During the last two decades investigations were undertaken to assess brackishwater seed resources along the east and west coast of the country. Though breakthrough has been achieved in producing hatchery bred P. monodon and P. indicus seeds, it is yet to be commercialised. Even when hatchery bred prawn and fish seed will be available in millions, dependence on natural seed for development of brackishwater aquaculture will still continue. It is, therefore, necessary to lay greater emphasis on seed prospecting to meet the demand of cultivable species as an essential prerequisite to promote brackishwater aquafarming.
Calender of availability
The distribution of cultivable commercial species of fishes and prawns in time and space has been charted out and the calender (Table-I) prepared for procurement of seed along the coasts of six maritime states of the country may be considered as a guide line.
Collection methods of brackishwater prawn and fish seed
All culturable brackishwater species of fish and prawns in the country, except Etroplus spp. are not known to breed normally in confined waters. Generally breeding ground of fish and prawn is restricted within the foreshore and offshore areas of the sea. During the season of abundance, the brackisbwater prawn and fish seed, in nektonic form, oscillate with the tides in estuarine and coastal areas in search of shelter and nursery ground. To negotiate with these prawn and fish seed, several types of gears are generally operated for collecting the stocking materials for brackishwater aquaculture.
Gears for collection of prawn seed
The most important gear used for prawn collection is shooting net. It is a funnel shapped bag net with 4-6 m length and 4-6 m width at the mouth and gradually tappering at the cod end where a detachable tail piece is tied. Synthetic net cloth having 1/2 inch or 1/16 inch mesh size is used for stiching such nets which are found to be very effective for prawn seed collection in tidal streams. The prawn seeds collected in the tail piece is hauled every 20-30 minutes. In West Bengal, P. monodon is the dominant penaeid prawn collected by such net.
Along Northern Circars, Coromondal and Malabar coasts, where tidal amplitudes and current velocities are not sufficient enough for operation of shooting nets, locally faqricated scoop nets and hand nets may be used in the shallow marginal sheltered areas along the coast line for collection of postlarvae and juveniles of Penaeid prawns i. e. P. monodon and P. indicus.
Gear for collection of fish seed
Cultivable fish seeds of importance like, L. calcarifer, mullets and C. chanos when entrapped in shooting net may escape from the tail piece when the current is feebale. Such fish seeds are known to harbour in shallow sheltered waters infested with weeds or mangrove vegetation and also in natural depression and irrigation pits where from the juveniles may be scooped out with the help of a rectangular sac-like
cloth piece popularly known as hapa.
Enclosure net is generally used in mudflats in tidal zone inundated during high tide and completely exposed in the receding phase of the tide. During high tide the area is encircled by drag net cloth with the help of bamboo props. When the encircled area is completely exposed during low tide, the entrapped fish seed, both fry and fingerlings, are handpicked and transported for stocking in fish ponds. Drag net with scare lines of palmyra leaves is commonly used along the Northern Circars and Coromondal coasts for collection of C. chanos fry. The gear is operated by two persons by encircling shallow coastal area. The scare line of leave is shaken at times over a cloth net and C. chanos fry is collected.
Transportation of prawn and fish seed
For carrying out successful brackishwater aquaculture throughout the year, the required quantity and quality of cultivable prawn seed may not be always available in the vicinity of farm site. While some sites are surplus in respect of some particular species of seed, others are deficient. Moreover, zoo-geographical pattern of fish seed distribution is not uniform in the east and west coasts. Therefore, i’t would be necessary to transport seed from collection sites of abundance to fish farms situated at a distance. To maintain adequate supply of seed at places of deficiency, improved methods of seed transportation has been evolved. The following techniques developed so far in this direction may be adopted.
Transport of penaeid prawn under oxygen packing
After collection and segregation of economic varieties of prawn from the net, the seed is kept in small containers in settled estuarine water for a duration of 2-3 hours. The seed is then transferred to plastic bags having 4-5 litres of water. Packing at a density of 500 to 2500 per litre of post larvae (10-12 mm) of P. monodon is done depending on the duration of transport. From 500 to 2500 seeds/litre can be packed under oxygen for transport durations of 36 to 12 hours respectively with survival of 85-90%
Transport without oxygen packmg
Taking into consideration of the dissolved oxygen concentration, temperature, etc. of the ambient water and the rate of oxygen consumption by the postlarvae per unit time, penaeid seed can be transported for short distance of 4-6 hours journey without oxygen packing. In aluminium containers of 20 to 30 litre capacity, 500 seeds/litre can be transported with less mortality.
Transport of fish seed
Seed of mullets, Etroplus sp., Chanos sp., Lates sp., etc. can be transported for long distance under oxygen packing by careful manipulation of seed density and fry size in the container.
- Central Inland Fisheries Research Institute, Barrackpore