A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Aquaculture and Aquatic Resources Management, School of Environment, Resources and Development

Thesis (M.Sc.) - Asian Institute of Technology, 2010

Three intensive tilapia ponds were studied to determine the variation of bacterial population in Hai Duong province, Vietnam. Bacteria were identified to genus or species level. Total bacteria count in sediment ranged from 6.49 x105 to 1.29 x106 cfu g-1. In total, 11 bacterial genera and 13 species were identified from pond sediments, in which both Gram-negative and Gram-positive rod-shaped bacteria dominated the population accounting for 52% and 48%, respectively. Bothbeneficial and harmful bacteria were found in sediments, in which the harmful bacterial species dominated (62.5%) and appeared more at the beginning of culture. The harmful bacteria were Pseudomonas fluorescens, Pseudomonas sp., Vibrio pectenicida, Enterococcus, sp.HD31, Enterococcus sp.HD1, Vibrio damsella, Aeromonas sp.HD5, Vibrio sp. HD7, Vibrio sp.HD331, Aeromonas sp.HD361, Enterococcus sp.HD371, Vibrio sp.V31, Aeromonas. Hydrophyla and Pseudomonas putida. In contrast, beneficial bacteria accounted for 20.8% only of total isolates. The beneficial bacteria were Carnobacterium inhibens, Lactobacillus plantarum, Bacillus cereus, Lactobacillus delbrueckii and Bacillus pumilus; they appeared more in the later stage of culture period. This study did not find any correlation between bacteria quantity and the concentration of NH4+, NO2-N, NO3-N in pond water and OC, TP in sediment (P>0.05). However, the significant correlations between NH4 and NO2, NO2 and NO3 concentration were found and the highest concentrations of nitrite and nitrate concentration were found in the fifth culture month. Moreover, this study also showed the highly significant difference (P<0.01) in nitrate reducing bacteria, sulphate reducing bacteria count, and nitrite oxidizing bacteria (P<0.05) during the culture period.