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).

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

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

Figure 64
Long-shaft aerator in action (Thailand)

SOURCE: HASSANAI KONGKEO

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

BOX 14

Definitions of farming intensity used in this manual

Level 1:
EXTENSIVE FRESHWATER PRAWN CULTURE
Extensive culture means rearing in ponds (but also in other impoundments such as reservoirs, irrigation ponds and rice fields) which produce less than 500 kg/ha/yr of freshwater prawns. They are stocked, often from wild sources, with PL or juveniles at 1-4/m2. There is no control of water quality; the growth or mortality of the prawns is not normally monitored; supplemental feeding is not normally supplied; and organic fertilisation is rarely applied.
Level 2:
SEMI-INTENSIVE FRESHWATER PRAWN CULTURE
Semi-intensive systems involve stocking PL or juvenile freshwater prawns (usually from hatcheries) at 4-20/m2 in ponds, and result in a range of productivity of more than 500 kg/ha/yr and less than that defined as intensive in this box.
Fertilisation is used and a balanced feed ration is supplied. Predators and competitors are controlled and water quality, prawn health and growth rate are monitored. This form of culture is the most common in tropical areas.
Level 3:

INTENSIVE FRESHWATER PRAWN CULTURE

Intensive culture refers to freshwater prawn farming in small earth or concrete ponds (up to 0.2 ha) provided with high water exchange and continuous aeration, stocked at more than 20/m2 and achieving an output of more than 5 000 kg/ha/yr. Construction and maintenance costs are high and a high degree of management is required, which includes the use of a nutritionally complete feed, the elimination of predators and competitors, and strict control over all aspects of water quality. This form of culture is not recommended in this manual because it requires more research, particularly on size management.

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

BOX 15

Systems of management in grow-out ponds for freshwater prawns

System 1:
THE CONTINUOUS SYSTEM

This involves regular stocking of PL and the culling (selective harvesting) of market sized prawns. There is no definable ‘cycle’ of operation and the ponds are therefore only drained occasionally. One of the problems of this form of culture, which can only be practised where there is yearround water availability and its temperature remains at the optimum level, is that predators and competitors tend to become established.
Also, unless the culling process is extremely efficient, large dominant prawns remain and have a negative impact on the postlarvae which are introduced at subsequent stocking occasions. This results in a lower average growth rate. The decline in total pond productivity (yield) that has been observed when this system has been used for a long time is, however, not confined to this management system and may also be a function of genetic degradation, as discussed elsewhere in this manual.
This results in less and less satisfactory animals being stocked. There are other major problems which occur when ponds are continuously operated (see Figure 65).
The various real or perceived problems of the continuous management system were not obvious when the original FAO manual on freshwater prawn farming was revised. In its first English edition (New and Singholka 1982) the authors mentioned the continuous system but specifically omitted any details about it because they thought that it might be wrongly interpreted as a recommendation for application in all circumstances.
However, following requests for details, the authors included detailed information on this topic in its revision (New and Singholka 1985); this information was also included in its French and Spanish editions. In view of the experience gained in the 17 years since this information was published, the long-term continuous management system is not now recommended and the annex providing details about it has therefore been omitted in the current manual.

System 2:
THE BATCH SYSTEM

At the other extreme to the continuous system is the batch system, which consists of stocking each pond once, allowing the animals to grow until prawns achieve the average market size, and then totally draining and harvesting it. This reduces predator and competitor problems. However, although dominant prawns cannot impact on newly-stocked PL (because there is only a single stocking), the problem known as heterogeneous individual growth (HIG) remains. This term (HIG) refers to the fact that freshwater prawns do not all grow at the same rate. Some grow much faster, tend to become dominant, and cause stunted growth in other prawns. This bland statement is a simple summary of a very complex phenomenon, which is explained in more detail in Annex 8.

System 3:
THE COMBINED SYSTEM

This provides the advantages of reduced predator and competitor problems of the batch system with the cull-harvesting employed in the continuous system, to reduce the problems of HIG. In the combined system, ponds are stocked only once. Cull-harvesting starts when the first prawns reach market-size (the exact size depends on the local, live sales, or export market requirements).
This removes the fast-growing prawns for sale, leaving the smaller ones to grow, with less HIG impact. Eventually, after several cullharvests, the ponds are drained and all remaining prawns harvested. The total cycle usually lasts about 9-12 months in tropical regions, depending on local conditions. This system is recommended in this manual.

System 4:
THE MODIFIED BATCH SYSTEM

This more complex management regime was developed in Puerto Rico (Alston and Sampaio 2000) and involved three phases. After 60-90 days in a 1 000 m2 nursery pond stocked at 200 to 400 PL/m2, 0.3-0.5 g juveniles were harvested and stocked at 20-30/m2 into empty (without any existing prawns present) ‘juvenile’ ponds. After another 2-3 months, seine harvesting of these juvenile ponds began and was repeated every month after this.
These harvests removed animals of 9 to 15g, which were then stocked into ‘adult’ ponds with existing populations of small prawns. The juvenile ponds were themselves then either converted to adult ponds, to allow remaining animals to grow to marketable size, or were drained and refilled for further use. According to the owner of the farm (J. Glude, pers. comm. 1998), drain-harvesting into a catch basin, instead of seining, would have reduced labour costs and increased survival. Further advantages could have been obtained if postlarvae had been held longer in the nursery ponds and then graded into at least two size groups before stocking into juvenile ponds.

also be used for disinfection (see Boyd and Zimmermann 2000) but it is not recommended because it is a much more expensive treatment.
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 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.

BOX 16

Application of rotenone and teaseed cake

ROTENONE:
20 g/m3 (200 kg/ha when the water averages 1 m deep) of rotenone powder (which contains 5% rotenone, usually from Derris roots, and thus equivalent to applying 1 g/m3 of pure rotenone) is the normal dose. Rotenone needs to be mixed in water and the solution kept well-mixed while it is applied.
TEASEED CAKE:
The application of teaseed cake (containing 10-13% of saponin) at a dose of 50-70 g/m3 (500-700 kg/ha when the water depth averages 1 m) is adequate to remove unwanted fish. Teaseed cake needs to be prepared by drying and finely grinding the seeds, soaking the powder in lukewarm water for 24 hours, and diluting the suspension before mixing it evenly into the pond water.

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

SOURCE: DERIVED FROM BOYD AND TUCKER (1998)

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.
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.
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 manure 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.

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

SOURCE: CLAUDE BOYD

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).

BOX 17
Measuring soil pH ake 10-12 samples of the upper 5 cm layer of the soil, before any soil treatment has been applied, dry them in an oven at 60°C, and pulverize them to pass a 0.085 mm screen. Bulk the samples together and mix 15 g of the pulverized soil with 15 ml of distilled water. Stir occasionally for 20 minutes and measure the pH, preferably with a glass electrode. The hand-held pH-soil moisture testers used by some farmers are not accurate enough (Boyd and Zimmermann, 2000).

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.

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

SOURCE: DENIS LACROIX

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 temper ature), and on the management system you are using. Older ponds tend to be more productive than new ones. Your decisions about stocking rate should con sider all these factors. Specific stocking densities are not recom mended in this manual because no guarantee can be given that a certain quantity of prawns will be produced! As stated in Box 14,

 

BOX 18

Reasons for not applying organic fertilizers
ORGANIC FERTILIZERS:

VARY in composition.

HAVE a low nitrogen and phosphorus content and therefore have to be applied in large quantities.

CREATE an oxygen demand in the pond water.

LEAVE organic residues on the pond bottom.

PROVIDE detritus that becomes a starting point for the growth of filamentous algae.

MAY contain high concentrations of heavy metals.

MAY be contaminated with antibiotics.

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

 

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.
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.