Effect of Stocking Density on Survival Rate and Bacterial Compositions in the Larval Rearing of the Giant Freshwater Prawn (Macrobrachium rosenbergii)
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Abstract
Larval rearing of the giant freshwater parwn, Macrobrachium rosenbergii using a clear-water system was studied at a commercial prawan hatchery located in Ratchaburi province, Thailand. The prawn larvae were stocked in circular fiberglass tanks containing 2.5 cubic meters (m3) of 15 parts per thousand (ppt) seawater at the density of 100,000/m3, 120,000/m3 and 140,000/m3. Water quality parameters and quantitative and qualitative analyses of bacterial populations in the water and postlarvae were studied. The average survival rates of postlarvae in the 100,000 /m3, 120,000/m3 and 140,000/m3 groups were 91%,87% and 74%, respectively.The were significant differences among the three density groups. The water quality throughout the rearing period was suitable for nursing postlarvae. The average total numbers of bacteria in water at the stocking densities of 100,000/m3 and 120,000/m3 ranged from 3.20x103 to 4.40x104 and 4.27x103 to 7.6x103 CFU/ml,respectively, and were significantly lower than for the 140.,000/m3 group in which the average total baceria ranged from 9.04x103 to 9.00x104 CFU/ml. The average total numbers of bacteria in postlarvae at densities of 100,000/m3, 120,000/m3 and 140,000/m3 were 8.94x10±9.77x10, 1.36x102±1.39x102 and 2.04x102±2.04x102 CFU/g,respectively. Five species of bacterial flora were identified from the water and postlarvae and the one most frequently isolated was the Vibrio community, represented by Vibrio vulnificus, V. alginolyticus, V. cholerae (non 01), V. mimicus and Aeromonas hydrophila.
There was an absence of mass mortalities during the 27-day rearing period in this study. No histopathological changes related to diseases were observed in any postarvae. External protozoans such as Epistylis sp. were found from the body cuticular surfaces of some of the postlarvae sampled, especially in the 140,000/m3 group. The results from this study indicated that direct involvement of bacteria alone was unlikely to cause mass mortalities of the prawan larvae. Environmental stressors from high stocking density affected the survival rate of prawan postlarvae.