Dynamics of spp. Infection in Ranched Southern Bluefin Tuna: First Observation of at Transfer.

Aporocotylid blood flukes and are an ongoing health concern for the Southern Bluefin Tuna (, SBT) industry, where infections can lead to morbidity and mortality in ranched SBT populations. This study compared blood fluke infection in SBT from two companies during the 2021 ranching season. Both companies administered the same dosage of praziquantel approximately 5 weeks after transfer, feeding with frozen baitfish daily; the only difference in the company's practices was that the pontoons were located 2.5 km apart. Infection severity was measured as prevalence and intensity by quantifying adult in SBT heart and copy numbers of and ITS-2 DNA in SBT heart and gills. Data from the 2018 and 2019 harvests of SBT were used to make comparisons with 2021 harvest data. was detected at transfer and no longer detected after treatment with praziquantel. spp. were present in 83% of sampled SBT in 2021. Both companies demonstrated similar patterns of infection, and Company A had higher prevalence and intensity of spp. infection. Based on ITS-2 DNA, infection intensity at harvest was significantly greater for both companies in 2021 when compared to 2018 and 2019. Continued monitoring of spp. in SBT and improvements in diagnostics contribute to our understanding of spp. epizootiology and the detection of changes in treatment efficacy.