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).

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.

Figure 69
When the temperature in the bag is the same as in your pond, the postlarval Macrobrachium rosenbergii can be released (Brazil)
Figure 70
Grass is invading the shallow areas of this pond (Brazil)

 

SOURCE: PATRÍCIA MORAES-RIODADES SOURCE: PATRÍCIA MORAES-RIODADES

BOX 19

Keeping rooted plants out of your ponds

DO NOT construct ponds with extensive shallow areas.

NEVER ALLOW a shallow amount of water to remain in a pond when it is not in use. Drain it properly. ‘Weeds’ grow much better in shallow water and predators such as crabs thrive.

MAINTAIN an adequate phytoplankton bloom in the pond by feeding and/or fertilisation. This will reduce the light intensity at the bottom of the pond.

CUT any vegetation at emergence level. Pulling up the roots usually causes dangerous levels of turbidity in the pond. This job is very time- and labour-consuming and should not be necessary if the pond has been well constructed and managed.

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.
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

Figure 71
Close-up of material used as pond substrate for Macrobrachium rosenbergii culture (USA)
Figure 72
Substrates have been placed vertically in this temperate zone rearing pond for Macrobrachium rosenbergii culture (USA)

SOURCE: CHARLES WEIBEL SOURCE: CHARLES WEIBEL

 

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

SOURCE: CHARLES WEIBEL

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.
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

BOX 20 Size grading

PLACE A FLOATING grader box (these are commercially available for finfish grading) into a holding tank.
Trial and error is necessary to select the size of the grader bars to use. Your choice will depend on the size of the animals you want to grade. The efficiency of the procedure is a function of the average size of the population to be graded, how variable the size range of that population is, and the average weight and proportion of the total that you wish to achieve in the two graded portions.
For example, prawns with an average weight of about 0.6 g can be separated into two portions with size #13 bar graders (13/64 inch; 5.16 mm) and #14 bar graders (14/64 inch; 5.55 mm).
Net the juveniles from the nursery tanks and pour them through the grader. Smaller animals will pass through the parallel bars of the grader and the larger ones will be retained above the bars.
The grading process can be speeded up by causing water movement (water flow, moving the box, airstones) but it is important not to overload the box because this will cause the juveniles to stack up and they will not actively try to swim out of the grader. Over-crowding may also cause mortalities to occur.
It is recommended that the juveniles be graded into equal (50:50) numbers of upper and lower sized individuals. These should be reared in separate ponds to achieve the best average yield of marketable prawns from the total area of the two ponds.

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).