Basically there are two methods of harvesting: culling (sometimes called cull-harvesting) and draining (drain-harvesting). The time to harvest depends partly on growth rate and the size of animals you want to sell. This, in turn, depends on your market requirements. It also depends on the pond management technique chosen. Cull-harvesting is used to harvest market-sized animals from the pond at intervals and removes the faster growing prawns. Annex 8 gives further information on size management. The rest of the prawns are caught when the ponds are drained at the end of the grow-out cycle.
In tropical ponds cull-harvesting usually starts 5-7 months after PL have been stocked, or sooner if juveniles have been stocked. After cull-harvesting commences, you should totally seine each pond once per month or partially seine it twice per month (i.e. seine half the pond twice per month or all of it once per month). Take out the market-sized animals and sell them. Keep the smaller ones and soft-shelled animals in the pond for further growth. After about 8-11 months, drain the pond and sell the whole harvest. In areas where water supply is short, some farmers utilize water drained during complete harvesting in other ponds to conserve water but, if you think of doing this, beware that you may be transferring a water quality or disease problem from one crop to the next. This practice is not recommended in this manual. After drain-harvesting, you can either prepare, refill and restock the pond immediately, or keep it empty until you have enough water available again and/or (in temperate zones) until water temperatures become suitable again for rearing.
Cull-harvesting is not very efficient in removing harvest-sized prawns. It does not maximize the total quantity of marketable prawns which could be achieved, partly because some marketable animals remain in the pond longer than necessary and partly because the smaller prawns do not get the maximum chance to grow faster than they would have if there were no dominant prawns left behind. In theory, the best management system would be to totally harvest the pond, remove all the dominant animals, and restock the others in the same or a different pond. Various management strategies to maximize pond utilisation and production rate have been described, for example the modified batch system developed in Puerto Rico, as mentioned earlier. A four-pond multi-stage rotational scheme, which includes the re-stocking of undersized prawns following harvest, is described by Karplus, Malecha and Sagi (2000). Some handling losses occur during the animal transfers necessary for these more complex management systems. These types of management may be feasible in large farms with many available ponds. However, cull-harvesting, followed by total drain-harvesting before re-stocking with a new batch, remains the most practical management scheme for small freshwater prawn farms.
You should carry out all harvesting operations as early as possible in the morning when it is cooler, to avoid having water levels too low when the sun is directly overhead. If you allow the water to become shallow, temperatures can quickly rise to danger level and the prawns will be subjected to low dissolved oxygen levels. This will cause many mortalities before you can finish harvesting all the animals.
In this technique a seine net is pulled through the pond to remove market-sized animals. The net may be a simple seine or one constructed especially for the purpose (Annex 7), usually made of monofilament nylon, and provided with floaters, sinkers and sometimes a bag. The size of the seine you use depends on the size of pond you are using it for. Nets which are 2.5 m high with a length equivalent to 1.6 times the pond width are suggested. The mesh you choose depends on the size of animal to be marketed. Although stretched knot mesh sizes of as low as 0.7 inches (1.8 cm) are sometimes used where there is a market for small prawns, the usual recommended mesh size is 1.5-2 inches (3.8-5 cm).
Care must be taken to ensure that the bottom of your seine is kept on the bottom of your pond. If you do not, many prawns will escape beneath it. Preferably you should pull the seine down the long axis of the pond (this is why rectangular ponds of 30 m maximum width are preferred) so that the ends of the net are pulled along the banks of the pond (Figure 83). Seining different halves of the pond once every two weeks avoids disturbing the whole of the pond at once.
The amount of prawns collected by the seine should not exceed that quantity which can be rapidly taken out of the seine, and transferred to a live box, a cage, or an impounded area for sorting. Many prawns, especially smaller ones, die when the bag of a bag seine is lifted out of the water if the quantity is too great. One way to ensure that the seine does not get overloaded because a whole pond is being seined is to stretch two seines across the middle of the pond. Firstly, while one seine is staked in position to retain the prawns in one half of the pond, pull the other seine to the end of the pond and harvest it. Then pull the first seine to the opposite end of the pond (Figure 84).
Ponds which are too wide to harvest by pulling each end of the seine down the long sides of the pond can also be seined but not so efficiently. You will have to repeat the seining operation several times, keeping one end stationary and pulling the other end in a semicircular fashion (Figure 85). If some of your labourers beat the surface of the water with sticks it will discourage the prawns from escaping the open end of the net as it is pulled toward the bank. Circular ‘breeding’ depressions are often seen in drained freshwater prawn ponds. These interfere with seining operations because large males can use them to evade the seine. Pond bottoms should be kept as even and smooth as possible. You should remove these depressions between rearing cycles.
Generally speaking, the more men you have available to help with seining, the more efficiently you can perform this task (provided there is one clear leader!). Three to five people can pull a seine net through a 30 m wide pond and seven to ten men can cope with a 50 m wide pond. A single-seined 0.2 ha pond takes about two to three hours to cull harvest using three or four people. Some farms claim that they can cull-harvest and sort the prawns caught from a 1 ha pond with 5 people in less than 2 hours but this may not maximize the catch of harvest-sized animals. You can put a temporary harvesting enclosure within the pond during seine-harvesting (Figure 86). This enables you to keep the prawns alive as long as possible (you may be going to sell them alive) and to sort them into different categories.
Transfer the prawns that you have seined to a holding tank or net (Figure 87) quickly. From this, you can sort them into different categories (by size, sex, berried females, etc.) to suit your market. Figure 88 shows a holding net being held near the pond inlet to maintain good water quality, and therefore high survival. Market sized animals should be retained for sending to the market; smaller animals should be returned to the original or to another pond. The measurement of market-sized animals during the sorting operation can be done by a quick check of total length. Tying rulers to the operators hands is a useful way of training them to do this but experienced harvesters quickly learn to judge sizes quite accurately. Remember that cull-harvesting is a good time to examine the health of your prawns (Figure 89).
Further details of seining techniques are contained in another FAO manual (FAO 1998,) which also illustrates other methods of partial harvesting, such as cast nets and lift nets.
The method and efficiency of drain harvesting depends on the design of the pond. As with any other method of harvesting, speed is important and harvesting should start very early in the morning while the temperature is cool. You can partially draw down the pond water level during the night before harvesting commences.
FIGURE 86
Sorting prawns while seine harvesting
SOURCE: SPENCER MALECHA
FIGURE 87
Market sized freshwater prawns can be kept alive while harvesting continues (Martinique)
SOURCE: MICHAEL NEW
If your pond has a ‘monk’ or sluice gate structure for drainage, you can include a harvesting sump (catch basin) in front of the gate (Figures 90 and 91) or outside the pond. Details of drain harvesting structures are given in another FAO manual (FAO 1998). Efficient total harvesting can best be achieved through the addition of a catch basin within a drainable pond. As you drain the pond, prawns will accumulate in the basin. You must be careful to avoid oxygen deficiencies in the sump; you can prevent this by providing aeration or by having a continuous flow of new freshwater through the catch basin. Minimizing prawn stress (e.g. caused by low dissolved oxygen levels, high temperatures, crowding) during harvesting is extremely important. Having the correct outlet structure dimensions becomes particularly and obviously important when drain harvesting takes place. If the outlet pipe of the pond is too small, the pond will take much too long to drain (Table 14) and many of your animals will die before harvesting is completed. If your pond has been properly constructed, very few prawns will become trapped on the pond bottom during draining. In ponds with added substrate, these habitats can be left in place. The prawns will move out of the substrate to the catch basin as the water level drops.
If your pond does not have a monk or sluice gate you will have to drain it by pumping. Prawns must be prevented from entering the pump by means of a screen. Long-tail pumps, as shown in Figures 60 and 61, can be used. If you are harvesting this type of pond you will need to catch most of the prawns by multiple seining of the pond while draining takes place (Figure 92). As the water level gets low, many prawns will retreat into the mud or become stranded in isolated pools of water. At this stage there is no substitute for catching by hand (Figure 93). If you have problems with stranded prawns at this time you will wish that you had been more careful to construct your pond with a good slope towards the drain and a well compacted, smooth surface. Another typical result of poor pond drainage is the target that stranded prawns provide for birds. Birds love harvest time (Figure 94)!
FIGURE 88
Prawns destined to be sold alive need clean and well-oxygenated water to keep them in peak condition (Martinique)
SOURCE: MICHAEL NEW
FIGURE 89
Cull-harvesting or sampling is an opportunity to check the health of your prawns
SOURCE: DENIS LACROIX
In temperate regions the harvesting of the total pond population of prawns must occur before water temperatures drop below 17°C. However, some animals may attain marketable sizes 4 to 6 weeks before final harvest. 2-3 cull-harvests may be beneficial to spread the marketing season over a longer period.
Good handling and processing really begin at the pond bank. You need to take special care during harvesting to avoid the ‘mushiness’ which processed freshwater prawns have often been accused of. This effect is not unavoidable; it is caused by poor harvesting and processing. Do not allow your prawns to become ‘stacked up’, either in a net or in a harvest basin; this causes damage to the internal organs which results in poor quality prawns when sold. If you are not going to sell your prawns live, you should immediately wash them in clean water, kill them in a mixture of water and ice at 0°C, and then wash them in chlorinated water (5 ppm active chlorine). This process, which preserves product quality, must be carried out near the pond, usually on the pond bank. Transporting live prawns to processing facilities before this treatment is carried out is not recommended because some prawns will die during transport and become ‘mushy’. No subsequent treatment can improve the texture of such prawns. When you are planning to sell the prawns live, take special care to reduce stress and minimize harvesting damage as much as possible, to ensure long-term post-harvest survival. You can achieve this by aerating or supplying a little clean water to the harvest basin (see Figure 91) and holding the prawns in clean, aerated holding tanks at temperatures preferably of about 20-22°C.
FIGURE 91
Harvesting Macrobrachium rosenbergii from ponds with an internal harvesting sump (Brazil)
SOURCE: WAGNER VALENTI
In general, the value of your harvested product will depend on its quality. Speed during and after harvesting, getting the prawns on ice and out of the sun, and care in handling to prevent physical damage, will all reap valuable dividends. Farmed prawns should be better than the wild-caught product in every way and it is up to you to see that your hard-won harvest does not deteriorate through poor harvesting and post-harvest procedures.
The handling of freshwater prawns after harvesting was thought to be beyond the scope of the original FAO manual on freshwater prawn culture; very few recommendations were made at that time. However, much more is now known about the effects of handling on prawn quality (and therefore value). The following section of the manual has been derived mainly from the work of Madrid and Phillips (2000) and especially from the experience gained in a Costa Rican farm operated by one of those authors.
FIGURE 92
Cull-harvesting freshwater prawns several times before drain harvesting increases the yield of market-sized animals in your crop (Martinique)
SOURCE: MICHAEL NEW
If you intend to sell your prawns fresh (instead of selling them alive or frozen) you will need to keep them very cool, after the pond-side pre-processing described earlier has been done. You should not allow your prawns to die from asphyxia by leaving them out of water. Harvesting mud as well as prawns is a source of microbial contamination. You should not place live prawns straight onto ice; this results in a slow decline in body temperature, causes stress, and accelerates the deterioration process which occurs after death. As noted in the harvesting section of this manual, prawns which are not going to be sold live should immediately be washed in clean water and killed in a mixture of water and ice at 0°C (Figure 95). To kill a batch of 50 kg of prawns, for example, immerse them in 50 L of water and 80 kg of ice for 30 minutes. Finally, you should wash them in chlorinated water (5 ppm active chlorine). If supplies are locally available, full-strength seawater which has been chlorinated has been found to reduce the incidence of ‘mushiness’.
FIGURE 93
The last few prawns may have to be caught by hand, especially when the pond does not drain well (India)
SOURCE: STEPHEN SAMPATH KUMAR
FIGURE 94
Bird predation causes problems during drain harvesting (Hawaii)
SOURCE: SPENCER MALECHA
After killing, remove your prawns from the cold water and immediately place them in isothermal boxes, with alternate layers of ice and prawns, placing ice in the first and last layers. Make sure your ice has been made from clean chlorinated water! Further information on the use of ice can be found in another FAO publication (Graham, Johnston and Nicholson 1993). You can then refrigerate your prawns at 0°C for short-term on-farm storage for sale as fresh prawns, or for transport to market or processing facilities (-10°C is not necessary and is much more expensive in terms of equipment and running costs). You are recommended not to keep prawns refrigerated at 0°C for more than 3 days; 5 days is the absolute maximum. Do not use large blocks of ice for storage or transport on ice because they will damage the prawns; use flaked or crushed ice.
If you are not going to sell your prawns within 5 days of harvest, which is considered to be their maximum practical refrigerated shelf life, you need to freeze them immediately. These prawns need the same care and attention as those sold fresh. Always remember that freezing does not improve the quality of the prawns; at best it will preserve them in the quality they show at the time of freezing. Freezing at temperatures below -10°C is essential; storage at -20°C or below is recommended; storage at -30°C is ideal. To avoid physical damage to the muscle structure of the prawns, it is recommended that the freezing temperature passes from -1°C to -5°C as rapidly as possible (not more than 2 hours). This decreases the production of ‘drip’ (leak) at the moment of thawing, and keeps the prawns looking and tasting the same as before freezing. If you freeze them more slowly it will cause large crystals to form in the water between the cells of the animals and increase ‘drip’. Keeping prawns frozen on-farm is generally not good practice, except on very large farms where specialist equipment has been installed. Otherwise it is best to sell them to professional processors who know how to care for the product properly.
Despite this advice, domestic freezers are often used by small farmers to freeze and store prawns. This does increase their shelf life but it can damage the texture of the flesh. You must not try to freeze prawns by placing them straight into domestic freezers because it results in the most frequent criticisms of prawns, namely that they are ‘mushy’. Domestic equipment should only be used for very small amounts of prawns that are already very cold when first placed into the freezer. It is important that you keep the prawns in contact with the base of the ‘freezer’, or with the shelves where the refrigerant liquid passes. As the thickness of the layers of prawns increases, the freezing time increases in a geometric proportion. When you withdraw prawns from a freezer, do not let any which you do not immediately intend to use thaw out. Replace them into the freezer before they thaw.
Newly frozen prawns should be put in the back of the storage facilities and the prawns at the front used first. This technique for ensuring freshness is sometimes referred to as ‘FIFO’ (first in, first out). You must always avoid the alternatives, which could be called ‘FILO’ (first in, last out) or ‘FISH’ (first in, still here).
FIGURE 95
If you are not marketing your freshwater prawns alive, you should kill-chill them in a bath of iced water immediately after harvesting to get the best quality (Puerto Rico)
SOURCE: DENIS LACROIX
If your farm is large and you intend to freeze your own prawns the best solution is to use special processing and freezing equipment and packaging (Figures 96 and 97). This topic is beyond the scope of this manual but an introduction can be found in Madrid and Phillips (2000) and more details in another FAO publication (Johnston, Nicholson, Roger and Stroud 1994).
If your market prefers it, you can pre-cook your prawns before freezing (Madrid and Phillips 2000). The appearance of cooked versus uncooked prawns is shown in Figure 98.
Sometimes you will want to sell your prawns alive, either at your farm gate or after transport to markets and (especially) restaurants. These prawns also require careful, but different handling; the techniques are similar to those used to sell other live aquatic products. You will need to change your holding and transport water regularly to eliminate ammonia build-up. Keep the dissolved oxygen level above 5 ppm with aeration. Prawns to be transported live should be washed in non-chlorinated clean water and then brought to the same temperature that can be maintained during transport to prevent thermal shock through sudden transfer into water of a totally different temperature. It is recommended that you keep the transport temperature at about 20-22°C. Use small amounts of ice, if necessary, to keep this temperature constant. Transport techniques similar to those used to transport prawn postlarvae from hatcheries to distant grow-out facilities by road transport (as described earlier in this manual) are suitable. In general, restaurants and shops or market stalls selling live prawns will have aquariums to display them. You are recommended, for best quality, not to keep live animals in these aquaria for more than five days before sale and consumption.
Detailed technical guidelines and essential requirements for the harvesting, processing and handling of prawns, which apply both to those that are caught from open waters or obtained through farming, is contained in the relevant international code of practice, which is continuously updated (FAO/WHO 2001).
THIS TOPIC WAS NOT DEALT with in the original FAO manual on freshwater prawn farming but it is hoped that its inclusion in this new manual will be useful. The information presented has been derived from Phillips and Lacroix (2000).
For marketing purposes harvested prawns are sometimes divided into a number of groups:
Good quality harvested prawns have a greenish or bluish tint with bright blue or orange chelipeds (claws).
FIGURE 96
Freshwater prawns need sorting while being processed (Brazil)
SOURCE: MICHAEL NEW
FIGURE 97
Package your prawns attractively (Mauritius)
SOURCE: YANN VON ARNIM
FIGURE 98
Freshly harvested (blue) Macrobrachium rosenbergii can be cooked (pink) at the pond-side to provide a tasty barbeque (Brazil)
SOURCE: MICHAEL NEW
