural limestone. The use of slaked lime, or quick lime (CaO) may increase the subsequent pH of the water above tolerance limits if prawns are stocked (as is recommended for other reasons later) soon after the ponds are filled. After adding agricultural limestone you should sun-dry the ponds for at least two weeks so that toxic gases such as hydrogen sulphide and methane are voided. Some freshwater prawn farms make a standard application of 1 000 kg/ha of agricultural limestone every time a pond is drained. Chlorination can also be used for disinfection (see Boyd and Zimmermann 2000) but it is not recommended because it is a much more expensive treatment.

FIGURE 62
Using paddlewheel aerators keeps the dissolved oxygen level high enough to increase stocking levels

SOURCE: CLAUDE BOYD

FIGURE 63
Power supplies are not always reliable. Loss of aeration at a critical time of the day and/or when ponds are heavily stocked. This Thai farm is using a mobile engine to drive long-shaft aerators in two adjacent ponds

SOURCE: HASSANAI KONGKEO

If your pond has previously been stocked with fish and you want to convert it to freshwater prawn culture, or if a lot of fish were present during your last prawn grow-out season, treat it with a piscicide after harvesting and while it still has water in it. Rotenone or teaseed cake are commonly used for eradicating unwanted fish between cycles. They are effective if spread evenly throughout the pond. However, the use of rotenone is banned in some countries because of environmental concerns: check before you use it. The quantities needed for treatment are shown in Box 16.

More powerful chemicals, such as insecticides, are sometimes used for pest eradication (in severe cases, where there are very stubborn predators or competitors that resist other forms of treatments and/or because of their cheapness). However, the use of insecticides to remove unwanted fish is not recommended in freshwater prawn farms; they are potentially toxic to prawns and may accumulate in prawn tissues, with consequential dangers to human health. Further reading on the eradication of predators is con tained in another FAO manual (FAO 1996).

FIGURE 64
Long-shaft aerator in action (Thailand)

SOURCE: HASSANAI KONGKEO

FIGURE 65
The sediment in continuously operated freshwater prawn ponds can become so deep that it reduces the water volume and depth and disturbs the drainage pattern; this pond had not been drained for many years (Hawaii)

SOURCE: SPENCER MALECHA

Acid soils may cause your pond rearing water to be too acid for good prawn productivity. These soils need treatment to improve the alkalinity of pond water. Liming will be necessary if the water in your pond is pH 6.5 or below at sunrise. If it is necessary to treat the soil, you must apply the lime before the ponds completely dry out, so that it dissolves and penetrates the soil. Routine liming should be sufficient to increase total alkalinity to about 40 mg/L. The quantity of lime required depends on the type of soil and the pH. Agricultural limestone is the best compound to use for increasing alkalinity. First, measure the soil pH as shown in Box 17.

Table 16 shows the quantity of lime to use in treating pond bottoms between cycles. Spread the limestone uniformly before fertilizers are applied. Liming may be necessary every time the pond is drained if it is managed with a rapid water exchange. Judge the need by testing the water before draining. If the pond water contains less than 30-40 mg/L of alkalinity it will be necessary to lime. If it is more than 60 mg/L it should not be limed.

You are not recommended to build ponds on suspected acid sulphate soils because making them usable is expensive, time consuming and laborious. Despite this advice, some people build ponds on such soils! If you have inherited or bought such ponds, you will find that correcting the pH by liming the pond bottoms is usually impractical, due to their high lime requirements. In such cases, liming should be limited to the banks of the pond and combined with the planting of acid resistant grasses, such as the African star grass. Continuous flushing of the water through the ponds and over the banks of the ponds, followed by drying, accelerates the reclamation process of this type of pond. The period required to correct pH may vary between a few months and several years, depending on soil and climatic characteristics.

So far, in this section of the manual, it has been low pH that has been discussed. Ponds having a high water pH can be improved by ‘ageing’. This means filling them with water 2-4 weeks before stocking and allowing natural biological processes to buffer the pH. However, doing so also increases predator and competitor problems, as discussed before.

FIGURE 66
The bottoms of ponds can be tilled with a disc harrow (USA)

SOURCE: CLAUDE BOYD

If your water supply is very soft, you can increase its hardness by adding calcium sulphate (gypsum). Information drawn from Table 5 suggests that a total hardness of around 50-100 mg/L (CaCO3) would be ideal for freshwater prawn grow-out. If the pond water before draining shows levels lower than this, gypsum should be added during pond preparation. 2 mg/L of gypsum is required to increase total hardness by 1 mg/L. Thus, if the total hardness is 20 mg/L before treatment, 600 kg of gypsum/ha (for ponds with an average water depth of 1 m) should be applied to correct it to 50 mg/L. No treatment is suggested for hard water but, if the procedures for site selection have been followed properly, excessively hard water should not be present in freshwater prawn ponds.

TABLE 16
Lime requirements for treating the bottom of ponds between cycles

SOURCE: DERIVED FROM BOYD AND TUCKER (1998)

Some soils may benefit from the application of nitrates to oxidize the soil and aid the decomposition of organic matter where pond bottoms cannot be completely dried out. For most ponds 150-200 kg/ha of sodium nitrate would be sufficient. Calcium peroxide is also sometimes used for this purpose but is less efficient and is not recommended.

Some farms use organic fertilisation, Manure is used for fertilising ponds, before and during the rearing cycle, where freshwater prawns are grown with silver and bighead carps in China. In Brazil, freshwater prawn ponds are often fertilized between cycles using 1 000-3 000 kg/ha of cattle man-ure or other organic material. This increases the benthic fauna, which become an important feed for PL and juveniles. However, this practice is not encouraged in this manual for the reasons shown in Box 18.

If you are really convinced that organic fertilisation between cycles is helpful, use plant meals, such as soybean meal or rice bran, not animal manures. Generally, the productivity of ponds improves as they get older and as a rich bottom area and grassy banks are established. Further reading on pond preparation can be found in Boyd and Zimmermann (2000).

FIGURE 67
There are some advantages in rearing freshwater prawns (Macrobrachium rosenbergii) to a larger (juvenile) size before stocking

SOURCE: DENIS LACROIX

Stocking

It is better to stock ponds immediately after filling them with filtered water. This has no predators and causes no photosynthetically-induced pH changes. There may be a slight reduction in growth from the initial lack of natural food, but increased survival will outweigh this factor. Stocking the ponds quickly reduces the amount of competitors and predators, which have less time to become established. Often postlarvae (only about a week or two old after metamorphosis) are used to stock grow-out ponds, where they will remain until harvesting. Some farmers prefer to use PL reared in a simple (in contrast to a sophisticated) hatchery, believing them to be more hardy because the strongest have been naturally selected. Juveniles are more tolerant of high pH and ammonia than PL and there are some advantages in stocking juveniles (Figure 67) instead of PL, even in tropical areas. Juveniles are more expensive to produce in nurseries, or to purchase from others, but the improved grow-out survival and shorter time to marketable size achieved should more than balance this out.

The transport of PL (or juveniles) to the grow-out site has already been described in this manual. On arrival at the pond bank you should take great care to acclimatize the PL to the temperature of the pond water by floating the transport bags in the pond for 15 minutes (Figure 68) before emptying them into the water (Figure 69). Severe mortalities can be caused not only by thermal shock but also by sudden changes in pH. You should measure the pH of the pond water before stocking. If it is more than 0.5 pH units different from the pH in the PL holding tank or the nursery ponds, acclimatize the PL to this pH level slowly (over a one-day period) in the hatchery-nursery before transporting and stocking them at the grow-out site.

The stocking rate you need to use depends on the size of the animals you will eventually be selling (and thus on the demand of the local, national, or international market that you are targeting), on the length of the growing season (determined by water availability and temperature), and on the management system you are using. Older ponds tend to be more productive than new ones. Your decisions about stocking rate should consider all these factors. Specific stocking densities are not recommended in this manual because no guarantee can be given that a certain quantity of prawns will be produced! As stated in Box 14, semi-intensive stocking rates vary between 4 and 20 PL/m2 (40 000-200 000/ha). The lower stocking rates will tend to result in prawns of a larger average size. Higher stocking rates tend to result in greater total productivity (mt/ha/crop) but smaller average prawn size. The stocking rate you choose should therefore be adjusted according to your previous experience in your farm or locality, and the size of marketable animals desired. If you are stocking juveniles, there are some advantages in grading them before stocking, as discussed later.

FIGURE 68
Sudden changes in temperature and pH can cause mortalities when prawns are stocked. Before their release, the bags containing the postlarvae should be floated in your pond to bring the temperature within them gradually to that of the pond. Any adjustments to the pH of the transport water should have been made in the hatchery, before transport (Brazil)

SOURCE: PATRíCIA MORAES-RIODADES

The postlarvae (PL) you have purchased and brought to your pond-site will have been counted into the transport bags at the hatchery. You may wish to be present at that time to ensure fairness. Normally, hatcheries will put more PL into the bags, rather than underestimate them. However, if you are receiving PL without having seen them packed, it is advisable for you to count the contents of one or two bags at random to check the accuracy of the delivery. If a standard number of PL are packed into each transport bag the stocking procedure will be easier because it is only necessary to count the number of bags to achieve the desired density.

In some countries (e.g. Bangladesh, India, Viet Nam), hatcheries currently have insufficient capacity to supply all grow-out requirements. In these cases, wild-caught PL or juveniles are often used for stocking ponds (New 2000b). This practice it not recommended because of the possibility of introducing prawns of other species, disease organisms, and predator fish, as well as the effect that excessive fishing of these young stock causes to the natural freshwater prawn fishery. Every effort needs to be made to increase hatchery capacity for a healthy freshwater prawn farming industry. However, it is recognized that catching prawn (and shrimp) juveniles provides considerable rural employment and any transition from the use of wild-caught to hatchery-reared PL and juveniles should be carefully phased to minimize socio-economic problems.

Postlarval freshwater prawns obtained from foreign hatcheries are sometimes used to stock grow-out ponds. Take care in making introductions from another locations; seek the advice of your local animal health expert on this subject before you do this.

Increasing surface area and routine pond maintenance

Ponds need to be well-maintained during the farming period. You should take special care about the prevention and treatment of pond bank erosion and the maintenance of water inlet and outlet structures, particularly the filters (screens, socks). You can increase the pond surface area available to the prawns by placing rows of netting, suspended from floaters and weighed down with sinkers, across the pond. You can also use the sort of substrate that is described in the next section of this manual, on culture in temperate zones. Twigs, pipes, bricks, etc. are often used as prawn habitats but they interfere with harvesting, and are not recommended.

As mentioned before, vegetation along the pond bank minimizes erosion. Below the water line, it also provides food and a habitat for the prawns. The plants Elodea spp. and Hydrilla spp. make a good substrate for prawns. You must be careful not to allow the growth of these plants to become so excessive that it interferes with harvesting. Maintain the pond depth at an average of 0.9 m. Do not allow extensive shallow areas to develop, or rooted aquatic plants will grow extensively on the pond bottom (Figure 70). The growth of rooted aquatic plants and benthic algae must also be discouraged by management practices that encourage significant growth of phytoplankton, thus reducing light penetration to the pond bottom. The tips in Box 19 will help you.

MONOCULTURE IN TEMPERATE ZONES

Special conditions apply to the culture of freshwater prawns in ‘temperate zones’, because of the short period during which the grow-out phase can be operated (usually about 4-5 months). A captive broodstock has to be maintained, an indoor heated hatchery operated, and postlarvae reared to juvenile size in indoor nurseries. This is necessary to provide larger animals for stocking grow-out facilities as soon as possible in the season, thus enabling the longest possible growing period. The highest possible average weight at harvest can be achieved in this way. These topics are fully discussed by Tidwell and D’Abramo (2000).

In the temperate zone culture of freshwater prawns, natural food, enhanced by feeding or fertilisation, is used until the prawn biomass reaches about 200-250 kg/ha. After that, supplemental feeding is essential. The use of a range of diets, both for initial fertilisation and as a feed for prawns, is discussed later in this manual. Aeration may be necessary to maintain satisfactory levels of dissolved oxygen. Although average water temperatures during grow-out in temperate zones may be much lower than in the tropics, the maximum may become quite high (over 30°C). Dissolved oxygen levels decline as temperatures rise (Table 7).

FIGURE 69
When the temperature in the bag is the same as in your pond, the postlarval Macrobrachium rosenbergii can be released (Brazil)

SOURCE: PATRíCIA MORAES-RIODADES

FIGURE 70
Grass is invading the shallow areas of this pond (Brazil)

SOURCE: PATRíCIA MORAES-RIODADES

Without using substrates to increase productivity, a stocking rate of about 4 juveniles/m2 (40 000/ha) is recommended for the monoculture of Macrobrachium rosenbergii in temperate zone ponds. There are some advantages in using larger juveniles for stocking. For example, it has been demonstrated that increasing the average stocking weight at 4 animals/m2 from 0.17 g to 0.75 g increases production at harvest by nearly 30%. However, this stocking size advantage does not apply indefinitely; research has shown that stocking 3 g animals did not improve production because the animals matured too rapidly.

Grading nursed juvenile prawns before stocking also has significant advantages. In temperate zones it has been found to increase average harvest size and total pond production. Size grading is a way of separating out the faster growing prawns and lowering the suppression of growth that they cause to other prawns; it can also result in improved feed conversion ratios (FCR). Some notes on size grading are given in Box 20 but you should note that this procedure is still in the developmental stage. You may need to experiment to refine the technique.

FIGURE 71
Close-up of material used as pond substrate for Macrobrachium rosenbergii culture (USA)

SOURCE: CHARLES WEIBEL

FIGURE 72
Substrates have been placed vertically in this temperate zone rearing pond for Macrobrachium rosenbergii culture (USA)

SOURCE: CHARLES WEIBEL

Another means of improving results in temperate freshwater prawn culture is to place artificial substrates in the ponds, which makes it feasible to increase stocking rates above the level recommended earlier for ponds without substrates. PVC fencing (such as is used to close off areas when roads are being resurfaced) forms an ideal substrate (Figure 71). This material can be expensive in some countries but the investment should be worthwhile, as the following information indicates. Substrate provision on a commercial scale (Figures 72 and 73) has resulted in production and mean harvest size exceeding 1 800 kg/ha/crop and 35 g respectively, from a stocking rate of 4 PL/m2, while yields exceeding 2 500 kg/ha/crop with average weights of >40 g have been consistently achieved at a stocking rate of
64 500/ha (Tidwell and D’Abramo, 2000). It is therefore suggested that you increase the stocking rate of juveniles from the 4/m2 (40 000/ha) recommended earlier for use without substrates to 6.5/m2 (65 000/ha) when you use either horizontal or vertical substrates. No extra labour (apart from its initial installation) is necessary if this form of substrate is used because it can be permanently installed in ponds equipped with catch-basins at the drain end. As the water is drained, prawns abandon the substrate and follow the water flow to the catch basin. You can spread the cost of the labour for installation, as well as the substrate material itself, over several production cycles. This new technology is still being developed but it clear that the use of substrates can markedly increase the productivity of freshwater prawn farming.

FIGURE 73
In this temperate zone rearing pond the substrates have been placed horizontally (USA)

SOURCE: CHARLES WEIBEL

Experimental trials of a combination of grading and the use of these substrates has recently (2001) shown that a production of nearly 3 000 kg/ha/crop can be obtained, with animals averaging 52 g (J.H. Tidwell, pers. comm. 2001). At the time this manual was being prepared (2001) work was ongoing to see if this research finding could be verified in commercial temperate-zone ponds.

These types of management make prawn production feasible in smaller, deeper ponds which were previously considered unsuitable. This is useful in hilly inland regions where suitable sites for large shallow ponds are very limited. Grading before stocking and the use of substrates has not been practised much in tropical monoculture yet but the advantages obtained in temperate culture should be transferable. One researcher believes that up to 9 mt/ha/yr of 20g prawns from three 4-month cycles might be achieved in tropical areas using the combination of grading and substrates (W. Valenti, pers. comm. 2001).

POLYCULTURE AND INTEGRATED CULTURE

A considerable, but unquantified proportion of global freshwater prawn production comes from polyculture and integrated culture. No detailed recommendations for the polyculture of Macrobrachium rosenbergii with other species, or its integration into other farming activities, have been provided in this manual. This is because there is no single recommendable process. Many different management techniques are possible. It is hoped, however, that you will be stimulated by the examples given below to try polyculture with locally available species, as well as integration with other farming activities in your specific location. Further reading on this topic is available in Zimmermann and New (2000) and New (2000b).

Polyculture

Records exist of the polyculture of various Macrobrachium species in combination with single or multiple species of fish, including tilapias, common carp, Chinese carps, Indian carps, golden shiners, mullets, pacu, ornamental fish, and red swamp crayfish. Other combinations may be feasible.

The inclusion of freshwater prawns in a polyculture system almost always has synergistic beneficial effects, which include:

• more stable dissolved oxygen levels;
• the reduction of predators;
• coprophagy (the consumption of fish faeces by prawns), which increases the efficiency of feed;
• greater total pond productivity (all species); and
• the potential to increase the total value of the crop by the inclusion of a high-value species.

However, the management of a polyculture system is more complex. This particularly applies to the harvesting of prawns. Some large fish can be cull-harvested from a polyculture pond but this interferes with the culture of the prawns. Prawn-fish polyculture systems are therefore normally batch-harvested. It is difficult to synchronize fish production with prawn production to achieve the maximum production of marketable animals. For this reason, most polyculture systems involving freshwater prawns concentrate management on the production of the fish and regard the harvested prawns as a high-value bonus.

The addition of prawns to a fish polyculture system does not normally reduce the quantity of fish produced. On the other hand, the addition of fish to a prawn monoculture system markedly increases total pond yield but may reduce the amount of prawns below that achievable through monoculture. Some problems have been reported. For example, tilapia which were inadvertently introduced into prawn ponds in Hawaii were described as a pest, causing serious competition for food. Escaped tilapia, which had been grown in cages in freshwater prawn ponds in Puerto Rico took years to eradicate. However, this problem could be avoided by the use of artificially incubated sex-reversed caged tilapia. The monoculture versus polyculture decision is site-specific and depends on economic factors, namely balancing the relative market values of the various species with the costs of a more complex management system.

Fish are faster than prawns in accessing any supplemental feed which is presented, so the feeding for polyculture systems is normally directed at the fish, not at the prawns. The prawns consume feed which falls to the bottom of the pond, as well as fish faeces and nutrients derived from detritus. Though commercial fish feeds are sometimes applied, tropical polyculture systems often use simple mixtures of rice bran with plant oilcakes, such as mustard and groundnut. Since there are so many potential combinations of fish and freshwater prawns, it is impossible to give firm guidelines on management in this manual. In the cases summarized in Table 17, the culture cycles ranged from 3 to 6 months and the water temperatures were 26 ± 4°C. This table also gives an indication of the productivity obtainable. The results of other published studies on prawn and fish polyculture have been reviewed by Zimmermann and New (2000). Much of the output of M. rosenbergii produced in China comes from polyculture systems. Examples are given in Box 21.

TABLE 17
Average stocking densities and yield of carps, tilapias and freshwater prawns reared in polyculture, based on a literature study

SOURCE: DERIVED FROM ZIMMERMANN AND NEW (2000)

Integrated culture

The wastewater from ponds containing prawns being reared in monoculture or polyculture with fish can be used for the irrigation of crops. Prawns can also be reared in paddy fields, without depressing rice production. This has proved especially valuable in Viet Nam, where it has been shown that the income from prawns in integrated rice-prawn culture can be two or three times as great as that from the cultivation of rice. The introduction of freshwater prawns reduces the area devoted to rice paddy (because deeper areas where prawns can shelter when the ricefield is dry have to be provided). It also reduces weeding costs (prawns eat weeds) and fertilisation costs. Figure 74 illustrates a Vietnamese rice-prawn farm where peripheral canals have been constructed for Macrobrachium culture and a bamboo structure has been erected on the canal dykes to support cucumbers.

Similar to polyculture, no single management strategy can be recommended for integrated culture because the potential combinations are almost infinite. However, examples from Viet Nam have been presented in Box 22.

Other forms of rearing prawns

The use of concrete ponds, cages (a floating structure, usually enclosed in nylon netting) and pens (an area of a larger water body, such as a reservoir or lake, which is separated off by the use of netting, bamboo or other structures) has not found favour in freshwater prawn culture, although there were some early attempts, especially in Thailand. However, nets are sometimes used in nursery systems, as noted earlier in this manual. Many attempts have been made to rear freshwater prawns under highly intensive grow-out conditions in tanks housed under environmentally controlled conditions in cool temperate zones, including the UK. Such ideas have been abandoned due to excessive costs, especially for heating. Indoor rearing in environmentally controlled conditions is now confined to broodstock and nursery systems designed to maximize production in the temperate zones of China and the USA, for example.

6.3 Feeding and fertilization

This section of the manual concentrates on practical feeding in the grow-out stage, and some farm-made feeds for freshwater prawns are described in its tables. The feeds and feeding strategies given apply equally to prawns reared in nursery facilities. Detailed information on the nutritional requirements of this species can be found in D’Abramo and New (2000), and on the digestive system in Ismael and New (2000). Feeding strategies for broodstock and feeds and feeding strategies for the larval stages of freshwater prawns have been discussed earlier in this manual.

FIGURE 74
Macrobrachium rosenbergii farming can be integrated with crop and other livestock production; in this case prawn culture is associated with rice culture and vegetable production (Viet Nam)

SOURCE: MARCY WILDER, REPRODUCED FROM NEW AND VALENTI (2000) WITH PERMISSION FROM BLACKWELL SCIENCE

It is necessary to maintain an adequate phytoplankton density, to provide cover and control the growth of weeds in freshwater prawn ponds. This is done by encouraging the growth of phytoplankton. However, it is often unnecessary to fertilize, because this is rapidly achieved by the feeding regime. However, ponds built in a sandy-clay soil may require fertilization for this purpose. Where necessary, 25 kg/ha/month of
triple superphosphate (Na3PO4) will keep the water green. Benthic fauna are very important features in the ecosystem of freshwater prawn ponds, forming part of the food chain for prawns. Fertilisation to encourage the development of benthic fauna is therefore recommended. Animal manures have been used for this purpose (e.g. 1 000-3 000 kg/ha of cattle manure) but the use of animal manure is not encouraged in this manual, for the reasons explained in Box 18. Animal manures can be substituted by other organic materials, such as distillery by-products or other plant resides. The rest of this section of the manual is devoted to the use of feeds.

FEED TYPE

You can get a small production level of freshwater prawns (perhaps 200-300 kg/ha/year, as shown in Box 14, Level 1) by relying on the natural productivity of the ponds. However, successful semi-intensive farming must involve supplementary feeding. Some farms claim to rely on fertilisation, rather than feeding, at the beginning of the rearing period. Some stimulate an initial algal bloom through the addition of an inorganic fertilizer (such as a liquid 0-36-0 formulation, applied to provide about 9 kg/ha of phosphorus). Others find that providing feed from the beginning of the rearing period improves performance and is cost-effective. However, the dividing line between the effectiveness of feed as a direct nutritional input to the prawns and what is acting as a fertilizer is blurred. Whether the feeds are pelleted mixtures or individual ingredients (such as distillery or brewery by-products), they actually act as both feeds and fertilizers. At the beginning their primary use may be as an organic fertilizer that enhances the availability of natural feeds in the rearing ponds. Later, as the prawns grow, the feeds become more and more directly consumed by the prawns. The application of feeds/fertilizers from the beginning of the rearing period not only increases the availability of natural food but also decreases the transparency of the water, therefore reducing the growth of weeds.

The types of feed used in freshwater prawn farming vary widely and include individual animal or vegetable raw materials and feed mixtures prepared at the pond bank; both of these are generally referred to as ‘farm-made feeds’. In addition, commercial feeds designed for freshwater prawns are available in some countries, sometimes from several aquafeed manufacturers. Freshwater prawns are omnivores and, so far as is known at present, their nutritional requirements are not very demanding. Some farmers utilize commercial feeds designed for marine shrimp in freshwater prawn nurseries or during the first few weeks of the grow-out phase when prawns are stocked as PL. Marine shrimp feeds have a much higher protein content than is needed for freshwater prawns, so cheaper commercial feeds that have either been specifically designed for freshwater prawns or for a species of fish (e.g. catfish) must be used in grow-out ponds stocked with nursery-reared juveniles, or substituted as soon as possible in those stocked with PL. You can assess the relative suitability of commercially available feeds by asking your local extension agent from your government fisheries department and checking with other freshwater prawn farmers.

FIGURE 75
If hatchery-reared Macrobrachium rosenbergii are not available, brushwood can be used to capture wild postlarvae (Viet Nam)

SOURCE: MICHAEL NEW, REPRODUCED FROM NEW AND VALENTI (2000) WITH PERMISSION FROM BLACKWELL SCIENCE

Commercial feeds may be the most productive and reliable to use but they are expensive, are not always available to the small farmer, and do not take advantage of locally available ingredients. You may also have problems in storing compound feedstuffs in humid conditions where deliveries cannot be made regularly in small quantities. If you make your own feeds, some of your ingredients can be locally available. You can also produce them with your own farm labour and simple equipment, such as small bakery mixers and meat mincers. Usually, no extra labour is required. Feed making just forms another job which can be fitted in between the other duties of your farm labourers.

Many different ingredients could be used in your farm-made feeds, either individually or combined into ‘compound feeds’. Commercial feeds for freshwater prawns tend to use ingredients which are available in large quantities; many of them are global commodities, such as fish meal or soybean meal. You can also include some of these ingredients in the feeds you make on-farm, as shown in the formulae given in this manual. In addition, you could include so-called ‘unconventional feeds’ (feeds not normally used in commercial feeds because they are only available in small quantities, often only locally and seasonally); some of these are listed in Table 18. In addition to ‘trash’ fish, molluscs and prawn wastes form valuable animal protein sources. Meal made from the leaves of the Ipil ipil bush (Leucaena sp.) has formed a constituent of shrimp and prawn diets but its use is cautioned by the toxicity of mimosine, which is a problem in its use for terrestrial animals. Some farmers add other materials to their ponds, including pig manure (added as a feed, not a fertilizer, where ethnically acceptable) and the mortalities from chicken farms, staked out around the periphery of the pond. Other locally available materials may also be satisfactory.

If you use individual raw materials (not made into a mixed and bound compound feed), especially wet materials (such as trash fish and beef liver), you stand more risk of causing your pond water to become polluted. Compounded feeds, especially when they are water-stable, cause less problems of this type. Compounded chicken and pig feeds, either unmodified, or re-extruded through a mincer with trash fish or prawn meal, have been used in freshwater prawn farming. Some are included in the formulae given in this manual. However, be careful about using chicken and pig feeds because they often contain growth promoters, antibiotics, and other substances which may have unpredictable effects on prawns. Their presence in prawn tissues may also make the product unacceptable.

TABLE 18
Examples of major ingredients either used individually or in mixed freshwater prawn grow-out feeds

Using water-stable feeds provides your prawns with a balanced ration. It also stops the prawns selecting individual ingredients. Using well-bound compounded feeds also results in less water pollution and makes your task of judging how much feed to give each day easier. Feeds can be made water-stable by including a wide range of naturally occurring and modified gums and binders, by adding pre-gelatinized starch, and by certain processing techniques used by feed manufacturers. Some typical formulae for freshwater prawn diets are given in Annex 9.

The methods for making farm-made feeds are not described in this manual because there are other publications available. Details are provided in another FAO manual (New 1987) and the use of farm-made aquafeeds generally is discussed in New, Tacon and Csavas (1995). If you are formulating your own diet it is necessary to determine [by analysis and/or consulting published information (Fonnesbeck, Harris and Kearl 1977; Gohl 1981; New 1987; Tacon 1987, 1993a, 1993b)] the composition of your locally available ingredients. Some specifications for freshwater prawn grow-out feeds are given in Table 19. Recently (Anonymous 2001a), it was reported that a farm-made aquafeed and feed mill unit has been launched in Cochin, India. Here, farmers can manufacture their own feed using communal equipment. This kind of development was recommended in a meeting in Thailand in 1992 (New, Tacon and Csavas 1995) and it is hoped that more units like this will emerge to assist small farmers to make their own, cheaper feeds. However, if you choose not to make your own feeds, consult your local aquafeed manufacturers and ask if they make feeds for freshwater prawns. You will find that they are keen to help you.

TABLE 19
Tentative specifications for semi-intensive freshwater prawn grow-out feeds

MEASURING FEED EFFICIENCY

You should not judge the value of a feed only by its unit cost (price per mt of feed). What you must consider is:

• what weight of prawns you will get by using this feed (mt/ha/crop) ?
• what proportion of the prawns produced will be marketable (the correct size for your market; good appearance, etc.) ?
• what will your total feeding costs be (this includes not only the cost of the feed itself, but how much does it cost you to store it, transport it to the ponds, feed it, and solve any problems it may cause in pond management) ?

The unit of measurement most commonly used on the farm is the feed conversion ratio (FCR). This is the actual weight of feed presented divided by the actual weight of animals produced (no adjustments are made for the differing moisture contents of the feed and the prawns). An FCR of 2:1 to 3:1 would be typical for a dry (~10-12% moisture) compounded diet. The FCR of wet feeds, such as trash fish, is much higher, perhaps 7:1 to 9:1. A semi-moist feed (typically with a moisture content of 35-40%), consisting of a mixture of dry and wet ingredients, might have an FCR of 4:1 to 5:1.

However, FCR is a rather crude measurement, because it only refers to total productivity. This is not the whole story. Time from stocking to harvest (the growth rate achieved), the prawn size and quality obtained, and the cost of storage and feeding are just three of the other factors that are important. For example, suppose that two feeds have an equal unit cost and the same FCR, the use of one may result in prawns reaching the average marketable size in 5 months, the other may take 6 months. The first is obviously the more efficient. FCR alone does not tell you this. This illustration is provided simply to make you think about your choice of feed more carefully.

FIGURE 76
Feed can be distributed within the pond by simple boats, which can be lifted from one pond to another; manual feeding along at least one side of the pond would be quite difficult in this case because of the method of construction, which has set a water channel in a very narrow pond bank (Thailand)

SOURCE: HASSANAI KONGKEO

FIGURE 77
Using a lift net for observing feed consumption (Puerto Rico)

SOURCE: HAROLD PHILLIPS

FEEDING RATE

There can be no exact general recommendation for daily feeding rates, because these depend on the size and number of prawns (and, in a polyculture system, fish) in the pond, the water quality, and the nature of the feed. Some farmers start feeding rates very high at first (perhaps as much as 100% of body weight at the PL stage). If juveniles are stocked, the rate might be 20-10% of body weight (depending on juvenile size) and it would decline gradually to about 2% by harvest time. This works quite well if the ponds are batch-harvested. However, if you are culling out the larger animals, this may result in some underfeeding for the others. It is also very difficult to calculate even a reasonably accurate estimate of the total body weight in your pond.

This manual recommends that you should start by feeding a fixed amount, which depends on the pond size, to encourage the growth of natural food (as measured by transparency, see below). Then, you should continue by feeding ‘to demand’ (in other words, giving as much feed as the prawns will eat but no more). Spread the feed around the periphery of the pond in the shallows, which are good feeding zones. Putting the feed in defined ‘feeding areas’ a few metres apart makes it easier to observe how much is consumed. This practice also leaves the areas in between the feeding zones clean, thus lessening pollution and promoting more healthy rearing conditions. Some farmers operating large ponds use boats to distribute feeds more evenly (Figure 76). Others use rafts, which are towed around fixed routes by means of a series of ropes guided by fixed wood or bamboo stakes within the pond or on its banks, for this purpose. Whether you confine feed to the periphery of your pond or distribute it more widely throughout your pond, the use of defined feeding areas, rather than general broadcasting, is recommended.

The best way of measuring food consumption is to use feeding trays (Figure 77), which can be lifted out of the water for inspection. You can construct lift nets from any netting with a mesh small enough to retain the feed particles. If you use this system, lift the tray out of the water for inspection to see how much feed has been consumed before you distribute the next feed. If there is no feed left on the following day, the feeding rate should be increased. If there is excessive food left, the feeding rate should be decreased. In cases of severe over-feeding, which may cause water quality problems, feed may even be omitted for a day. The need for the operator to be able to see the unused feed after 24 hours highlights one of the advantages of a water stable diet.

Where you have enough water available to allow it to flow through the pond all the time, you could adjust the phytoplankton density by altering the water flow rate. Even if your pond is normally static you could flush the pond if the phytoplankton density becomes too high (or there are other reasons to suspect poor water quality) by partially draining and refilling the pond. However, the best means of controlling phytoplankton density without wasting water (and money!) is to carefully monitor the effect of feeding rate and aeration (which is often used in nursery ponds and always in intensive grow-out) on water transparency, and make alterations as necessary. By this means panic situations caused by gross over-feeding can be avoided.

Exact daily feeding rates are site and management specific. However, an example is given in Box 23, which also describes how to adjust feeding according to water transparency. This example assumes that PL are stocked directly into the final grow-out pond. The use of feedstuffs to induce phytoplankton growth may seem rather expensive (compared to using fertilizers) but it is simple and effective. However, rearing PL to juvenile size in a nursery system, as described earlier in this manual, is a more efficient way of using feeds for the first 2 months or so after metamorphosis.

6.4 Health, predation and disease

WATCHING FOR SIGNS OF PROBLEMS

Continuous exchange of a small proportion of the water is the normal way of maintaining good water quality. However, some farmers change water more suddenly every two weeks, and in much larger proportions, because this tends to make the prawns moult. The more that moult (and are therefore soft-shelled) at the same time, the less potential losses there may be due to cannibalism. A scum of phytoplankton may cover the surface of the pond. This will cause low DO2 problems at night and should be controlled by a reduction in feeding and by exchanging water. Low DO2 should be suspected if prawns begin to crawl out of the ponds or congregate at the edges of the pond in daylight. If this problem occurs, flush the pond. The need to do this in emergency situations illustrates the importance of having sufficient water available. Very high pH levels in freshwater prawn ponds can cause prawn mortalities, both because of the direct effect of the pH itself and because of the greater solubility of waste ammonia at high pH. High pH is often caused by dense phytoplankton blooms.

If you see sudden heavy mortalities, or observe small numbers of mortalities over a period of time, you should carefully investigate the cause. Prawns covered with algae or showing signs of not having moulted recently may indicate either that culture conditions are poor or that the animals are not healthy. Poor farm management, resulting in poor water quality and/or disease may be to blame. However, external factors may also be responsible. The most likely source of external water pollution is from pesticides and herbicides. For example, pesticides used on neighbouring banana farms and herbicides used for the elimination of water hyacinths in irrigation canals have been blamed for prawn mortalities in the Caribbean and Thailand respectively. Thus the importance of site selection and water source is obvious. Further reading on this topic is contained in Boyd and Zimmermann (2000), Correia, Suwannatous and New (2000) and Daniels, Cavalli and Smullen (2000).

FIGURE 78
Measuring transparency can be very simple, even when the design of the Secchi disk is unconventional (Peru)

SOURCE: OSCAR ORBEGOSO MONTALVA

DEALING WITH PROBLEMS OF PREDATION

Predation is one of the greatest problems for any aquaculture enterprise, including freshwater prawn farming. Predation is caused mainly by other aquatic species, birds, snakes and humans. Two of the greatest sources of loss in freshwater prawn farming are human predation and operator error.

Freshwater prawn farms are more prone to human predation than many fish farms because of the high value of the product and because prawns are relatively easy to catch. The temptation to catch a few kilograms of prawns by cast-net at night (a kilogram of which may be as valuable as a tenth of a month’s individual income to some) is sometimes too great to resist. You cannot eliminate any form of predation, including human poaching. However, you must minimize it by good management. Perimeter fences, dogs, lighting, and reliable watchmen help. If your farm is big enough to financially support it, you may be able to achieve some protection from human predation if you stock some PL into local public waters, thus generating a positive attitude towards your farm. If you own a small farm you may find it useful to form a cooperative with other farmers within the community. The activities of such groups are normally protected by the local community. You may also lose prawns through operator error and poor management. For example, water levels may be allowed to become too low and therefore temperatures too high, or DO2 levels may be allowed to fall too low. Both errors will cause animals to die. Not maintaining outlet structures properly allows prawns to escape.

Normally, insects (mainly dragonfly nymphs), carnivorous fish and birds are the most serious predators in freshwater prawn farming. In the past, chemicals have been used to kill dragonflies and other insects but this is not recommended because it may negatively affect the pond ecosystem. Mosquito fish (Gambusia affinis) and related species were also once stocked in freshwater prawn ponds to control insects. M. rosenbergii postlarvae themselves, if they are stocked before the insects hatch, can control the dragonfly population. You can effectively control unwanted fish by using rotenone or teaseed cake between cycles, as discussed earlier in this manual. You can prevent the entry of fish and some insects by passing the intake water through suitable screens or gravel filters (Figure 79). Most commercial prawn farms rely on simple net filters. If fish eggs and larvae do get into your ponds (which they will!), it is not a complete disaster because, by the time they get to a dangerous size, many will be seined out during the cull-harvesting of prawns. The ideal would be to exclude all predators but this is not possible. The most important thing is to stock the prawns very soon after each pond is filled, so that predators and competitors have less chance to become established. The presence of many frogs and toads in a pond usually indicates that predatory fish have been fairly efficiently excluded.

It is suggested that you use small 60 cm high netting fences around ponds for the prevention of invading catfish and snakehead fish (Figure 80). These fences may also prevent the entrance of amphibians, reptiles and some mammals. You will also find that the seining that you do during the cull-harvesting of prawns can remove large predators, such as fish, turtles and snakes. Birds are very difficult to repel or control. Netting or string can be stretched across the top of ponds as a deterrent. You can use various bird-scaring devices. In general, you should not shoot invading birds because you may be breaking local bird conservation regulations [a list of endangered birds is available in the IUCN Red List (IUCN 1996)]. The use of dogs as bird scarers may be more efficient and cheaper than shooting them.

Competent management of prawn competitors and predators includes stocking prawns as soon as the pond is filled, seining periodically, and totally draining and treating the ponds at least once per year.

FIGURE 79
A simple gravel filter on a farm supply system helps to exclude predators (Brazil)

SOURCE: WAGNER VALENTI

FIGURE 80
Netting can be used to protect freshwater prawns from predators that arrive overland (Brazil)

SOURCE: JULIO VICENTE LOMBARDI

COPING WITH DISEASES AND OTHER PROBLEMS

Diseases in freshwater prawn ponds are relatively unusual, compared to other forms of aquaculture. However, this may be a function of the relatively low stocking densities used so far. If stocking rates are increased, more problems may occur in future. Furthermore, diseases have been known to occur in freshwater prawn grow-out when the quality of the water (either of the intake or within the pond itself) is poor. Proper attention to the possibility of disease and other problems is therefore essential. The potential problems are discussed in this section of the manual.

The general issues of health, defence mechanisms against disease, and diagnosis have been dealt with by Johnson and Bueno (2000); their review also contains general information on sanitation, quarantine and therapeutic treatment. There are number of other problems in freshwater prawn grow-out, which include the result of nutritional deficiencies, fouling or parasites.

Diseases of known cause

A summary of the infectious diseases currently known to affect freshwater prawns during the grow-out phase is given in Table 10, while Table 11 lists some actions that can be taken to reduce the incidence of these problems (prevention). Some treatments that have been used, often experimentally, after the appearance of diseases are also listed in Table 11. However, treatment is not normally practical. The continuous use of antibiotics and other chemicals is also not recommended, either in hatcheries or grow-out systems. It is not thought practical to treat prawns in commercial grow-out facilities at this moment (practical and environmentally acceptable treatments may evolve in the future).

Prevention (through good management) is always better than attempted cure. There are potential human health hazards and food safety issues concerning the use of antibiotics. Some, such as chloramphenicol, are banned substances in aquaculture. If you do use antibiotics, you must consult your local aquatic animal health specialist and only use approved substances in the correct dosages. You must also follow the specialist’s advice on how long before harvest to stop using the product, to make sure that there are no residues in your harvested prawns.

Disease problems may originate during the transfer of animals from one site to another, including the introduction of animals into a location where they are not indigenous. Comments on the care which should be taken with introductions have already been mentioned in this manual.

Diseases of non-specific cause

All life stages of freshwater prawns are also subject to a disease known as muscle necrosis. Affected prawns show a whitish colour in the striated muscle of the tail and appendages. The necrotic areas may increase in size and become reddish, a colour similar to cooked prawns, due to the decomposition of the muscle tissue. Secondary pathogens (bacteria and the fungus Fusarium) have been found to be associated with muscle necrosis (see Table 10). Prawns suffering from chronic muscle necrosis do not survive. Population mortality rates vary from insignificant to 100%. This disease is associated with poor management and occurs particularly when stocking rates and handling stress are high and when environmental conditions are poor (low dissolved oxygen level; temperature fluctuations; and, in the hatchery, salinity fluctuations). Follow the good management practices suggested in this manual and you will minimize the occurrence of such problems.

Other diseases of uncertain origin affect freshwater prawn larvae; these have been described earlier in this manual.

Parasites

Parasites seem to be quite rare in cultured M. rosenbergii. Freshwater prawns have been found to be hosts for the bopyrid isopod Probopyrus. These attach themselves to the interior of the gill chamber, usually resulting in a visible swelling. This would normally only be a problem if it became common in a captive broodstock, because it is reported to interfere with egg production. The only other problem that might occur if this parasite became common in grow-out would be its affect on the appearance of prawns sold head-on; so far this has not been described.

Wild-caught freshwater prawns of various species have been observed to be intermediate hosts for trematode worms. Prawns have also hosted the Asian lung fluke but are thought to have an unimportant role in its transmission to mammals.

Fouling

The general body surface of the prawn can serve as a substrate for filamentous bacteria and algae, and single or colonial protozoa. More information about the specific organisms that cause fouling (which include Zoothamnium, Epistylis, Vorticella, Leucothrix and many others) and references to further reading on this topic are given in Johnson and Bueno (2000). Moulting temporarily frees prawns from these fouling micro-organisms. The
problem is particularly noticeable in large animals, especially blue-claw (BC) males, which moult less often.

Although these organisms do not invade the tissues they make it difficult for the prawns to move and to feed, particularly in the larval and postlarval phases. Extreme infestation on the gills can impair their function, and may cause mortalities in juvenile or adult prawns. Heavy infestation over the exterior surface can also reduce the market value of prawns. Infestation by filamentous algae has been observed to occur in grow-out ponds with high transparency (above 40 cm). This problem can be therefore be reduced by encouraging lower water transparency through feed management.

You can cut down the incidence of problems in hatcheries caused by fouling organisms by good management, especially the correct treatment of incoming water, the proper cleaning of tank bottoms, and the treatment of Artemia cysts. In both hatcheries and ponds the avoidance of over-feeding and increased water exchange help to minimize the fouling of animals. A number of chemical treatments against fouling organisms have been suggested (Johnson and Bueno 2000) but are not recommended in this manual.

6.5 Monitoring performance and record keeping

The growth rate and survival of each population of prawns depends on many factors, including density, predation, feed and temperature. Since these factors are so site- and operator-specific it is not wise to predict what they will be in this manual, for fear of causing expectations which may not be realized. However, Box 24 gives examples of growth and production rates that have been reported in the scientific literature. Survival rates during the grow-out period should not fall below 50%. Under the semi-intensive management system described in this manual, productivity ranges of 1 000-3 000 mt/ha/yr are typical but can be exceeded.

As your farm operates, you will develop your own experience of growth rate and productivity during grow-out. You can only achieve this by careful monitoring and record keeping. It cannot be over-stressed that you should keep adequate written records of such things as water quality, stocking rate and date, daily feeding quantities, dates on which water changes are made (and how much), harvesting dates and quantities, etc. Only in this way can you build up a picture of how each pond behaves under a certain management regime (and every pond is different) and accurately apply your experience to future pond management in order to operate your farm profitably. This applies equally to hatchery management.

Methods of monitoring feeding rate and phytoplankton density and for the control of the latter have been dealt with earlier in this manual. It is good practice, if possible, to monitor other water quality parameters such as pH, temperature and dissolved oxygen routinely, so that you can link production rates with the environment of each pond and the way in which you manage it. This will give you the information you need to take actions to prevent a recurrence of problems (such as low dissolved oxygen levels, for example).

Ideally, you would like to determine the average size and the number of prawns in your pond at any time. In this way you could tell whether growth and survival rates are satisfactory, or not, and determine a daily feeding rate based on a percentage of the pond biomass. Unfortunately, there is no accurate way known of determining the standing crop of freshwater prawns in a pond unless the pond is regularly seined. Even when a reasonable estimate of pond biomass can be obtained, daily feeding rates based on a percentage of biomass should not be applied blindly but should be tempered by observations on consumption and phytoplankton density.

If you are growing freshwater prawns for the first time, you must realize that individual prawns within a population grow at different rates. Some will grow very fast, others hardly at all. This normal characteristic of the animal has been described in Annex 8. The disparity in growth rate is more pronounced among males than females and in mature populations of freshwater prawns.

FIGURE 81
A large BC Macrobrachium rosenbergii broodstock male from the CAUNESP (Aquaculture Center, São Paulo State University, Brazil) being measured in the ‘scientific’ way (from behind the eye orbit to the tip of the telson)

SOURCE: DEBORAH ISMAEL

You should regularly measure growth rate, either by weight or total length. Measurement of length from the eye orbit to the tip of the telson (Figure 81) is the most reliable technique (because the rostrum of some animals becomes shortened by damage) but, in farming practice, total length from the tip of the rostrum to the tip of the telson is usually measured, often by ruler. Figure 82 gives the relationship between total length and live weight for a mixed-sex population of freshwater prawns. Males weigh slightly more than females of the same length, but not markedly so. A method for sexing small (juvenile) prawns is shown in Figure 3. The differences between larger females and the various male morphotypes have also been described earlier in this manual.

If you have never grown freshwater prawns before you may notice that you do not see the prawns after they are stocked; they are difficult to see and to catch at this stage. Do not be discouraged! At this time, you will be giving quite large quantities of food but, after a while, you may begin to think that the feed (and your money!) is being wasted. You cannot see many prawns, so you wonder: have they all died, escaped, or been eaten by predators ? Do not decrease the amount of feed or stop feeding altogether. About two months after stocking, you will begin to see (by now quite large) prawns again. If you wait until the stage when you can see the prawns in the pond to start feeding again, the productivity of your crop will have been permanently reduced. This is a common experience of new farmers of this species. Have patience, and examine the perimeter of your ponds by night, with the aid of a flashlight.

¹⁰ Shrimp and prawn processing wastes (heads, shells, etc), sometimes used in farm-made feeds, may be virus disease carriers, if used raw (not processed). It is better to use commercially available shrimp meals. The diseases they carry may not produce obvious symptoms in freshwater prawns but could induce them to transfer the disease to other crustaceans. Meat and bone meal is a banned feedstuff ingredient in some countries (because of BSE); prawns reared on aquafeeds containing meat and bone may face consumer resistance. Compound feeds made for other species may contain antibiotics and/or levels of other substances harmful to prawns. Ipil ipil contains the toxin mimosine. Some other high-protein plant ingredients also contain toxins but these are removed by adequate procesing.