The selection of sites for indoor nurseries should follow the same pattern as for hatcheries. Site selection for outdoor nursery facilities should be similar to that for grow-out ponds.
After rearing your freshwater prawns in your hatchery, you need to be able to hold them until you stock them in your ponds or sell them to other people. 50 m3 concrete tanks are convenient for holding postlarvae (PL) prior to transport for stocking in ponds. However, you can also use other sizes and types of tanks, similar to hatchery tanks. There are no special requirements, except that they must have supplies for freshwater and air. You can use branches and nets suspended from floats in the tanks (both referred to as ‘substrates’ in this manual) to increase the surface area available to the PL but this may make the normal operations of feeding, cleaning etc. (similar to hatchery operation) more difficult. Figure 37 shows PL utilising a nylon screen inside a holding tank.
Tanks for indoor freshwater prawn nurseries (Figure 38) can be constructed from concrete or fibreglass. The use of asbestos cement tanks is not recommended. The shape of nursery tanks is not important and their size, usually from 10 to 50 m2 with a water depth of 1 m, depends on the area of the outdoor ponds which you (or those you sell juveniles to) are eventually going to stock with your product. You can use artificial substrates of various designs and materials to increase surface area; these provide shelter and increase survival rates.
Prawns tend to use the edges of substrates, whether they be natural (e.g. leaves, branches) or artificial. Layers of mesh can therefore be used to increase the amount of surface edges available to the prawns in both vertical and horizontal planes (Figure 39). Plastic netting can be placed in several layers over wood, aluminium, or PVC pipe frames. Suspend these 10 cm above the bottom of the tank, so that it can be cleaned. Hanging the mesh vertically allows the prawns easy access to the tank bottom to search for feed and allows detritus to fall to the tank bottom, where it can be siphoned out. Other substrate designs are feasible but you must be careful to think about the effect of the substrates that you use on your ability to manage the tanks (feeding, observation, cleaning, etc.).
FIGURE 37
Postlarval freshwater prawns can use a nylon screen as an additional surface area in holding tanks (Brazil)
SOURCE: EUDES CORREIA
The water supplies for indoor nurseries can be flow-through or recirculating. For flow-through, allow the water to continuously enter from above the tank water level and exit from the lowest part of the tank through a vertical standpipe with an outside sleeve (pipe with a larger diameter) extending higher than the water surface. Cover the standpipes with a 1.0 mm mesh screen to prevent PL and juveniles from escaping. This drainage system draws water from the tank bottom where food waste and detritus settle.
If you wish to operate your primary nursery tanks on a recirculation system it can be similar to those used in recirculation hatcheries.
Nursery ponds are similar to grow-out ponds in design and facility requirements; these are described later in this manual. They usually vary in area from 300 to 2 000 m2. Some operators cover their nursery ponds with plastic netting to avoid predators, especially dragonflies, whose nymphs predate on freshwater prawn PL. Do not fill the ponds earlier than 2 days before PL are stocked; this will prevent such predators becoming established before the prawns arrive. You can use artificial substrates to increase the surface are available to the prawns, as described in the manual section on grow-out in temperate areas. Make sure that ponds are treated between cycles, as described in the section on grow-out. Figure 40 shows an internal turn-down drain in use during pond draining and sediment removal.
FIGURE 38
Overhead air and water distribution systems are used to supply these indoor nursery tanks (USA)
SOURCE: CHARLES WEIBEL
Postlarvae can also be nursed in cages but research on the best ways to manage cages for this purpose is not yet complete enough to recommend this in this manual. The scientific literature describes (for example) the use of 1 m2 cylindrical cages constructed from 0.64 cm mesh galvanized hardware placed in the mud bottom of a pond (although these were described as cages, they are really pens). In this case, animals weighing almost 2 g were stocked. PL or juveniles would require a much smaller mesh size. 2 x 1 x 1 m cages made of iron with a 1.0 mm nylon mesh, suspended above the pond bottom (real cages) have also been described for stocking PL. Such fine mesh would need careful cleaning to ensure proper water exchange and the mesh size would need to be increased as the animals grow.
FIGURE 39
Substrates can be used in nursery tanks to increase the surface area available to juvenile prawns; this substrate consists of the material used to make barriers around roadworks (USA)
SOURCE: Charles Weibel
You should not retain PL in your holding tanks for more than a week or two prior to stocking in nursery facilities, grow-out ponds, or sale. The length of time you hold them depends on the demand for PL. If you have to retain the PL longer, you must reduce the density of animals. You can then sell the prawns as juveniles, which have a higher value than PL, reflecting the increased costs of holding them longer. Whilst the PL are in the holding tanks you must continue to exchange the rearing water (40-50% every 2-3 days) and provide aeration. You can maintain PL at densities of up to 5 000 PL/m2 for one week (Note: once animals become PL, it is normal to refer to density on an area rather than a volume basis, that is per m2, not per m3), or up to 1 500-2 000 PL/m2 for one month under these conditions. If you need to hold them for one month, you could improve survival if you reduce the density to 1 000/m2. Using substrates can help you maximize the stocking density, thus reducing other equipment and labour costs.
You do not need to continue feeding BSN after metamorphosis. You can immediately use the same diets as are used for pond feeding. However, some hatcheries prefer to use a floating diet in the holding tanks. This makes it easier to visually judge the quantity to feed. The young PL, although they tend more and more to cling to and crawl on surfaces, still swim quite actively in the water and utilize a floating diet well. A floating catfish diet or even an expanded pet food is adequate. Some hatcheries continue to use egg custard based diets for a few days.
Many grow-out farmers prefer to stock production ponds with juveniles instead of PL. If they do not want to have nursery ponds themselves, you will need to provide this facility on your own site. The management of nursery facilities is dealt with later in this section of the manual.
FIGURE 40
The standpipe drain in this pond, normally vertical, is turned down to allow water to flow out (USA)
SOURCE: CHARLES WEIBEL
Cooled and aerated fish transport tanks are ideal for transporting freshwater prawn postlarvae (PL) from the hatchery holding tanks to the pond site but they are rarely available or affordable. For journey times of up to one or two hours to the pond site, you can use aerated garbage cans. A 100 L trash can, holding 50 L of water, will hold 50 000 PL. You should insert baffles in the container to prevent excessive water movement during transport. Larger, open plastic tanks (1 m3), containing about 500 L of aerated water, can hold about 500 000 PL during a short journey.
For longer distances you can use the method employed for transporting aquarium fish. Place them in double plastic bags containing 1/3 water and 2/3 air or oxygen (Figure 41). You can put about 250-400 PL in each litre of water. A 45 x 80 cm bag holding 8 L of water, for example, will take 2 000-3 000 PL. Higher or lower transport densities are used by some hatcheries. It is suggested that, if you have not done this before, you carry out some simple experiments to determine the optimum density for your conditions (length and duration of journey, climatic conditions, etc.). Round off the corners of the bags with rubber bands to prevent animals getting trapped there. Twist the top of the bag and bend it over, sealing it tightly with a rubber band after you have inflated it with air or oxygen.
These inflated bags can be used to transport PL very long distances (up to at least 16 hours travelling time by road). If you put them into insulated ‘styrofoam’ boxes you can ship PL by air most effectively. If they are in non-insulated boxes you can send them on night (cool) journeys by rail, for example. For long day-time (hot) journeys, you can stack the plastic bags on shelves in a home-made transport box mounted on a truck. The transport truck (Figure 41) should be insulated. Place garbage cans filled with ice on the floor, to keep the temperature down.
Lowering temperature during transport reduces metabolic activity and improves survival. You should also use water from the holding tank to fill the plastic transport bags. If you place the PL into ‘new’ water for transport, many will moult during the journey and many with be lost through cannibalism. Some hatcheries add a very small amount of seawater to the transport bags, claiming that survival rates are better in brackishwater than in freshwater. You can try this for yourself but remember that increasing salinity will lower the dissolved oxygen content of the water and, if extreme, may make it necessary to adjust the salinity again before stocking the animals in the rearing water. A transport temperature of about 20-25°C is recommended for journeys of less than 6 hours. Keep it down to 20-22°C for longer trips. A one ton truck can transport up to 500 000 PL in plastic bags. You can keep the temperature steady throughout the truck by using electric fans driven from the truck engine or batteries. You can get very good transport survival in this way.
To facilitate stocking at the pond site it is normal to standardize the number of prawns in each transport bag. It is therefore necessary for you to estimate the quantity of PL as accurately as possible when the bags are filled (Annex 6). It is not necessary to be exactly accurate. The value of accuracy must be set against the losses of PL which would be caused by excessive handling. However, you must make reasonably accurate estimates because it is upon this figure that pond stocking and feeding rates will be based and charges for PL levied.
The survival rate of 7 day old (after metamorphosis) PL during shipping is much higher than 1 day old animals. It is not good practice to ship PL of widely different age groups. However, because of the method of larval culture, the age (post-metamorphosis) of the PL will inevitably vary by a day or two.
Many grow-out farmers stock M. rosenbergii postlarvae (PL) obtained from hatcheries directly into their ponds; the age of these PL varies because they will have been in holding tanks for different periods of time. Other farmers prefer to stock larger juveniles that have been reared from PL in their own nursery facilities or bought from commercial hatchery-nurseries. Since nurseries can be stocked at higher densities than grow-out enclosures, there are potential savings in space, labour, feed and cost. In addition, the early mortalities will have already occurred before grow-out facilities are stocked, and strong juveniles are therefore being selected. Poor survival rate in a particular batch is easier to notice in a crowded nursery pond than it is in a production pond. It is much cheaper to replace a poor batch of PL at this stage than not to realize the problem until grow-out harvest time. On the other hand, some prefer to avoid too many nursery stages, in order to minimize the losses that occur through frequent animal transfer and handling, and that occur through the greater risk of disease problems in high-density culture.
Postlarvae (here defined as young juveniles of 7-10 mm in length and 6-9 mg in weight) can be cultured at high densities from metamorphosis to juveniles in nursery systems. Nursery facilities include primary (indoor) nurseries, secondary (outdoor) nurseries in ponds, and cages. Indoor nurseries are used primarily in temperate regions, where outdoor culture is only possible during a limited season (say 6-8 months); this extends the total time available for prawn growth. Nursery management may be single-phase, two-phase, or multi-phase operations. One- and two-phase operations may combine outdoor and indoor systems. A full description of these systems is contained in Alston and Sampaio (2000).
General management
Tanks for holding postlarvae (PL) are a form of indoor nursery. However, their purpose is not really to grow the PL to a larger size before stocking but simply to be able to maintain them before sale. Sometimes hatcheries use holding tanks to acclimatize their PL to the pH and temperature of the rearing facilities where they are to be stocked. True indoor nurseries contain tanks where PL are intentionally reared to a larger size before transfer to outdoor nurseries or grow-out ponds.
Nursery tanks require aeration and may be operated as flow-through or recirculating systems, like hatcheries. Siphon the tank bottoms regularly to remove food wastes, faeces, and decomposing organic matter. Some nurseries allow organic matter to accumulate to enable PL to graze on ‘lab-lab’ but this may be difficult to manage without getting into water quality problems. Between cycles, you should dry out the tanks, disinfect them (the same way as hatchery tanks), and leave them to dry out for at least 48 h to minimize problems with pathogens. Do not forget to flush them out well to remove all traces of chlorine.
Keeping the water quality good
General water quality requirements for indoor nurseries are similar to those for freshwater in hatcheries. Maintain the optimum temperature (27-31°C) by heating the water in the system or the building in which they are housed, if necessary. If you are operating a recirculation nursery system, a turnover rate through the biological filter of 12 times a day is suggested.
Stocking rates
The best stocking density for indoor nursery tanks depends on the length of time the animals will remain in the tanks before transfer to an outdoor nursery or grow-out facility. You are recommended not to exceed a stocking density of 1 000 PL/m3 in tanks without substrates. You can stock 2 000 PL/m3 in tanks where substrates are provided (Figure 42). These stocking densities assume that the indoor nursery rearing time is not more than 20 days. You would need to reduce the density if you are going to keep the prawns in your indoor nursery for longer periods (e.g. in sub-tropical and temperate regions, where prawns are usually maintained in indoor nurseries until the grow-out ponds reach temperatures of at least 20°C). Maintaining prawns in commercial indoor facilities for periods longer than one month may prove too expensive, although they are often maintained longer than that for research purposes.
Feeding strategy
Feeding once or twice per day is sufficient. You should adjust the quantity of feed based on observing the actual consumption. It should normally be about 10-20% of the total weight of the prawns in the tank. Grow-out feeds can be used but enhanced results may be obtained by supplementing them with other materials, such as beef liver, egg custard based diets (EC), or minced fresh fish. However, you must take great care if you use fresh feeds. Fresh feeds, which usually break down more easily than pelleted diets, may rapidly cause water quality problems. This could overload recirculation systems or mean that you would need to have a much greater water exchange in flow-through systems (this is not such a problem as in hatchery systems, because nursery water is not brackish; however, it would increase pumping and other costs). Adult Artemia (Artemia biomass) have also been used as a nursery feed for Macrobrachium rosenbergii in countries where it is readily available as a fresh (live) product from salt farms. Recently (2000), a freeze-dried version of this product has also become commercially available.
Growth and survival rates
When prawns are stocked as postlarvae, their weight will be about 0.01 g. After 20 days in nursery tanks, the juveniles should be about 0.02 g, and they should reach about 0.3-0.4 g after a total of 60 days at reduced densities. Cannibalism, competition and poor water quality are the main causes of mortality in indoor nurseries. However, survival rates of about 90% can be obtained up to 20 days.
Harvesting juveniles
You can use dip nets (3 mm mesh) for catching juveniles from indoor nurseries. Estimates of the numbers of juveniles present must be made, using the technique given in Annex 6. Estimates of average weight should also be recorded. It is only by keeping records like this that you can compare the success (or failure!) of different batches and your management procedures. The method of transporting animals to ponds has been discussed earlier in this manual.
FIGURE 42
The substrate material shown dry in Figure 39 can be seen beneath the water surface in this nursery tank (USA)
SOURCE: CHARLES WEIBEL
Outdoor nurseries are similar to grow-out ponds and can be stocked with newly metamorphosed postlarvae (PL) from hatcheries, or with juveniles from primary nurseries. In the secondary nurseries you can rear them until they reach 0.8-2.0 g. This may take 4-10 weeks, depending on the source used for stocking.
General management and water quality
Supplemental aeration is ideal but may be too expensive. If you use substrates you can improve performance. This is discussed in the section of this manual that deals with grow-out in temperate areas. Between rearing cycles, you should disinfect your nursery ponds by applying 1 mt/ha of burnt lime or 1.5 mt/ha of hydrated lime to kill unwanted pathogens. Water quality and its management in secondary nursery systems is similar to that employed for grow-out.
Filling and stocking ponds
Postlarvae are especially sensitive to the effects of algal blooms (excessive quantities of algae; the methods for controlling these are described in the grow-out sections of this manual) and high pH. Some operators allow natural food to build up, and pH to stabilize, over a period of 10-14 days after pond filling, before stocking. However, this causes predators and competitors to become established, with consequential effects on prawn survival. It is not recommended in this manual. Stock your ponds immediately (within 2 days) after filling them with filtered water, which has no predators and causes no photosynthetically-induced pH changes. You may not get quite such high initial prawn growth rates from ponds with little natural food but increased survival will outweigh this factor.
It is difficult to recommend exact initial stocking rate in outdoor nurseries because this is site-specific (e.g. temperature profile over time; prawn size at stocking; the length of time the prawns will remain in the nursery ponds; the presence or absence of substrates and aeration; the amount of predation and whether the ponds are covered to reduce this, etc.). If your nursery ponds have no substrates or aeration, do not exceed stocking rates of either 1 000/m2 of PL, 200/m2 of small juveniles (0.02 g), or 75/m2 of 0.3-0.4 g juveniles. You can increase these stocking densities if you provide substrates, aeration and predator protection.
Feeding strategy
Normally, outdoor nurseries use grow-out feeds, which may be either bought from commercial sources or made on the farm. Feeding once or twice per day is sufficient. You can also add some supplementary fresh feeds but you must be careful about water quality problems, as mentioned earlier in this manual. The quantity of feed should be adjusted after observing the actual consumption but should be about 10-20% of the total weight of the prawns in the pond.
Survival and growth rate
Some mortalities (10-20%) will occur soon after PL are stocked, even when the conditions are ideal. To determine the rate of survival, a sub-sample of the animals should be evaluated within a mesh bag (cage) suspended above the pond bottom. If the survival is poor after 24 to 48 h, stock more PL, unless poor water quality was the cause of the mortalities. Over-stocking is much easier to remedy later than under-stocking. Total survival from stocking (or re-stocking) until removal from the nursery ponds should be at least 75%. The weight of the prawns at the end of the outdoor nursery period should be about 0.8-2.0 g, but the time taken to reach those sizes will depend on local conditions.
Harvesting, grading, and transport
You can harvest juvenile prawns by seining your ponds two or three times with a 5 to 6 mm mesh seine, or by emptying them completely. If you use drainage, the juveniles should be trapped in a large catch basin or box at the end of the outlet. The catch structure should not stress the prawns. Polypropylene boxes or tanks filled with water from the nursery pond and kept aerated, can be used to transport the juveniles to the grow-out ponds if they are close by. More care needs to be taken if you are taking them to sites further away (see the section on transport of PL). You should estimate the numbers of juveniles harvested (Annex 6) and transported to the grow-out ponds. There are some advantages in grading the juveniles into two or three groups, depending on their average weight, before stocking them into separate grow-out facilities. This decreases competition in grow-out ponds by reducing HIG (Annex 8) and increases productivity. Grading is discussed in the grow-out section of this manual.
Multi-phase nursery systems
A number of multi-phase nursery systems have been developed for research and commercial systems. The simplest system, developed in Israel, involved stocking ponds with newly metamorphosed PL at 1 000 to 10 000/m3 in the first phase. 15 to 30 days later they were transferred into second phase ponds at 100 to 200/m2 for a further 60 day period. Survival rates of 92% (phase 1) and 85% (phase 2) were achieved. Other multiphase systems have been modelled or commercially applied but are not described here because they are complex and/or their true value has not been adequately demonstrated. Further details can be found in Alston and Sampaio (2000).
Nursing in cages
Some research work on nursing postlarvae in cages has been carried out. These involved the rearing of newly metamorphosed PL (stocked at initial densities of 2-10 PL/L) in 2 x 1 x 1 m and 1 x 0.5 x 0.7 m cages for 20 days. The prawns grew to 50 mg at the lowest and 30 mg at the highest stocking densities. Survival rates were not significantly different up to a stocking density of 8 PL/L. Another experiment involved the stocking of 0.19 g juveniles at 50 and 100 prawns/m2; the prawns grew to 3.2 and 2.4 g in 2 months, with survival rates of 86% and 75%, respectively. A further experiment stocked with 0.05g juveniles at 100-800/m2 and reared them for 60 days in similar cages; the prawns reached 0.35-0.79g.
Research on this topic is sparse and the results need to be confirmed on commercial farms. At the time this manual is being prepared, the operation of cage nurseries is therefore not yet recommended for commercial practice. This does not mean that the practice has no potential value, simply that no clear recommendations can be made at this time.
