The efficiency of depuration of the Indian backwater oyster Crassostrea madrasensis (Preston, 1916) using the fill-draw method (static method) with high-loading density was evaluated in this study. Depuration experiments were conducted with cartridge-filtered and UV-treated seawater at a salinity of 30.3‰, a pH of 8.3, and a temperature of 29.5°C. The oysters located in trays on the surface and on the bottom were compared for microbial loads. Samples were taken at 0 h, 8 h, 16 h, 24 h, 36 h, and 48 h of depuration. The results showed that in winter monsoon-sampled nondepurated oysters, the most probable number of fecal coliforms and Escherichia coli were greater than the limits according to NSSP and European Union regulations. The surface held oysters took 24 h to reduce the coliforms and E. coli levels to below safe limits whereas for bottom held oysters it took 48 h. The species Salmonella was never detected in the oysters sampled, whereas Vibrio spp. were present in the nondepurated oysters and were eliminated completely after 8 h of depuration. Variation in depuration of total coliforms, fecal coliforms, E. coli, total plate count, and fecal streptococci in oysters were significant (P < 0.05) between surface and bottom oysters. The study results recommend a loading density of 2 oysters/L water stacked in 1 layer as the optimum loading density for commercial depuration completed within 24 h.
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Vol. 33 • No. 2