POLYCULTURE AND INTEGRATED CULTURE

A considerable, but unquantified proportion of global freshwater prawn production comes from polyculture and integrated culture. No detailed recommendations for the polyculture of Macrobrachium rosenbergii with other species, or its integration into other farming activities, have been provided in this manual. This is because there is no single recommendable process. Many different management techniques are possible. It is hoped, however, that you will be stimulated by the examples given below to try polyculture with locally available species, as well as integration with other farming activities in your specific location. Further reading on this topic is available in Zimmermann and New (2000) and New (2000b).

Polyculture

Records exist of the polyculture of various Macrobrachium species in combination with single or multiple species of fish, including tilapias, common carp, Chinese carps, Indian carps, golden shiners, mullets, pacu, ornamental fish, and red swamp crayfish. Other combinations may be feasible.

 

The inclusion of freshwater prawns in a polyculture system almost always has synergistic beneficial effects, which include:

• more stable dissolved oxygen levels;
• the reduction of predators;
• coprophagy (the consumption of fish faeces by prawns), which increases the efficiency of feed;
• greater total pond productivity (all species); and
  the potential to increase the total value of the crop by the inclusion of a high-value species.

However, the management of a polyculture system is more complex. This particularly applies to the harvesting of prawns. Some large fish can be cull-harvested from a polyculture pond but this interferes with the culture of the prawns. Prawn-fish polyculture systems are therefore normally batch-harvested. It is difficult to synchronize fish production with prawn production to achieve the maximum production of marketable animals. For this reason, most polyculture systems involving freshwater prawns concentrate management on the production of the fish and regard the harvested prawns as a high-value bonus.
The addition of prawns to a fish polyculture system does not normally reduce the quantity of fish produced. On the other hand, the addition of fish to a prawn monoculture system markedly increases total pond yield but may reduce the amount of prawns below that achievable through monoculture. Some problems have been reported. For example, tilapia which were inadvertently introduced into prawn ponds in Hawaii were described as a pest, causing serious competition for food. Escaped tilapia, which had been grown in cages in freshwater prawn ponds in Puerto Rico took years to eradicate. However, this problem could be avoided by the use of artificially incubated sex-reversed caged tilapia.
The monoculture versus polyculture decision is site-specific and depends on economic factors, namely balancing the relative market values of the various species with the costs of a more complex management system.
Fish are faster than prawns in accessing any supplemental feed which is presented, so the feeding for polyculture systems is normally directed at the fish, not at the prawns.

TABLE 17
Average stocking densities and yield of carps, tilapias and freshwater prawns reared in polyculture, based on a literature study

SOURCE: DERIVED FROM ZIMMERMANN AND NEW (2000)

The prawns consume feed which falls to the bottom of the pond, as well as fish faeces and nutrients derived from detritus. Though commercial fish feeds are sometimes applied, tropical polyculture systems often use simple mixtures of rice bran with plant oilcakes, such as mustard and groundnut. Since there are so many potential combinations of fish and freshwater prawns, it is impossible to give firm guidelines on management in this manual. In the cases summarized in Table 17, the culture cycles ranged from 3 to 6 months and the water temperatures were 26 ± 4°C. This table also gives an indication of the productivity obtainable. The results of other published studies on prawn and fish polyculture have been reviewed by Zimmermann and New (2000). Much of the output of M. rosenbergii produced in China comes from polyculture systems. Examples are given in Box 21.

Integrated culture

The wastewater from ponds containing prawns being reared in monoculture or polyculture with fish can be used for the irrigation of crops. Prawns can also be reared in paddy fields, without depressing rice production. This has proved especially valuable in Viet Nam, where it has been shown that the income from prawns in integrated rice-prawn culture can be two or three times as great as that from the cultivation of rice. The introduction of freshwater prawns reduces the area devoted to rice paddy (because deeper areas where prawns can shelter when the ricefield is dry have to be provided). It also reduces weeding costs (prawns eat weeds) and fertilisation costs. Figure 74 illustrates a Vietnamese rice-prawn farm where peripheral canals have been constructed for Macrobrachium culture and a bamboo structure has been erected on the canal dykes to support cucumbers.
Similar to polyculture, no single management strategy can be recommended for integrated culture because the potential combinations are almost infinite. However, examples from Viet Nam have been presented in Box 22.

Other forms of rearing prawns

The use of concrete ponds, cages (a floating structure, usually enclosed in nylon netting) and pens (an area of a larger water body, such as a reservoir or lake, which is separated off by the use of netting, bamboo or other structures) has not found favour in freshwater

BOX 21

Polyculture of freshwater prawns with carps in China

GENERAL MANAGEMENT CONDITIONS:
The pond size ranges from 0.2 to 0.7 ha, with a water depth of 1.2-1.5 m.
Dissolved oxygen is maintained at about 3 ppm. Ponds are treated between cycles by sun-drying for 3-5 days and the application of quicklime (CaO) at 900-1 125 kg/ha for pest eradication. Fermented organic manure (often chicken manure) is applied to the ponds at 750-1 500 kg/ha, 7-10 days before stocking.
Additional quantities of the same manure are applied one or two times each month. The amount is adjusted according to the fertility of the water and the climatic conditions. Shelters for the prawns, in the form of aquatic weeds, grasses and tree branches are placed in the ponds. Carnivorous and omnivorous fish are not grown with prawns. Bighead and silver carp are the usual species of choice.
The time of stocking depends on the location. It is done when water temperatures reach 20°C; this occurs in mid-April in southern China and mid-May in central China.
The rearing period is 4-6 months (one cycle per year). Partial seine harvests are taken but the ponds are totally drained before the water temperature drops below 18°C. An average prawn market size of 20 g is sought. Fish are removed with large mesh size nets before drain harvesting occurs. Feeds include soybean meal, groundnut cake, wheat bran, a 35% protein pelleted feed, trash fish, molluscs, silkworm pupae, earthworms, and animal entrails. Feeding rates vary from 15-20% of body weight when the prawns are <1 g, decreasing gradually to 5-6% when the prawns are >10 g. 70% of the daily feed ration is given in a late afternoon feeding and 30% in the morning. The food is spread evenly around the pond about 2 m from the bank.

STOCKING AND PRODUCTION RATES WHEN THE EMPHASIS IS ON FRESHWATER PRAWN (M. ROSENBERGII) PRODUCTION:
Freshwater prawns are stocked as 1.0-1.2 cm juveniles at 16.5-22.5/m2, or as 1.5-2.0 cm juveniles at 15-18/m2. Silver and bighead carps are stocked at 1 500-1 800/ha at a size of 12-15 cm. Production of prawns ranges from 1 500 to 3 000 kg per crop. Production of carp ranges from 750 to 1 500 kg per crop.
STOCKING AND PRODUCTION RATES WHEN THE EMPHASIS IS ON CARP PRODUCTION:
Freshwater prawns are stocked as PL at 24-30/m2, or as 1.0-1.2 juveniles at 4.5-9.0/m2, or as 1.5-2.0 cm juveniles at 3-6/m2. Silver and bighead carps are stocked at a size of 3-4 cm at 16.5-21/m2. Production of prawns ranges from 450 to 750 kg/ha/crop. Production of carp ranges from 5 000 to 7 500 kg/ha/crop of fingerlings (12-15 cm body length).

prawn culture, although there were some early attempts, especially in Thailand. However, nets are sometimes used in nursery systems, as noted earlier in this manual. Many attempts have been made to rear freshwater prawns under highly intensive grow-out conditions in tanks housed under environmentally controlled conditions in cool temperate zones, including the UK. Such ideas have been abandoned due to excessive costs, especially for heating. Indoor rearing in environmentally controlled conditions is now confined to broodstock and nursery systems designed to maximize production in the temperate zones of China and the USA, for example.