THE BRACKISH-WATER PRAWN CULTURE
Prawns are cultured in the mild saline-water in various places. There are some well built enclosures are connected with the feeder canals through sluice gates. These sluice gates are manipulated mechanically. Inside the sluice gate, split bamboo screens in the form of inverted 'V or 'W are fixed. The screens are fixed in such a way that the larvae of prawn can enter inside the enclosure along with the incoming water of feeder canal but cannot escape outside the inclosure.
Sluice gate is opened and the tidal water enters to the enclosure through feeder canal. During this high tide of water, the huge number of prawns enters to the brackish-water but during the low tide, the water of the enclosures comes out to the feeder canal ; but fry of prawn is fail to escape from the enclosure due to inverted screen on the mouth of enclosure. In this way, the sluice gates are operated and as a result, the enclosures become full of tidal water. The enclosures now contain huge number of stocked fry of prawn. Due to presence of plenty of organic materials in there, there is rapid development of fry of prawn. Now these are filled up with tidal water and sluice gates are closed and the place are kept undisturbed.
Thus the larvae of prawn remain undisturbed and growth of prawn continues rapidly. Then the prawns of become matured and they become marketable size. Now the water of the Brackish-Water prawn culture place is allowed to go out as far as practicable by opening the sluice gates. So, large scale harvesting of prawn is done with the help of drag nets in the Brackish-Water prawn culture place.
ADVANTAGES OF IN BRACKISH-WATER PRAWN CULTURE
Following are the advantages of brackish-water prawn culture—
(i) Prawn can live easily on normal environment.
(ii) They can tolerate to some extent the variation of salinity in water.
(iii) Their rate of growth is good.
(iv) They are more or less omnivorous.
(v) Fry of prawn is easily collected from the natural environment.
(vi) There is least possibility of diseases.
PROBLEMS OF BRACKISH-WATER PRAWN CULTURE
Different types of problems of Brackish-water prawn culture are given below—
Mechanical problems of Brackish-water prawn culture:
(i) Damage of embankment: Plant growth in saline soil is scanty. As a result, soil embankments of the enclosure, especially in rainy season become loose and so there is every possibility of damage of the embankment during this period.
(ii) Problems of sluice gate: Functional activity of sluice gates gradually becomes decrease due to deposition of mud in the sluice gates. Thus the problem arises to control the sluice gates during incoming and outgoing of water. Thus there is possibility of flood of water in the enclosures especially during the heavy rains in the monsoon.
(iii) Erosion and deposition of soil: There is erosion of soil of the enclosures due to presence of sodium in the saline soil. This eroded soil can disturb the prawn culture due to deposition of this soil at the bottom of enclosure or at the sluice gate or at the feeder canal.
Physical problems of Brackish-water prawn culture :
(i) Turbidity of water: If there is heavy turbidity of water in the enclosure, then the rays of the sun will be decreased at the bottom of the water. Thus its productivity will also be decreased.
(ii) Temperature of water :In summer season, the prawn culture is hampered due to increase of temperature of the water in the enclosure.
Chemical problems of Brackish-water prawn culture :
(i) Deficiency of dissolved oxygen: There is every possibility of decrease of dissolved oxygen of water in the enclosure if there is increased salinity of water as well as increases of the number of plants and animals. Thus the prawn culture is hampered due to deficiency of oxygen.
(ii) Pollution of water :There is heavy damage of prawn culture if the water of the feeder canal is polluted, so also of the enclosure water.
Biological problems of Brackish-water prawn culture :
Prawn culture is also disturbed by various types of carnivorous fishes, such as Bhetki etc. These carnivorous fishes ingest the fry of prawns. In addition to this, some parasitic organisms create some diseases to prawn.
DISEASES CONTROL OF BRACKISH -WATER PRAWN CULTURE
Brackish-water prawns are attacked by various kinds of organisms, such as Virus, Bacteria, Fungus etc. As a result, different kinds of diseases are produced to prawns. Important diseases, their symptoms and control measures are given below.
1. Viral diseases :Causative organism—Virus.
Symptoms—Due to attack of the virus, post larva and mature prawns take less amount of food with less mobility and also growth is reduced.
Control measures—Disease free larvae are to be reared scientifically, diseased prawns are to be destroyed.
2. Bacterial diseases :
Causative organisms—Aeromonus, Vibrio etc.
Symptoms—Black spots on the carapace and erosion of the carapace occur. Black spots also appear on gills and in other appendages of prawn.
Control measures—Care to be taken on the quality of water. Antibiotics on water or with food may be used.
3. Fungal diseases :
Symptoms—Mycelium of the fungus spreads inside the body of the larvae of prawn; thus death may occur to the larvae.
Control measures—Disease free larvae are to be cultured and observe the growth of larvae regularly.