Production of Giant Freshwater Prawn

The giant freshwater prawn, Macrobrachium rosenbergii, native to India and South East Asia commonly known as ’Scampi’ is the biggest among freshwater prawns. It is a candidate species for aquaculture owing to its very fast growth rate, market demand, hardness, euryhaline nature and its compatability with Indian major carps. It is sluggish by nature and remains half buried during the day time in the sediment or hidden in shelter which is well protected from direct sunlight. Usually, it seeks shallow regions rich in aquatic vegetation and organic detritus. It breeds throughout the year even in captivity with peaks during monsoon season. It needs freshwater and brackish water habitats to complete the lifecycle. The larva feeds on zooplankton, while the juvenile and adult are benthic omnivores. Scampi grows to a size of 30-32 cm (350-450 g) and half of the body is occupied by its cephalothorax.

Macrobrachium rosenbergii

Mature male prawn is noticeably larger (heterogenous individual growth-HIG) than the female and the second chelipedes are significantly larger and thicker. The head of the male is also proportionately larger, and the abdomen is narrower. The genital pores of the male are placed between the fifth walking legs at the basal region. There are three distinct male morphotypes namely small male (SM), orange claw male (OC) and blue claw male (BC). The second pair of peraeopods of BC male is blue in colour and extremely long, those of OC male is golden coloured and those of SM male is small, slim and almost translucent. Interestingly, a number of intermediate male forms have also been recognized, including weak orange claw (WOC), strong orange claw (SOC) and transforming orange claw (TOC) males.

The female genital pore is situated at the base of the third walking legs. The pleura (overhanging sides of the abdominal segment) is longer in female than in male and the abdomen itself is broader. These pleura of the first, second and third tail segments of female form a brood chamber in which the eggs are carried during the incubation period ie. between laying and hatching. A ripe or ovigerous’ female can easily be identified because the ovary can be seen as large orange-coloured masses occupying a large portion of the dorsal and lateral parts of the cephalothorax. Female prawns are sometimes referred to as virgin females (V or VF), berried (egg carrying) females (BE or BF) and open brood chamber (spent) females (OP). In India, the technique of mass rearing of larvae on commercial basis was first developed at Azhikode hatchery of State Fisheries department, Kerala.

Seed Production

Broodstock development

The sub-adult/ adult is collected from wild or grow-out pond (weighing not less than 60 g) and transported carefully in an aerated plastic container or polythene bag (after capping the rostrum, telson and chelipedes). On arriving at hatchery, it is disinfected with 0.3 ppm

CuSO₄ for 30 minutes and stocked in an earthen pond of 0.2-1 ha water spread area with a water depth of 1 m. The stocking density of 0.5-1 no./m² with a male to female sex ratio of 1:4 is usually adopted. Procuring adults from different locations would help to maintain the genetic quality of the broodstock and offspring. Proper quarantine protocols and health check-up for the presence of virus, bacteria and parasites need to be followed to avoid the disease occurrence. The prawns can be fed thrice daily @ 3-5% of the body weight with a commercial pelleted feed containing 35-40% protein.

Mating

The mating of adults result in the deposition of a gelatinous mass of spermatozoa (spermatophore) on the underside of the thoracic region and between the walking legs of the female's body. Successful mating can only take place between hard-shelled males and soft-shelled ripe females, which have just completed their pre- mating moult usually at night. All of the various types of males are capable of fertilising females but their behaviour can be different. Under natural conditions, mating occurs throughout the year with regional climate linked variations. Also, there are sometimes peaks of activity related to environmental conditions like monsoon showers, saline ingress and temperature. In temperate areas the breeding takes place generally in the summer.

Within a few hours of mating, eggs are extruded through the gonopores and guided by the ovipositing setae (stiff hairs), which are present at the base of the walking legs, into the brood chamber. During this process, the eggs are fertilised by the spermatozoa attached to the exterior of the female's body. The eggs are held in the brood chamber (stuck to the ovigerous setae) and kept aerated by vigorous movements of the pleopods/swimmerets. The length of time that the eggs are carried by female for incubation is normally three weeks with slight temperature dependent deviations. The temperature of 28-30°C can be considered as ideal for the satisfactory incubation.

The fecundity depends on the size of the female and ranges from 80,000 to 100,000 eggs during one spawning when fully mature. However, first brood (i.e. those which are produced within their first year of life) often lay 5,000-20,000 numbers of eggs only. The thumb rule is that 1 g of a healthy female can yield about 500-1000 larvae.

Seed production

The berried female bearing dark grey coloured eggs are sourced from the brood stock pond or natural waters and is held in PVC pipes or cylinders capped on both sides with netting to prevent puncturing of the bag. The rostrum and telson are capped with protective rubber tubes. Temperature is controlled using ice bags in the container during transportation. Starving of prawn for a few hours before packing reduces accumulation of metabolites during transport. Immediately after arrival at the hatchery, the berried prawn is given a bath in 100 ppm formalin for 10 minutes followed by rinsing in freshwater to eliminate the epifauna, if any. The berried female is kept individually in separate FRP tanks of 500 l capacity with 300 l filtered water at 6 ppt salinity and fed with oyster or clam meat. Mild aeration is provided continuously. Left over feed and metabolic wastes are removed from the tank and half of the water is replaced during every morning hours.

Maturity stages

Initially, the colour of the egg is yellow, then it changes to bright orange to pale grey, and further it darkens to slate grey by the time of hatching. Once, the egg colour turns dark grey, hatching will be started within 48-72 hours at 26-3l⁰C. The larvae are collected in the early morning using a scoop net. Soon after hatching, female is carefully shifted back to stock tank.

Rearing of larvae

The larva passes through 11 zoeal stages before metamorphosis into post-larva (PL) which is carried-out normally in an FRP tank of 500 1 capacity but varies as per convenience and capacity of the hatchery. The hatched larvae are stocked @ 100-300 no./1. Water quality parameters required for the larval rearing are given below.

• Temperature : 29 ± 2ºC
• pH : 7-8.5
• Salinity : 12±2ppt
• DO : >5 ppm
• Alkalinity : 80-100 ppm
• Photoperiod : 12/12 hr L /D
• Turbidity : Nil
• TAN : <0.1ppm
• NO₂-N : <0.01ppm
• Iron : <0.02 ppm
• Hardness : <120 ppm
• TDS : <200 ppm 

Filtered seawater and freshwater are mixed to prepare 12 ppt saline water and it is chlorinated (35 ppt) with sodium hypochlorite solution, which is then aerated for 24 hours for de-chlorination. Excess chlorine is removed by treating with sodium thiosulphate, if necessary. The larvae are fed with live Artemia nauplii, egg custard and a formulated feed. Feeding is done with extreme care to avoid over/under feeding and the details of daily ration are given in Table 25.1. Daily, 60-80% of the water is replaced. On every morning, left over feed, metabolic wastes, detritus, shell and dead larvae are removed by turning-off the aeration and siphoning-out the settled particles from the tank bottom. The metamorphosis is non-synchronous and undergoes 11 larval stages within 16-28 days according to the temperature and water quality. Up to stage-V, the healthy larva swims at the water surface, while the un- healthy larva accumulate at the tank bottom.

Artemia nauplii requirement

Rearing of Post-Larvae (PL)

The metamorphosed PL (7-9 mm) is more benthic and resembles the juvenile which rest or crawl on the tank surface. It is kept in cement tank (20 t capacity) with a stocking density of 40-60 no./1 with continuous aeration and gradually acclimatised to freshwater. Submerged artificial shelters (tiles, PVC pipes etc.) are provided to prevent cannibalism. It is fed with a pelleted feed in fine crumble form 3-4 times daily @ 10-20% of the body weight. Pelleted feed can be replaced with egg custard and minced fish/mollusc/shrimp flesh for 1-2 times daily. A water exchange of 60-80% should be done daily. It attains 16-21 mm size by next 15 days with a survival rate of 70-80%. Since the PL are cannibalistic in nature, proper feeding plays a key role.

Harvest and transportation

PL is harvested from the tank by scooping-out. The left-over PL get concentrated near the illuminated area of the rearing tank covered with dark sheets. Before packing, the salinity of the rearing tank is reduced to desired levels. The PL is spooned-out to polythene bag @ 1000-2000 numbers depending on size of PL and duration of transportation. Other aspects are similar to that of the tiger shrimp.

Pond Farming

Site selection and pond construction

Rectangular ponds of 0.2-0.4 ha area with a water depth of 100-150 cm is ideal for freshwater prawn culture. The pond should have a smooth bottom and gradual slope from water intake to outlet. Sandy- clay or sandy-loam soil is suitable for farming. The banks of the pond must be high enough to protect from floods. The intake water should be free from contaminants. Other aspects are almost similar to that explained for the major carps.

Preparation of pond

The pond after complete draining is dried under sunlight for about 2 weeks until the soil cracks which destroys pathogen and increase the soil fertility. The pond bottom should be tilled for the oxidation of organic matter and to enable the escape of foul gases. The rest of the pond preparation activities are carried out as explained for the farming of major carps.

Water quality requirements

The optimum range of water quality parameters are given below:

• Temperature : 26-31ºC
• pH : 7-8.5
• Transparency : 30-40 cm
• Salinity : 0-7 ppt
• DO : >4ppm
• Alkalinity : 80-120 ppm
• TAN :<0.1ppm
• NO₂-N :<0.01ppm
• NO₃-N :<10 ppm
• Calcium : 50-100 ppm
• Iron :<1 ppm
• Hardness: 40-100 ppm 

Stocking

The seeds (PL-15) are stocked at the density of 2-3 no./m². Healthy seeds swim with straight body, display a complete array of appendages, respond rapidly to external stimuli and actively swim against current when the water in the container is rotated. When the current subsides, it will tend to cling to the sides rather than being swept into the centre of the container. In order to ensure maximum survival rate, initially the seed can be reared in a small nursery pond at a density of 20-25/m² till reaching a size of 3-5 g. About 5-10% of the total grow-out area could be used as nursery pond.

Feeding

The prawn is fed twice daily with a sinking formulated feed containing 25-30% protein and 8-10% fat @ 8% of the body weight initially and further decreased to 2% towards the end of the culture period. It can also be fed with clam meat, if available at a low price.

Care and monitoring

The water taken from natural open source is filtered through a filter net having mesh size of 60 pm to avoid the risk of pathogens and carrier organisms. Fortnight assessment of growth and well-being of animal and water quality are essential. Sudden changes in the physio-chemical parameters of pond water adversely affect the health. Left over feed, metabolic wastes or sudden crash of plankton may lead to the rise of ammonia and result in mass mortality of the animal. The species exhibit high rate of cannibalism and hence cut branches of trees, nylon screen, pipes, etc are placed as hide-outs.

Harvesting

Culling of large individuals on regular basis is a common practice in freshwater prawn farming. The entire crop is harvested at the end of the culture period after the prawn has grown to the marketable size of 50- 100 g. The culture period is normally 6-8 months.

Integrated Farming with Paddy

Fresh water prawn is one of the suitable species for rotational culture with paddy where not more than one crop of paddy is possible. After filling the field with 30-60 cm water, 1000 kg cow dung, 500 kg poultry droppings and 100 kg superphosphate per hectare is added. The water level is slowly raised to 100 cm by the next 14 days. The prawn seed is initially reared in nursery pond for 30 days and released, two weeks after rice harvest, at a stocking density of l no./m² for grow-out rearing. In this system, a production of 300-500 kg/ha is achieved within 6 months.

All Male Culture of Scampi

All male culture has several advantages over the conventional culture like good FCR, survival, individual size and overall economic output from farming. Juveniles reared in nursery pond for 45-60 days are harvested using a seine net or cast net and males are manually segregated by skilled labour. Manual segregation is carried out by visual examination of the last peraeopods where the gap between the bases of the two walking legs is less in the case of male. The segregated male juveniles are stocked at the density of 1-2 no./m² in grow out pond. If the pond bottom is firm enough, after 75-120 days (depending on the size of the prawn), seining is carried out for “cull harvesting” to remove all the fully grown blue clawed males (80-100 g) and any female (30-40 g) that might be accidentally present. About 20% of the large prawn is thereby removed at this stage, while the smaller prawn is returned to the pond, allowing them to grow further. After 30 days, the second seine netting is carried out; which is the major cull harvest in which 60% of the full sized males are caught. Final harvest is carried out after another 30 days completing the culture period of 6-8 months.

Source : Department of Fisheries, Government of Kerala