en aquaculture aquaculture پژوهشي Orginal research papers Real-time quantitative PCR (qPCR) was performed to elucidate the abundance of <a name="OLE_LINK103"><em>Aeromonas hydrophila</em></a> and <em>Pseudomonas fluorescens</em>, which are among the most widespread fish pathogens in ponds. Both pathogens have three different breeding pattern<a name="OLE_LINK77"></a><a name="OLE_LINK67">s, namely, (a) gibel carp (<em>Carassius auratus gibelio</em>), (b) yellow catfish (<em>Pelteobagrus fulvidraco</em>), and (c) black carp (<em>Mylopharyngodon piceus</em>), over a production season.</a> Results revealed that pond sediments have significantly higher bacterial levels of <em>A. hydrophila</em> and <em>P. fluorescens</em> (<a name="OLE_LINK31"></a><a name="OLE_LINK30">10⁵&ndash;10⁶</a> copies &micro;l^-1 DNA) than pond water (10³&ndash;10⁴ copies &micro;l^-1 DNA). In addition, independent regression models revealed that environmental variables influence the levels of pathogenic bacteria. The occurrence of <em>A. hydrophila</em> and <em>P. fluorescens</em> were significantly positively correlated to <a name="OLE_LINK114"></a><a name="OLE_LINK104">dissolved oxygen</a> and water temperature, respectively. On the contrary, both pathogens were negatively correlated to total nitrogen. In this study, the prevalence of pathogenic bacteria and their relationships with physicochemical factors in different pond environments were investigated for the first time through a molecular method. Furthermore, although we did not include fish diseases occurring during the production season, our results can provide useful theoretical information for fish breeding especially with regard to the prevention of related bacterial diseases. Aeromonas hydrophila, Pseudomonas fluorescens, Freshwater fish, qPCR, Ponds 111 124 http://jifro.ir/browse.php?a_code=A-10-2226-1&slc_lang=en&sid=1 X.M Li 100319475328460032471 100319475328460032471 Yes Key Laboratory of Freshwater Biodiversity Conservation, Ministry of Agriculture of China, Yangtze River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Wuhan, China Y.J Zhu 100319475328460032472 100319475328460032472 No Key Laboratory of Freshwater Biodiversity Conservation, Ministry of Agriculture of China, Yangtze River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Wuhan, China E Ringø 100319475328460032473 100319475328460032473 No Norwegian College of Fishery Science, Faculty of Biosciences, Fisheries and Economics, UiT The Arctic University of Norway, N-9037, Breivika, Norway D.G Yang yangdg@yfi.ac.cn 100319475328460032474 100319475328460032474 No Key Laboratory of Freshwater Biodiversity Conservation, Ministry of Agriculture of China, Yangtze River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Wuhan, China