Seed supply
When required for hatchery use, female broodstock are usually obtained from grow-out ponds but also sometimes from capture fisheries. Normally, "berried" (egg-carrying) females are only used once. Commercial farms in tropical regions do not normally maintain captive broodstock for breeding purposes but adults are over-wintered indoors in temperate regions in order to stock ponds with PL as early as possible in the short grow-out season. The typical male to female ratio in broodstock holding systems is 1-2 BC males or 2-3 OC males per 20 females, at a total stocking density of 1 prawn per 40 litres. Within a few hours of copulation, fertilization occurs externally, as the eggs are transferred to the brood chamber beneath the abdomen. The eggs remain adhered to the female during embryonic development, which lasts about 3 weeks. At hatching, free-swimming zoeae are produced. Between 5 000 and 100 000 eggs are carried, depending on the size of the berried female. Eggs are orange until 2-3 days before hatching, when they become grey-black.
Some seed (PL; juveniles) is obtained from the capture fishery where
M. rosenbergii is indigenous, typically in the Indian sub-continent, but most is now hatchery-reared. First stage zoeae are just under 2 mm long and grow, through 11 larval stages, to almost 8 mm at metamorphosis into PL. Individual metamorphosis can be achieved in as little as 16 days but usually takes much longer, depending on environmental conditions. In commercial hatcheries, most larvae metamorphose by day 32-35 at the optimum temperature (28-31 °C). Larval rearing typically occurs in 12‰ brackishwater, and hatcheries are either flow-through (where a proportion of the rearing water is regularly replaced) or recirculating (where a variety of systems involving physical and biological filters are used to minimise water use). Either type of hatchery may be inland or coastal. Inland hatcheries produce brackishwater by mixing freshwater with seawater transported from the coast, brine trucked from salt pans, or artificial seawater. Some flow-through hatcheries use a "greenwater" system, which involves fertilization to encourage the growth of phytoplankton (mainly
Chlorella spp.), which is believed to improve water quality and increase larval survival; others operate a "clearwater" regime. Feeding systems vary widely but typically include brine shrimp (
Artemia salina) fed several times per day at first, reducing to a single daily feed by larval stage 10. Prepared feed (usually an egg custard containing mussel or fish flesh, squid, or other ingredients) is introduced at stage 3 and its feeding frequency is increased towards metamorphosis. Some hatcheries are integrated with nursery and grow-out facilities.
Nursery
Although some farmers stock grow-out ponds with young PL, many either purchase larger juveniles or rear PL in their own nursery ponds before transfer to grow-out ponds. In temperate areas with a limited grow-out season, environmentally controlled indoor nurseries are used to increase animal size before stocking outdoors as soon as temperatures become high enough. Indoor nurseries are stocked at 1 000-2 000 PL/m³, depending on whether substrates are used or not. Outdoor nurseries may be stocked with newly metamorphosed PL or with juveniles from an indoor nursery. Typically, stocking rates are 1 000/m² PL, 200/m² small juveniles (0.02 g) or 75/m² of 0.3-0.4 g juveniles, but increased densities are possible if substrates are used.
Ongrowing techniques
Freshwater prawns are reared in a variety of freshwater enclosures, including tanks, irrigation ditches, cages, pens, reservoirs, and natural waters; the commonest form being earthen ponds. Normal rearing methods comprise various combinations of the formerly used "continuous" (ponds operated indefinitely, with regular cull-harvesting and restocking) and "batch" (single stocking, single harvesting) systems; these are known as "combined systems". Most systems involve monoculture, but the polyculture of freshwater prawns with finfish and sometimes other crustaceans also occurs, particularly in China (with carps). Integration of freshwater prawn culture with crop production also occurs (typically in Viet Nam).
Pond stocking densities in tropical monoculture vary widely. In extensive rearing systems (typically producing <500 kg/ha/yr), PL or young juveniles are stocked at 1-4/m²; semi-intensive systems (producing 500-5 000 kg/ha/yr) are stocked at 4-20 PL or young juveniles/m². Rarely, some small intensive systems also exist, which stock >20/m² to achieve >5 000 kg/ha/yr. In temperate areas with a limited rearing window of opportunity about 5-10 PL/m² or 4 juveniles/m² are stocked; levels can be increased in presence of substrates.
The prawns are fed commercial or "farm-made" feeds, the latter being single or mixtures of ingredients, often extruded through mincers and either fed moist or (usually) after sun-drying. Feeds with 5 percent lipid and 30-35 percent protein are commonplace and an FCR of 2:1 or 3:1 is achieved with dry diets. Average growth rates depend on many factors, particularly the way in which male HIG is managed. The growth rate of SM is stunted by the presence of BC males; in their absence SM metamorphose into OC and ultimately into BC males. Thus the way in which grow-out ponds are managed (for example, the frequency of culling out large prawns, mostly males) influences total productivity.
Harvesting techniques
Harvesting is either total (in "batch" rearing) or partial (in "continuous" or "combined" rearing). Total harvesting is achieved by gravity drain-down or water removal through pumping, while seine nets are used for regularly culling larger animals. Stretched knot mesh sizes of 1.8 cm are use to harvest small prawns and from 3.8-5.0 cm for large prawns. The time and frequency of harvesting depends entirely on the volume and characteristics (the animal size) of market demand.
Handling and processing
Careful handling is essential from harvesting onwards to ensure good quality products. Freshwater prawns tend to go "mushy" if not handled and processed correctly. Firstly, it is essential to prevent prawns from becoming crushed during harvesting. Secondly, if they are not going to be sold live, they should be killed in a mixture of water and ice at 0 °C immediately (at the pond bank), and washed in chlorinated tap water. Prawns for live sale can be transported in aerated water at 20-22 °C. Prawns sold fresh must not be kept on ice for more than 3 days. Prawns for sale frozen must be quick-frozen at -10 °C (not simply placed in a "domestic" freezer) and stored at -20 °C or below.
Production costs
Providing production cost data is difficult, not only because the information is usually proprietary but also because it is site-specific. For example, one review of hatchery operating costs (all figures were normalised to 1997 USD) gave an average of USD 10.6/1 000 PL but the range was USD 1.1 to 17.0/1 000 PL. Hatchery investment costs ranged from USD 12.4-19.0 per 1 000 PL production capacity. Nursery investment costs for producing 4 million 2 g juveniles in Latin America, for example, has been quoted as USD 50 000. Nursery operating costs have been quoted as USD 6 per thousand 2 g juveniles and USD 50 per thousand 10 g juveniles in Thailand. Typically, feed expenses represented at least 40 percent of total nursery production costs. Investment costs for grow-out facilities have been cited as ranging from USD 400/ha (yielding 600 kg/ha/yr) in India to nearly USD 64 000/ha (yielding 2 270 kg/ha/yr) in the USA. Similarly, grow-out operating costs from less than USD 2 to more than USD 15/kg/yr have been cited. The average cost split was 30 percent feeding, 20 percent seed, 15 percent labour and 35 percent other expenses.
The major disease problems affecting
Macrobrachium rosenbergii generally occur because of poor intake water treatment, poor husbandry, overcrowding, poor sanitation, and non-existent or inadequate quarantine procedures. The measures to combat these problems are referred to as improved husbandry (IH) in the table below, which records some of the more important diseases. In some cases antibiotics and other pharmaceuticals have been used in treatment but their inclusion in this table does not imply a recommendation.
In some cases antibiotics and other pharmaceuticals have been used in treatment but their inclusion in this table does not imply an FAO recommendation.
MMV (Macrobrachium Muscle Virus) |
Parvo-like virus |
Virus |
Infected tissue becomes opaque, with progressive necrosis; affects juveniles |
IH |
WSBV (White spot Syndrome BaculoVirus) |
Baculovirus |
Virus |
White spots; affects juveniles |
IH |
Unnamed viral disease |
Nodavirus |
Virus |
Whitish tail; affects larvae |
IH |
Black spot; brown spot; shell disease |
Vibrio; Pseudomonas; Aeromonas |
Bacteria |
Melanised lesions; affects all life stages, but more frequently observed in juveniles & adults |
IH; oxolinic acid; nifurpurinol |
Bacterial necrosis |
Pseudomonas; Leucothrix |
Bacteria |
Similar to black spot but only affects larvae, especially stages IV & V |
IH; nifurpurinol; erythromycin; penicillin-streptomycin; chloramphenicol |
Luminescent larval syndrome |
Vibrio harveyi |
Bacterium |
Moribund & dead larvae luminescent |
IH; chloramphenicol; furazolidone |
White postlarval disease |
Rickettsia |
Bacterium |
White larvae, especially stages IV and V |
IH; oxytetracycline; furazolidone; lime, prior to stocking |
Unnamed fungal infection |
Lagenidium |
Fungus |
Extensive mythelial network visible through exoskeleton of larvae |
IH; trifluralin; merthiolate |
Unnamed fungal infection (often associated with IMN - see below) |
Fusarium solani |
Fungus |
Secondary infection; affects adults |
IH |
Unnamed yeast infections |
Debaryomyces hansenii; Metschnikowia bicuspidata |
Fungi |
Yellowish, greyish or bluish muscle tissues in juveniles |
IH |
Protozoan infestations |
Zoothamnium; Epistylis; Vorticella; Opercularia; Vaginicola; Acineta; Podophyra; etc. |
Protozoans |
External parasites that inhibit swimming, feeding and moulting; affect all life stages |
IH; formalin; merthiolate; copper-based algicides |
IMN (Idiopathic Muscle Necrosis) |
environmental disease |
Unknown |
Whitish colour in striated tissue of tail and appendages; when advanced, necrotic areas may become reddish; affects all life stages |
IH |
MCD (Mid-Cycle Disease) |
undetermined aetiology |
Unknown |
Lethargy; spiralling swimming; reduced feeding and growth; bluish-grey body colour; affects larvae, especially stages VI and VII |
IH; hatchery disinfection |
EED (Exuvia Entrapment Disease), sometimes known as MDS (Moult Death Syndrome) |
undetermined aetiology |
Unknown but probably multiple causes, including nutritional deficiency |
Localised deformities; failure to complete moulting; affects late larval stages; also seen in postlarvae, juveniles & adults |
IH; dietary enrichment |