Dynamics of Vibrio populations in the rearing water of tiger puffer (Takifugu rubripes)

Members of Vibrio spp. are important risk factors in aquaculture, acting as opportunistic pathogens in marine fish. Therefore, we investigated, using quantitative polymerase chain reaction (qPCR) with Vibrio- specific primers, the behavior of Vibrio spp. during short-term rearing of tiger puffer (Takifugu rubripes). The results showed that the abundance of Vibrio spp. and total bacterial counts increased markedly from the first week after the fish were placed in the tank, and that these values decreased when the fish were removed. The abundance (copies/mL) of Vibrio spp. increased linearly with total bacterial counts, but their density (cells/mL) was estimated to be about two orders of magnitude lower than total bacterial counts. Vibrio spp. in the rearing water were less abundant than other bacterial groups, indicating that many other non-Vibrio microbial communities exhibit similar dynamics. The increase in total bacterial counts, including Vibrio spp., during the rearing period may reflect the spread of pufferfish gut bacteria in the rearing water via the fish feces, along with growth of the bacteria on organic matters derived from mucus, feces, and uneaten food. These results strongly suggest that effective control of vibriosis in fish farms requires surveillance of Vibrio spp., not only in rearing water but also in the fish gut and live diet. The qPCR method used in this study is suitable for surveillance of Vibrio spp. in aquaculture because it is highly accurate and results can be obtained in 5 – 6 h.