In late 2018, a commercial Australian black tiger prawn (Penaeus monodon) farm suffered unprecedented mortality events within the hatchery and subsequently earthen pond grow-out production. Complete loss of over half of its hatchery tank populations and grow-out pond populations occurred, with total cumulative mortality in excess of 200 million animals. Investigations performed during the mortality events failed to identify a definitive infectious cause. A project was developed from the need to better understand the range of potential trigger(s) and determining factors that led to the mortality events in both the hatchery and grow-out stages of production. Next Generation Sequencing (NGS) were employed to investigate potential emerging pathogens and corresponding qPCR were developed for the three selected bacterial toxin genes; YaFO, RtX, and zon occludens (also referred to as ZOT). Further testing of archived hatchery post larval samples was conducted to determine the potential significance of the identified toxin genes. A prospective trial was conducted using three commercial grow-out ponds to allow for multidisciplinary diagnostic testing of samples that were purposively collected every 2-3 days to be performed (including microscopy, necropsy, histopathology, and molecular biology), should mass mortality re-occur in these ponds. Passive water sampling devices were also deployed in one of the prospective trial ponds to explore the possible involvement of agri-chemicals pollutants in the mortality events. Finally, an epidemiological analysis of hatchery and grow-out farm (stocking and mortality records) as well as test data was performed to identify potential risk factors associated with mortality events encountered during the 2018/19 commercial production. A better understanding the possible cause(s) of the mass mortality event is likely to be of value to all farms across the Australian prawn farming industry, which may encounter similar events in the future.
The presence or apparent quantity of the YaFO, RtX, and zon occludens toxin genes was not found to be significantly associated with the hatchery or grow-out stage mortalities. No other infectious organism (i.e. virus, bacteria, fungi or parasite) could be identified as a likely cause of the mortality events either in the hatchery or grow-out stage of production. None of the three prospective trial ponds experienced the mass mortalities equivalent to the previous season on the farm. Retrospective epidemiological analysis identified a range of risk factors, associated with the mortality events in the hatchery and grow-out, which helped the farm implement system changes for the following season. Further project findings will be presented.
The project was funded by the Australian Prawn Farming Association (APFA) and Fisheries Research and Development Corporation (FRDC).