54 SEPTEMBER 2014 • WORLD AQUACULTURE • WWW.WAS.ORG Oyster Stomach Contents Analysis of the contents of the digestive tract of Crassostrea lugubris in Nha Phu Lagoon indicated that oysters mainly consumed organic humus. Additionally 25 species of phytoplankton in 5 genuses, 3 families, 2 branches and 2 classes were identified. The branch Cyanophyta had 2 species represented (9 percent) and Heterokontophyta had 23 species represented (91 percent). Some zooplankton were also found in the gastrointestinal tract. Food Safety of Oysters Samples of adult oysters, sediments, and water in the areas of the oyster growth study were collected every three months from 2003 to 2005 to analyze the density of E. coli, Salmonela, Shigella, Vibrio, Hepatitis A and B according to the standard of hygiene and food safety requirements of the European Economic Community. The results showed that: • Among seven standards of food safety regulated, there are three standards for organisms that cause cholera, dysentery, typhoid and gangrene (Vibrio cholerae, Clostridium perfringer and group Salmonella-Shigella). These were absent in eight months of monitoring, except the sediment sample collected in September 2003, when C. perfringer was found (13 cells/g). • The density of E. coli, S. aureus and group Vibrio was 0-50 cells/mL in water during eight months of monitoring. In sediment, densities of E. coli, S. aureus and group Vibrio were high (500 cells/g). • In the gut, considering the bacteria present in water and sediment, the density of coliforms was very high (3000 cells/g). To assess depuration capacity, oysters were collected and stored in clean salt water for 3-5 days. Bacteria that cause disease (S. aureus, E. coli) were thoroughly removed from the oyster body and gut. After 5 days, Vibrio density was <5 cells/g and coliforms decreased by 10-15 times, making the product safe for consumption. Main Findings There is considerable interest from management agencies in developing oyster aquaculture in Nha Phu Lagoon and other lagoons in central Vietnam. Our study provides the first description of oyster growth, survival and reproduction in the Nha Phu Lagoon. • Crassostrea lugubris spawns all year but mainly from March to June and again August to October. • The size at first spawning of Crassostrea lugubris is 59 mm, or about 8 mo of age. • Average growth in one year is 80 ± 4 mm or 56 ± 4 g. • Oysters cultured in the intertidal zone have a slightly greater growth rate than oysters cultured in the subtidal zone. • For hygiene and food safety for consumers, oysters should be placed in clean water after harvest for 3-5 days before marketing. Notes Cao Van Nguyen*, Nguyen Tac An, Ho Van The Institute of Oceanography, Vietnam Academy of Science and Technology, 01 Cau Da Street, Nhatrang, Khanh Hoa, Vietnam Acknowledgments We thank Vietnam Academy of Science and Technology for funding to support this study. We thank Nguyen Van Minh and Nguyen Van Hau for protecting the oyster farm in Nha Phu Lagoon. References Angell, L.C. 1986. The Biology and Culture of Tropical Oysters. International Center for Living Aquatic Resources Management. Gosling, E. 2003. Bivalve Molluscs, Biology, Ecology and Culture. Fishing New Books, Blackwell Science, New York, NY USA. Lam. K. and B. Morton, 2004. The Oysters of Hong Kong. The Raffles Bulletin of Zoology 52 (1):11-28. National University, Singapore. Nguyen, C.V. and T.A. Nguyen. 2006. Spat collection and development of growing out the oyster (Crassostrea lugubris) in Nha Phu lagoon, Khanh Hoa province. Collection of Marine Research Works 15:201-209. Quayle, D.B. and G.F. Newkirk. 1989. Farming Bivalve Molluscs: Methods for Study and Development. Advances in World Aquaculture, Vol. 1. Department of Biology, Dalhousie University, Canada. TOP, FIGURE 7. Growth of Crassostrea lugubris in one year in Nha Phu Lagoon. BOTTOM, FIGURE 8. Comparative growth rate of Crassostrea lugubris in intertidal and subtidal environments in Nha Phu Lagoon.
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