Detection of Miamiensis avidus (Ciliophora: Scuticociliatia) and Cardicola spp. (Trematoda: Aporocotylidae) DNA in biofouling from Southern Bluefin Tuna, Thunnus maccoyii pontoons off Port Lincoln, South Australia

Presence of biofouling on pontoons and other structures can have adverse effects on fish health, both by affecting water quality and acting as a reservoir for pathogens. This study focused on three species of parasites affecting Southern Bluefin Tuna (Thunnus maccoyii) (SBT): the blood flukes Cardicola forsteri and Cardicola orientalis and the scuticociliate Miamiensis avidus. Blood flukes are the main health concern for SBT. They have two free living stages (miracidium and cercaria) and their intermediate host can be present in biofouling. Miamiensis avidus (Ciliophora: Scuticociliata) is an opportunistic pathogen thought to be the causative agent of swimmer syndrome in SBT. To determine if biofouling is a reservoir for blood flukes or M. avidus six perspex plates and six net pieces were deployed in two SBT pontoons at one and four meter depths and sampled one and three months after deployment. Biofouling samples were rarely positive for blood fluke DNA based on qPCR detection, but they were most frequently detected on plate samples at 3 months. Prevalence of Miamiensis avidus based on qPCR detection increased from 38% at one month to 89% at three months. No significant difference was observed between depths at which the plates were deployed. Miamiensis avidus was detected from total DNA extracted from a wide range of taxonomic groups collected from the fouling samples. Results suggest that biofouling may act as a reservoir for M. avidus in aquaculture. Monitoring environmental reservoirs may be an important non-destructive surveillance tool. Further optimization of the detection in biofouling may provide insights into host-pathogen interactions which will inform aquatic animal health management. This approach could also be applied to the surveillance of other potential aquaculture pathogens affecting species farmed in marine pontoons.