Protecting valuable stock: Non-lethal sampling approaches for Cardicola spp. monitoring in ranched Southern bluefin tuna

Reliance on lethal sampling to monitor Cardicola spp. infections in ranched Southern bluefin tuna (Thunnus maccoyii, SBT) constrains investigative time points and number of fish available for sampling, restricting health monitoring to commercial harvest periods. For these reasons six non-lethal sampling strategies from three biologically relevant sites in SBT were evaluated for their ability to detect Cardicola spp. infection severity relative to lethally collected gill necropsies, which are currently considered the diagnostic gold standard. From harvested SBT, gill biopsies demonstrated the highest sensitivity to reliably detect Cardicola spp., with higher infection prevalence than gill necropsies. Despite the appeal of blood and gill mucus as non-lethal samples, they were not effective for detecting Cardicola spp. in this study. This is likely related to the biology of the parasite, with DNA in these fluids representing migrating or residual stages of the parasite rather than established infections. The ability to assess fish health without sacrificing valuable stock represents an important methodological advance in aquaculture health monitoring and management. Gill biopsies are anticipated to be used in conjunction with other sampling techniques to enhance overall monitoring of SBT health, supporting improved blood fluke monitoring and assessments of treatment efficacy that inform management decisions. However, targeted welfare investigations are required prior to routine implementation.