Scientific freshwater prawn culture in North East India

Tonmoy Gon Choudhury / Pampa Bhattacharjee *

Introduction

Macrobrachium Rosenbergii, also known as the giant river prawn, is a species of freshwater prawn native to the Indo-Pacific and northern Australia. This is commercially important among the species from fresh water aquaculture for its value as a food source.

Prawns being a high value commodity and processing great potential for export offers great scope for their large scale adoption, both under monoculture and polyculture. Northeast India refers to the easternmost region of India consisting of the contiguous Seven Sister State, Sikkim, and parts of North Bengal. Scientific culture of freshwater prawn has high potential in North east India due to high productivity of water and market demand.

Site selection

Selection of suitable sites is a prerequisite for establishing a prawn farm. But it is a critical step for northeastern states due to hilly land topography in most of the states. Farming site should be connected to an approachable road for transportation of seed, fertilizers, feeding materials, farm equipment.

In addition, information about environmental conditions such as temperature, rainfall pattern, water source, etc. should be available. Sandy-clay or sandy-loam soil is suitable for the construction of the ponds.

Preparation of pond

Pond with a proper drainage system is suitable for removal of unwanted fish. Otherwise Mahua oil cake can be applied in a dose of 2500 kg/ha to kill the unwanted fishes. Local toxic plants available in North eastern states can also be used for removal of unwanted fish. In a newly excavated pond 5-7 tonnes cow dung or pig manure and 200-500 kg lime/ha should be applied.

To maintain the planktonic populating in the pond, organic and inorganic fertilizers like poultry droppings, pig and cow manure, biogas slurry etc. may be applied after stocking of juveniles at periodical intervals.

Urea and superphosphate should be applied in desired quality depending on the soil and water quality of the pond. To maintain Ca+ balance in water 50 kg lime/week should be applied which helps in the molting process. During starvation strong and big sized prawn attack smaller, for that large palm leaves, tires, cement or plastic pipes, small asbestos sheets etc. can also be used.

Stocking

For north eastern states monoculture is not economically viable. Polyculture of prawn can be done along with carp species like rohu, silver carp, and grass carp. Bottom dwelling fishes like mrigal, calbasu, common carp, singhi, magur, tilapia, etc. are avoided For polyculture of prawn with carps stocking density is 15,000-25,000 prawn seed and 2,500-3,5000 fish fingerlings, in which both catla and rohu may form equal share.

Availability of seed is the main constraint for developing the scientific culture of freshwater prawn in North Eastern States. As North east states are landlocked, no sea/brackish water available for developing prawn hatchery. But using artificial sea water prawn hatchery has been developed in states like Tripura, Assam which shows the promise for overcoming the problem of unavailability of seed.

Supplementary feeding

The production capacity of culture pond largely depends on the quality of feed provided. The grow-out feed should contain 30% crude protein, out of which 50% should be from animal origin and 50% from plant origin. Locally available feed ingredients like rice bran, mustard oil cake, groundnut oil cake, coconut oil cake, soybean meal, maize, sorghum, barley and other ingredients obtained from plants are used.

Beside fish meal, there are other ingredients from animal origin such as poultry viscera, slaughterhouse waste, mussel or snail meat, etc. Along with these, vitamin and mineral mixture in desired quantity are added to help the prawn to overcome environmental stress.

The feed pallets should be the desired size and it should be water stable for a few hours. Readymade feed is also available in the market for polyculture of prawn along with carp. The feed is provided at 3-10% of standing biomass/day in two rations during morning and evening.

Water quality management

The culture pond essentially requires maintenance of suitable water conditions, viz. water temperature 20pC-32pC, pH 7-8.5, dissolved oxygen>2.5 ppm, total hardness 100-150ppm,NH4+-N 0.02-0.20 ppm, calcium 30-80 ppm, phosphorus 0.01-0.9 ppm and Nitrogen 0.05-0.5 ppm.

Growth and production

The growth, survival and production rate of prawn in the pond depends upon the suitable water quality, supply of sufficient nutritious feed, suitable environment, etc. Differential size is a common phenomenon with some individuals growing much larger than others, and some of them remaining very small, which is due to their territorial behavior, social hierarchy and cannibalistic nature.

Therefore after 5 month, when they attain marketable size, they should be harvested. Survival levels of 40-60% are normally obtained in grow-out culture and within a culture period of 6-8 months production of 750-1500 kg/ha/6-8 months can be obtained.

Disease control

As aquaculture expands and intensifies, more and more new diseases will emerge. Until recently, freshwater prawn, Macrobrachiumrosenbergii was regarded as disease resistant. The recent report of white tail disease (WTD) in freshwater prawn hatcheries and farms has sent shock waves through the prawn aquaculture industry in India.

Whitish appearance of the tail is the prominent clinical sign, and therefore, the disease is named as white tail disease. As there are no treatments to viral pathogens, only through adoption of better management practices in hatcheries and farms, the spread and impact of White tail disease can be minimized.

Vibriosis is a problem for fresh water prawn in which affected prawn shows black or brown cuticular lesion, muscle opacity, and black lymphoid organ melanisation of appendages tips. Black gill disease is another bacterial disease, in which affected prawn shows low growth and mass mortality. Maintenance of water quality and supply of quality feed in sufficient quantity is the only prophylactic measure for all this bacterial disease.

Harvesting

Prawns are harvested after draining out or pumping out of the water. However, dredging or cast netting can also perform to harvest the prawn. Compared to finishes, prawns are difficult to be harvested from ponds through ordinary drag net, therefore special nets are prepared using glass beads or iron weights at its bottom to make pockets, in which once the prawns get caught, may not be able to escape.


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