Microbial water quality in pangasius and tilapia aquaculture systems in Bangladesh

In this study, physicochemical parameters, water quality indicator organisms (total coliforms and E. coli), toxic microorganisms (microcystin, saxitoxin, toxic Vibrio sp.), and off-flavour producers (geosmin and 2-MIB) were evaluated in pangasius and tilapia aquaculture systems in Bangladesh. To maintain good water quality and to produce quality and healthy fish, the systems depend on a diverse microbial community involved in different processes. PCR techniques were applied to identify and quantify the presence of potentially pathogenic microorganisms. The analyses showed that species of Vibrio bacteria were present in 80% of the pond samples and included V. parahaemolyticus, V. vulnificus and V.cholerae. The human pathogen type of V. cholerae (carrying the ctxA gene) was not found. The severe fish and prawn pathogen species V.parahaemolyticus was not detected in the pond samples, while V.vulnificus and V.choleraeoccurred in ponds in Khulna, Myrnensingh and Bogra region. Coliform bacteria were found in all fish ponds and varied from few (60 bacteria per ml) to more than 160,000 bacteria per ml, while E. coli ranged from none to 25,000 bacteria per ml. The highest numbers were found in ponds in the Myrnensingh and Bogra region, while the lowest numbers occurred in the Jessore region. No differences between ponds with pangasius and tilapia farms were obvious, but high abundances of coliform bacteria co-varied with high numbers of E. coli bacteria. The results showed that microbes producing the off-flavour geosmin were common in pond water in all regions, yet our studies of fish only revealed tainting of fish with geosmin at few sites. Genes encoding the toxin microcystin was dominant in pangasius aquaculture system, while saxitoxin-encoding genes were not detected in any of the farms. Based on our observations, we recommend the establishment of regular monitoring systems to control the occurrence of faecal coliforms and microcystin in the aquaculture in Bangladesh, since the extensive and semi-intensive aquaculture systems are very vulnerable to expose of pathogens and toxins.