The prawn production in India accounts for about 15% of the total world production of prawn and shrimps. For marine prawns, the percentage of Indian production to the world production is about 20%. The major commercial prawn species reared in India are Macrobrachium. rosenbergii and M. malcolmsonii.
Macrobrachiumrosenbergii, also known as the giant river prawn or the giantfreshwater prawn, is native to the Indo-Pacific and northern Australian Regions. The adult is found in freshwater, while its larval stages live in brackish water after the juvenile stage. During mating, the male attaches its spermatophore on the ventral side of the abdomen of female’s body and the eggs coming out of female genital opening are fertilised by the sperms derived from spermatophores.
The fertilised eggs are held in the brood chamber or egg basket, which is made by the interlocking appendixinterna of the pleopods and are aerated by vigorous movements of the swimmerets for 2-3 weeks. This is in contrast to shrimps, whose fertilized eggs are released into the sea. Females can lay 80,000-100,000 eggs during one spawning and eggs take an average of 20 days at 28°C to hatch into larvae.
After hatching, larvae are dispersed by the rapid movements of the abdominal appendages of the female. Larvae are planktonic and swim upside down actively with tail first posture and feed on small planktons. Larvae complete development in 15-20 days and metamorphose into post larvae, which resemble miniature adults and generally feed near bottom and then begin to migrate upstream into freshwater rivers within one or two weeks after metamorphosis and are soon able to swim against the rapidly flowing currents (contranatant behaviour) and can also crawl over the stones in shallow waters.
HATCHERIES AND NURSERIES
Freshwater prawn hatcheries need supplies for both freshwater and sea water; the latter can be drawn from areas where the salinity is 30 to 35 ppt. The brackish water derived from the mixture of seawater, brine or artificial sea salts mixed with freshwater should have salinity of 12-16 ppt, pH of 7.0 to 8.5 and dissolved oxygen level of 5 ppm.
The prawn farm site should also have the following facilities:
OBTAINING BERRIED FEMALE PRAWNS
Berried females are those that carry fertilised eggs in their egg basket. They can be obtained from rivers, canals, lakes and estuaries, where they are most abundant in the beginning of rainy season. In the tropics, berried females can be obtained all the year round from farm ponds containing adult animals. Selecting fast-growing, berried females from ponds has a positive effect on the weight of prawns at harvest.
In the tropics, where berried females are readily available, special brood holding facilities are not required but in temperate areas, indoor brood stocking facilities are essential. Brood stock is disinfected by placing into freshwater containing 0.2-0.5 ppm of copper sulphate or 15-20 ppm of formalin for about 30 minutes. Prawns should be fed daily at the rate of 1-3% of total biomass.
Berried females can be collected from the holding system and placed in tanks where the eggs will hatch into first instar larvae, which are collected by netting. The hatching tanks should be covered to prevent bright sunlight to reach larvae for which the inner side of the tanks should also be painted with black epoxy-resin paint.
LARVAL REARING TANKS
Different designs of containers can be used to grow freshwater prawn larvae, which may be circular flat-bottom tanks, circular conical-bottomed plastic tanks, plastic-lined wooden tanks, rectangular concrete tanks, concrete-faced brick tanks and earthen water jars. Good drainage system is essential as water has to be removed from tanks at harvesting time. Mixing tanks are also required for preparing the brackish water to be used in the hatchery as well as storage tanks. Aeration of water is also essential which can be done through PVC pipes, with holes cut at one foot intervals.
Larvae should not be exposed to direct sunlight, for which 90% of the tan area should be covered and shady. Some natural light is essential for good larval survival, which can be provided through transparent roofs over the hatcheries. Physical filters that include sand filters, drum screen filters, and medium filters should be easy to clean and designed to minimize water loss.
Water needs to be chemically treated before it can be used in rearing tanks and also should be physically filtered by passing through the sand bed before transferring it to another tank for treatment. Mix the seawater or brine with freshwater to form 12 ppt of brackish water. The optimum temperature range for M. rosenbergii is 28-31°C. Below 24-26°C the larvae will not grow well and the time taken for them to reach metamorphosis will be longer.
A wide range of feeding material is used by different hatcheries, which includes nauplius larvae of shrimps, freshwater cladocerans, fish eggs, squid flesh, frozen adult Artemia, rotifers, fish flesh, egg custard, worms and commercial feeds available in the market. The quantity of food to be given depends on the utilization of feed by larvae that vary from place to place. The quantity of feed consumed will increase as the larvae grow.
HARVESTING POST LARVAE
When post larvae are about 7-8 mm long, they can withstand transfer from 12 ppt water into freshwater. However, they should not be harvested from the larval tanks and transferred directly into holding tanks containing freshwater but should be acclimatized to fresh water in the larval tanks itself. When majority of larvae have metamorphosed, water level in tanks should be reduced to about 35 cm. Then gradually the tank should be flushed with freshwater over a period of 12 hours. The post larvae can then be collected and transferred or the larval tanks can be refilled to 70 cm with fresh water and the animals temporarily held in them. The best way to harvest post larvae from the larval tanks is to reduce the water level and then remove them by nets.
HOLDING POST LARVAE BEFORE SALE
Post larvae cannot be held in holding tanks for more than a week or two prior to stocking in nurseries. When the post larvae are in the holding tanks, the rearing water should be changed every 2-3 days) to provide aeration. Post larvae can be stocked at densities of about 5,000/m2 for one week, although survival increases by reducing the density.
Pond size should be such that can be managed easily. Generally most farms have ponds of around 0.2-0.6 ha size. Large ponds are normally wider than 30 m and often drained for harvesting. The average depth of water in freshwater prawn ponds in tropical areas should be about one meter; with a minimum of 0.75 m and a maximum of 1.2 m. Deeper ponds are used in colder areas to maintain more stable water temperatures. The banks of the ponds or embankments or bunds must be high enough for the highest water level expected in the pond, which generally should be 1-2 feet higher than water level. The flow of water into each pond must be controlled by valves, stop-logs or plugs. Paddle wheels are the most efficient method of increasing dissolved oxygen levels in the pond water.
Stocking the ponds quickly reduces competitors and predators, which have less time to become established. Often post larvae that are a week or two old after metamorphosis are used to stock ponds, where they remain until harvesting. A stocking density of about 40,000/ha is recommended for the monoculture of Macrobrachiumrosenbergii. Using larger juveniles for stocking increases the survival rate as well as the average weight of the animals by as much as about 30%.
Natural productivity of the ponds generally gives small production from the ponds. Therefore, intensive farming must involve supplementary feeding to increase productivity. Some farms claim to rely on fertilizers, rather than feeding at the beginning of the rearing period, which stimulates algal bloom and lot of micro flora and fauna in the ponds. Others find that providing feed from the beginning of the rearing period improves performance and is cost-effective. Commercial feeds are the most productive and reliable to use but they are expensive and unaffordable to small farmers.
HARVESTING MARKETABLE PRAWNS
Basically there are two methods of harvesting: culling and draining. The time of harvesting depends partly on the growth rate and partly on the size of animals for market requirements. Culling is used to harvest market-sized animals from the ponds to remove faster growing prawns which increase density quickly. In tropics culling usually starts 5-7 months after post larvae have been stocked to take out the market-sized animals for selling and keeping the smaller ones and soft-shelled animals in the pond for further growth. After about 8-11 months ponds are drained and all animals are sold. In cull harvesting, a seine net is pulled through the pond to remove market-sized animals, while in drain harvesting, a harvesting sump is installed in front of the gate or outside the pond, in which prawns will accumulate while water is being drained.
Several diseases affect freshwater prawn larvae as well as adults. Some hatcheries use formalin at the rate of 200 ppm daily as an effective remedy for protozoan and hydrozoan parasites and fungal diseases. Formalin can also be used at a lower level of about 30 ppm for longer periods, followed by water change after 24 hours. Larvae can also be transferred to disinfecting tanks every 5-10 days to get rid of diseases and parasites. Daily dip of larvae in Malachite green (0.2 ppm) for 30 min has also been used for treatment. Also, dipping in copper sulphate 0.4 ppm solution for 6 hours is recommended. Antibiotics and sulfa drugs are sometimes used to control filamentous bacteria and some hatcheries use lime (CaO) as a prophylactic measure.