Influence of Cage Shapes on Growth, Survival and Production of White Shrimp Penaeus indicus (H. Milne Edwards) in Vellar estuary

Abstract

Owing to the increasing demand for shrimps in world market, aquaculture is considered as extreme focus area by many countries. Much emphasis is also given to this fast moving commodity, because of its production potential, earning foreign exchange and scope for employment generation. Therefore last two decades, shrimp farming has witnessed tremendous technological advancements, the world over. Shrimps are cultured mainly in ponds, cages, pens and raceways. Out of which ponds and raceways are land based rearing methods, and cages and pens are water based rearing methods. Pond and raceway culture are found suitable in coastal areas and brackish water areas. Cage and pens are suitable in bay, lagoon, back water and open seas. Although previous workers used different type of cages [1-6] for shrimp culture but their suitability in terms of production in the estuaries was not attempted. Hence, in the present study, three type of cages viz, rectangular, square and circular cages were tried to find out their suitability for culturing Penaeus indicus in Vellar estuary. Materials and Methods Cage erection The present study was carried out in Vellar estuary during one year period from January 2003 to December 2004. Three cages were used in the present study each covering 50 m 2 water spread area with different shapes. Of these one was rectangular (10 m×5 m×1.5 m), one was square (7.072×7.072×1.5 m) and remaining one was circular (Diameter-7.98 m; height-1.5 m). To avoid fouling problem on the top portion of the cages, the height of all the cages were uniformly raised to 1.5 m to expose the cage even during high tide. The entire cages were erected manually with the support of casuaria poles. For circular cage, to maintain the circular shape, a collar made up of bamboo materials were used inside the top portion of the cage. In the remaining two cages the collar made up of thick HDPE materials. All these cages were tied with the respective casuarinas poles by using foot and head ropes. All the three cages were erected on the bottom soil substrate and provided hide-outs. Each cage bottom was provided with sixteen bicycle tyres as hide-outs. Seed transportation The seed purchased from the hatchery was transported to culture site in Vellar estuary by oxygenated polythene bag and were kept in styrofoam boxes. Before transportation, the qualities of seeds were Abstract To observe the influence of cage shape on the growth, survival and production of Penaeus indicus, three types of uniform size (50 m 2 water spread area) cages were erected on the bottom soil substrate and provided with hide-outs. Among these one was rectangular (10×5×15 m), one was square (7.072×7.072×1.5 m) and the remaining one was circular (Diameter=7.98 m; Height=1.5 m). The culture period was 100 days and the juveniles ranging from 3.3 to 4.1 g were stocked uniformly at the rate of 20% for all the cages. The higher growth of 22.1 g, survival rate of 92% and production rate of 406.6/m 2 were reported for the rectangular cage. So among the three cages used for the present study, rectangular cage is the most suitable for the culture of P. indicus especially in Vellar estuary. examined by taking the seeds in a plastic container to ensure uniform size and good health. Post larvae of penaeid shrimps are small, fragile and are sensitive to change in water conditions. So the post larvae purchased from the hatchery were acclimatized to estuarine condition before stocking to avoid heavy mortality. In order to increase the survival rate and prevent the escape from the cage, the acclimatized seeds were released into already erected small hapa of 2×1×1 m size in the estuary and the seeds were reared for 10 days. Feeding The pellet feeds were provided to all cage cultured shrimps. Initially 12% feed were provided to shrimps and then slightly raised and finally reached by 14% because of auto entry biomass in the cages. Feed was given daily in three installments at dawn, midday and dusk. But the two fourth of the feed were provided to dusk since shrimps are nocturnal animals. Feed was provided only through the feed trays of 1 m 2 area. The feed trays were tied to the casuarinas poles and allowed to submerge in the water column with help of a stone; five such trays were used per cage for feeding. Culture period The culture was carried out for 100 days. The study started during the premonsoon season and lasted in monsoon season. The fresh water inflow was high during later periods. The water was also turbid during the period. During the experimental period periodic sampling, environmental parameters, survival rate were regularly assessed. The shrimps were harvested once the culture period was over.