Fresh Water Prawn

Farming systems

Basically there are three different farming techniques used for Macrobrachium rosenbergii :

A) Extensive Freshwater Prawn Culture

Extensive culture means rearing in ponds (but also in other impoundments such as reservoirs, irrigation ponds and rice fields), which produce less than 500 kg/ha/yr of freshwater prawns. They are stocked, often from wild sources, with PL or juveniles at 1-4/m2 . There is no control of water quality; the growth or mortality of the prawns is not normally monitored; supplemental feeding is not normally supplied; and organic fertilisation is rarely applied.

B) Semi-Intensive Freshwater Prawn Culture

Semi-intensive systems involve stocking PL or juvenile freshwater prawns (usually from hatcheries) at 4-20/m2 in ponds, and result in a range of productivity of more than 500 kg/ha/yr and less than that defined as intensive in this box. Fertilisation is used and a balanced feed ration is supplied. Predators and competitors are controlled and water quality, prawn health and growth rate are monitored. This form of culture is the most common in tropical areas.

C) Intensive Freshwater Prawn Culture

Intensive culture refers to freshwater prawn farming in small earth or concrete ponds (up to 0.2 ha) provided with high water exchange and continuous aeration, stocked at more than 20/m2 and achieving an output of more than 5000 kg/ha/yr. Construction and maintenance costs are high and a high degree of management is required, which includes the use of a nutritionally complete feed, the elimination of predators and competitors, and strict control over all aspects of water quality. This form of culture is not recommended in this manual because it requires more research, particularly on size management.

Based of the management practice employed there are four different management systems being adopted for freshwater prawn farming in India.

System 1: The Continuous System

This involves regular stocking of PL and the culling (selective harvesting) of market sized prawns. There is no definable ‘cycle' of operation and the ponds are therefore only drained occasionally. One of the problems of this form of culture, which can only be practiced where there is year-round water availability and its temperature remains at the optimum level, is that predators and competitors tend to become established. Also, unless the culling process is extremely efficient, large dominant prawns remain and have a negative impact on the postlarvae, which are introduced at subsequent stocking occasions. This results in a lower average growth rate. The decline in total pond productivity (yield) that has been observed when this system has been used for a long time is, however, not confined to this management system and may also be a function of genetic degradation, as discussed elsewhere in this manual. This results in less and less satisfactory animals being stocked.

System 2: The Batch System

At the other extreme to the continuous system is the batch system, which consists of stocking each pond once, allowing the animals to grow until prawns achieve the average market size, and then totally draining and harvesting it. This reduces predator and competitor problems. However, the problem known as Heterogeneous Individual Growth (HIG) remains. This term (HIG) refers to the fact that freshwater prawns do not all grow at the same rate. Some grow much faster, tend to become dominant, and cause stunted growth in other prawns.

System 3: The Combined System

This provides the advantages of reduced predator and competitor problems of the batch system with the cull harvesting employed in the continuous system, to reduce the problems of HIG. In the combined system, ponds are stocked only once. Cull harvesting starts when the first prawns reach market-size (the exact size depends on the local, live sales, or export market requirements). This removes the fast-growing prawns for sale, leaving the smaller ones to grow, with less HIG impact. Eventually, after several cull-harvests, the ponds are drained and all remaining prawns harvested. The total cycle usually lasts about 9-12 months in tropical regions, depending on local conditions. This system is recommended in this manual.

System 4: The Modified Batch System

This is a more complex management regime involving three phases. After 60-90 days in a 1000 m2 nursery pond stocked at 200 to 400 PL/m2, 0.3-0.5 g juveniles were harvested and stocked at 20 to 30/m2 empty ‘juvenile' ponds. After another 2-3 months, seine harvesting of these juvenile ponds begin and this is repeated every month. These harvests removed animals of 9 to 15 g, which were then stocked into ponds with existing populations of small prawns. The juvenile ponds were themselves then either converted to adult ponds, to allow remaining animals to grow to marketable size, or were drained and refilled for further use. Further advantages is obtained if post larvae are held longer in the nursery ponds and then graded into at least two size groups before stocking into juvenile ponds.

Ponds need to be well maintained during the farming period. Special care should be taken for prevention and treatment of pond bank erosion and the maintenance of water inlet and outlet structures, particularly the filters. Pond surface area can be increased by placing rows of netting, suspended from floaters and weighed down with sinkers, across the pond. 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. 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). These types of management make prawn production feasible in smaller, deeper ponds, which were previously considered unsuitable.

Polyculture Culture

Farming Macrobrachium species in combination with single or multiple species of fish, including Tilapias, Common carp, Chinese carps, Indian carps, Ornamental fish etc are common. 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.

Prawn-fish polyculture systems are therefore normally batch-harvested. 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.

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. The introduction of freshwater prawns reduces the area devoted to rice paddy (because deeper areas where prawns can shelter when the rice field is dry have to be provided). It also reduces weeding costs (prawns eat weeds) and fertilization costs.