What is prawn?
Prawns are belong to the phylum arthropoda, class Crustacea is considered by most people a fish. But it is not a fish; it is an aquatic Crustacea of economic importance. The prawn is a good source of protein and constitutes highly nutritive diet for human beings. Due to its nice taste, it is a cherished food-item throughout the world.
What is Prawn culture ?
Generally the larval forms of prawn is allowed to enter the different types of the culture fields, rear them there and to capture them for marketing. And this is the system which is called the prawn culture.
Habit, Habitat and food of Prawns
Before going for the prawn culture we should have some knowledge about the habit,habitat and food of prowns. Prawn inhabits all sorts of water, much as in sea-water, estuaries and fresh-water. They are generally living at the bottom of water and are avoiding sun-rays. The marine and brackish water species spawn in sea. The hatchlnings are incapable to swim, so they are drifted along with the current to the coastal waters or estuaries where they undergo development till they reaches the juvenile stage. The post larvae feed upon the dead organic matter of plants and animals and upon small benthonic organisms.
The juvenile prawn has to enter the sea. The fresh-water species like Macrobrachiumsp. spawn in fresh-water, then they are drifted to estuaries and after attaining the juvenile stage swim back to fresh-water.
Prawns are consuming the organic substances, microscopic animals and plants as their food material. Among the animals, minute insect, snail, larvae of mollusca and echinodermata as well as aquatic weeds, algae, moss etc., are taken as their food material.
Types of Prawns used for prawn culture
Cultivable prawns are divided mainly into two groups, such as: (i) Saline-water species and (ii) sweet-water species. Saline-water species are placed in the family Penaeidae, so thay are known as Peneid and sweet-water species using to the family Palaemonidae, hence they are known as Palaemonid or non-peneid.
External features of Prawn
Body of the prawn is elongated and it is covered by hard covering known as carapace. The body is mainly divided into cephaiothorax and abdomen. Cephalothorax is composed of head and thorax and this cephaiothorax is covered by a hard carapace. There is a saw-like structure on the anterior end known as rostrum which acts as a defensive organ. Near about the base of rostrum, there is a pair of compound eyes. A pair of antenna which is a sensetive organ is placed at the anterior end of the body. On the ventral side of the cephaiothorax, six pairs of walking legs are situated for walking on the substratum. These walking legs are known as pleopods.
The abdomen is subdivided into few segments and each segment is covered by exoskeletal structures. The last segment of abdomen becomes triangular which is known as telson. On the ventral surface of the abdomen, there are few swimmerets which help in swimming. These swimmerets are known as pleopods.
Methods of Prawn Culture
There are various methods of prawn culture. The prawn can be cultured in fresh water as well as in salty water. In sweet water as in pond, different types of prawn can be cultured, such as Macrobrachiumrosenbergii, Macrobrachium rude, Macrobrachiummalcomsoni, Palaemonstyliferusetc. But in saline water as in paddy fields and brackish water prawns are cultured mainly of different species of Penaeus or Tiger prawn, such as Penaeusindicus, Penaeusmonodonetc. Out of the various species of prawn Penaeusmonodonbecomes more popular as well as high demand in the market. Hence this species of prawn is cultivated most extensively and rearers are also financially benefited.
PRAWN CULTURE IN PADDY FIELDS
Extensive prawn cultivation in paddy fields is going on in different regions of the world..
Method of prawn culture in paddy field
In those paddy fields which are fed by irrigation canals are most favourable for Prawn culture. In summer months, the water level of feeder canals of a river remains at a level about 30 cm below the level of the paddy fields. At the time of onset of monsoon, the paddy fields are manure and then seedlings are planted.
During monsoon, the rain water accumulates in the paddy fields as well as in the irrigated canals. As a result, water level of irrigated canals gradually becomes high and this water in feeder canals becomes low in salinity. During this period, water of the feeder canals is allowed to enter the paddy fields. In the meantime, prawns lay large number of eggs in the low saline water of the feeder canal. Thus the huge numbers of larval stages of prawns enter to the paddy field along with the canal water. When the paddy fields are flooded with the water of feeder canal then the cut ends of paddy fields are closed. In the manure paddy fields, the large number of food particles are developed and these food materials are consuming by the young prawns. These young prawns are developed rapidly during the wholeperiod of paddy cultivation. After four months, just before the harvesting of paddy, prawns are collected from the paddy field. These prawns are now of marketable size.
In the coastal region, paddy cultivation is going on once in a year in the rainy season. Prawn culture is also going on successfully in such areas. After harvesting of paddy, these low-lying fields are utilized for prawn culture. Before culture of prawn, sluice gates of the paddy fields are repaired and strengthened. The sluice gates are now controlled manually. In the meantime algae, fry of fishes, different types of copepods, crustaceans and also larva of mollusca are available in this confined water of the paddy field. These creatures are utilized by growing prawns as their food material. Now the sluice gates are opened during the high tide of the sea; as a result, millions of larval stages of prawn enter in these fields along with the incoming of sea-water. When the paddy fields are filled up with tidal water, the sluice gates are closed for a day. In the next day, sluice gates are opened during the low tide and the water of the field goes out. In order to prevent the escape of larvae of prawn during low tide from the paddy field, nets attached in rectangular frames are fixed at the sluice gates. This process of incoming of sea-water during high tide is repeated, so that the number of larvae of prawn increases in the paddy field. The captured young prawns are allowed to grow in this paddy. After that, harvesting of prawns begins and continues and after harvesting, the soil of the field is again prepared for the next cultivation of paddy.
For brackish water prawn culture click