Abstract

Bacteria are very vital organisms in aquaponic system decomposing and converting the toxic components of the fish waste into usable vegetable nutrients; however, some unwanted bacteria could cause fish diseases. The purposes of this research were to compare the amount of bacteria in water from aquaponic catfish cultured in different stocking densities and Denaturing Gradient Gel Electrophoresis (DGGE) of Polymerase Chain Reaction (PCR)-amplified 16S rDNA was applied to characterise the bacterial populations. Three different stocking densities; 80, 100 and 120 fish/m² of hybrid catfish were cultured in aquaponic system. Each treatment contained 3 replications; Treatment 1, 360 catfish (80 fish/m², without lettuce); Treatment 2, 360 catfish (80 fish/m², without lettuce); Treatment 3, 450 catfish (100 fish/m²); and treatment 4, 540 catfish (120 fish/m²). This research was conducted for 4 months. Water was sampled two times before fish stocking, it was found that the number of bacteria in the first week were higher than the third week. The average number of bacteria in water from treatment 1, 2, 3 and 4 were (2.49 ± 0.40) ×10⁷, (2.43 ± 0.23) ×10⁷, (2.48 ± 2.22) ×10⁷ and (2.29 ± 2.38) ×10⁷ Colonies forming unit (CFU)/ml respectively. According to four-time samplings after catfish were stocked, the average number of bacteria in water from treatment 1, 2, 3 and 4 were (1.85 ± 0.53) ×10⁷, (2.73 ± 0.15) ×10⁷, (2.45 ± 0.30) ×10⁷ and (2.24 ± 0.19) ×10⁷ CFU/ml respectively. The average number of bacteria in water after lettuce harvest from treatment 1, 2, 3 and 4 were (0.185 ± 0.04) ×10⁷, (0.26 ± 0.04) ×10⁷, (0.30 ± 0.01) ×10⁷ and (0.29 ± 0.05) ×10⁷ CFU/ml respectively. There were no differences in bacterial counts among treatments (p > 0.05). In addition, no significant difference was observed in the number of bacteria in filter tanks (p > 0.05) and the number were quite similar to the ones in catfish tanks. Aeromonas hydrophila and Flavobacterium columnare infected catfish at the beginning of the experiment; however, it got better and no more death found after acclimation. The DGGE analysis of PCR-amplified 16S rDNA fragments revealed that bacterial populations in water were dominated by Aeromonas spp., Pseudomonas spp. and Staphylococcus spp. Antibiotic sensitivity test indicated that enrofloxacin might be the best option for bacterial infection treatment. The results indicate that aquaponic system could reduce the wastewater from catfish culture; however, maintaining a good water quality environment for fish culture should take into account.

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