How To Do Freshwater Prawn Farming

Amita Saxena and Satesh Vasave

College of Fisheries, Gbpuat, Pantnagar 263145 India

The words 'prawn' and 'shrimp' are often used synonymously. Actual use is geographically dependent. Animals of the genus Macrobrachium are referred to as freshwater prawns in Australia and freshwater shrimp in the United States of America (USA). In its statistical data, FAO refers to the genus Macrobrachium as freshwater prawns but also uses the word prawn for many species of marine shrimp, including the banana prawn (Fenneropenaeus merguiensis), the giant tiger prawn (Penaeus monodon) and the kuruma prawn (Marsupenaeus japonicus) (FAO 2001).

Although several species of freshwater prawns are currently being cultured, the major commercial species (Macrobrachium rosenbergii), is indigenous to South and Southeast Asia, parts of Oceania and some Pacific islands. M. rosenbergii imported into many other tropical and subtropical areas of the world and is the species most favored for farming purposes. Modifications need to take account of the different environmental requirements of the other species, especially in the larval stages.

The most commonly cultured species in India is Macrobrachium rosenbergii, a hardy species, by virtue of its ability to adapt to various types of fresh and brackish-water conditions. The peak season is September to November and May to July. The breeding takes place in low saline waters which is also needed for larval and post larval development after incubation. Breeding of M. rosenbergii takes place in estuaries. Though seed may be available in natural sources to a limited extent, for large scale culture there is a need to ensure regular supply of seed. For ensuring availability of quality seed in predictable quantity freshwater prawn hatcheries should be encouraged, technology for which is already developed. Freshwater prawn hatcheries are coming up in many states.

It is a good candidate species for export oriented aquaculture.

Importance of Giant Freshwater Prawn (Scampi) (Macrobrachium rosenbergii)  

1. It is very tasty and it's protein is of very high quality.

2. It's growth rate is fastest in comparison to other freshwater prawns.

3. It is suitable for culture in tropical and sub-tropical regions.

4. It can be cultured in freshwater or saline water (salinity less than 10 ppt).

5. This can be cultured alone in monoculture or mixed culture with carp fishes.

6. Compared to marine prawns it has better resistance against diseases and hardy.

7. It's market rate is always higher compared to fishes.

Mature male prawns are considerably larger than the females and the second chelipeds are much larger and thicker. The head of the male is also proportionately larger, and the abdomen is narrower. The head of the mature female and its second walking legs are much smaller than the adult male. A ripe or 'ovigerous' female can easily be detected because the ovaries can be seen as large orange-coloured masses occupying a large portion of the dorsal and lateral parts of the cephalothorax.

During rainy season the mating (copulation) of adults results in the deposition of a gelatinous mass of semen on the underside of the thoracic region of the female's body (between the walking legs). Within a few hours of copulation, eggs are extruded through the gonopores and guided by the ovipositing setae (stiff hairs), at the base of the walking legs, into the brood chamber. During this process the semen attached to the exterior of the female's body fertilizes the eggs and kept aerated by vigorous movements of the swimmerets more than three weeks. Female prawns of M. rosenbergii have 80000 to 100000 eggs during one spawning. Incubation period range 18-23 days at 26-28 ⁰C. Eggs are slightly elliptical with a long axis of 0.6-0.7 mm, and are bright orange in colour until 2-3 days before hatching.

Larva becomes grey-black till the yolk sac absorbed. After hatching, rapid movements of the abdominal appendages of the parent disperse the larvae. Larvae are planktonic and swim actively tail first upside down. Larvae require brackishwater for survival. The larvae go through 11 distinct stages before metamorphosing into post larvae. Stage I larvae (zoeae) are just under 2 mm long (from the tip of the rostrum to the tip of the telson). Larvae swim upside down by using their thoracic appendages and are positively attracted to light. By stage XI they are about 7.7 mm long. Newly metamorphosed post larvae (PL) are also about 7.7 mm long and are identified by their movement and swimming like adult prawns. They are translucent and have a light orange pink head.

On completion of their larval life, freshwater prawns metamorphose into post larvae (PL). They show mainly crawling rather than free-swimming. Post larvae exhibit good tolerance to a wide range of salinities. Post larvae migrate upstream into freshwater conditions within 7-12 days after metamorphosis and swim against rapidly flowing currents and utilize larger pieces of organic material. Post larval freshwater prawns are omnivorous can also be cannibalistic in absence of food material.

The farming of the giant freshwater prawn Macrobrachium rosenbergii has been expanding in India recent years. Scampi/prawn farming gained momentum after the set-back in shrimp farming due to disease outbreaks and other factors. Hygiene becomes important factor in farming system. The existing culture system includes both monoculture and polyculture with Indian major carps in ponds. The ideal size is 0.2 to 1 ha. & rectangular in shape with 1 to 1.5 m in depth The pond should be provided with inlet and outlet (guarded by screen to avoid the unwanted fish, larvae, eggs and escape of prawns) and connected to water supply and drainage system respectively. They exhibit territorial behavior. To enhance the more surface for clinging, branches of trees, hanging nylon screens, hume pipes etc. have been kept. To increase the dissolved oxygen in pond the aerators or regular exchange of water were the best method. The pond either new or old may be provided with 250 to 400 kg/ha lime and cattle dung 1000 to 1500 kg/ha. and some inorganic fertilizers (if necessary) for the production of plankters. When the water turns brown more water is filled up to 1.2 m depth.

Water quality requirements in pond for culture of prawn.

pH is very important factor. The pH difference between pond water and seed containing water causes heavy mortality. The About 10% to 15% of the total pond area can be covered with the additional substratum. Grow out stocking densities of range from 0.5-2.5 post larvae of prawn per m² in polyculture and 1-5 per m² in monoculture. The culture period is 6-8 months starting at the beginning of southwest monsoon (June-July, 27-30°C). The scampi are fed with farm-made or commercially available feeds to get good growth of them. Diets with about 35-40% protein and gross energy level of about 3.2 kcal/g diet and protein:energy ratio of about 125-130 mg protein/kcal are suitable for growth of M. rosenbergii. Protein/starch ratio of 1:1 is known to be effective for better feed efficiency and growth rate. Dietary glucosamine (an amino sugar and intermediary between glucose and chitin) facilitates molting followed by enhanced growth with 30% dietary fiber. The prawn requires 60-150 mg vitamin C/kg diet. Levels of 60 mg ascorbic acid and 300 mg tocopherol per kg diet are considered sufficient for proper reproduction and offspring viability in prawn broodstock. Calcium concentration at 5 ppm and vitamix and mineral mix at 1 and 2% of the feed respectively given for good results. So add these above ingredients in the pellets/feed to achieve good growth and survival.

Periodic harvesting has to remove marketable size 50 gm and above allowing the rest to grow further. The production levels comes to 2.0 to 2.5 tons/ha (depends upon stocking) in monoculture and 600 to 700 kg/ha in polyculture. The time of harvest depend upon growth rate and market size desired. In continuous culture cull harvesting is practiced. Cull harvesting ponds are not drained. Larger prawns are harvested by seining in batch culture, batch harvesting or drain harvesting is followed.

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