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. • Evaluated non-lethal samples to detect Cardicola in harvested Southern bluefin tuna • First large-scale application of non-lethal gill biopsy for Cardicola monitoring • Gill biopsy outperformed necropsy (gold standard) for Cardicola detection • Gill mucus and blood not reliable for monitoring, but useful for other applications • Non-lethal sampling supports earlier and repeated health assessments in live SBT
Carabott et al. (2026) studied this question.