5.4 Managing nurseries

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 hatcherynurseries.

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, twophase, 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).

INDOOR (PRIMARY) NURSERIES

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.

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

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.

OUTDOOR (SECONDARY) NURSERIES

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 growout 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 photosyntheticallyinduced 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.

OTHER SYSTEMS

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.