Muscle and liver tissues are widely used in trophic ecology studies of fish but often require lethal sampling, raising logistical, ethical, and conservation concerns. We investigated the use of pectoral fin clips as a non-lethal, minimally invasive alternative for studying the trophic ecology of southern bluefin tuna (Thunnus maccoyii), an endangered species of economic and ecological value. Muscle, liver, and pectoral fin clips were analysed for stable isotope composition (δ13C, δ15N, C:N), lipid content, and fatty acid profiles across their migratory range of southern Australia, accounting for variation by region and year. This approach assessed whether the trends in spatiotemporal variation in biochemical markers from muscle and liver were detectable with fin clips. We found significant regional and temporal variation in isotopic composition, lipid content, and fatty acid profile, with muscle tissue most effective at capturing these trends, while liver tissue showed more variability. Fin clips did not capture spatiotemporal variation in isotopes and lipid content as effectively as other tissues, limiting their value for detailed dietary or environmental analyses. However, they were useful for detecting variation in fatty acids, particularly polyunsaturated fatty acids EPA and DHA, which are important for human nutrition. Although fin clips may not fully capture spatiotemporal variation in isotopes and lipid content, they provide a less invasive option for robust fatty acid analysis, supporting nutritional monitoring in captive settings (aquaculture and aquariums) and conservation of endangered pelagic teleosts. This study highlights the value of evaluating multiple biochemical approaches to assess the efficacy and limitations of different tissue types in trophic ecology.
Aporocotylid blood flukes Cardicola forsteri and C. orientalis are considered one of the most significant health concerns for Southern Bluefin Tuna (SBT) Thunnus maccoyii ranched in Australia. There is a need for rapid point-of-need diagnostics to detect Cardicola spp. in SBT to allow the industry to make timely management decisions. Recombinase polymerase amplification (RPA) is an isothermal technique which operates at constant low temperature (25-42˚C), and when coupled with a lateral flow (LF) strip, makes an ideal diagnostic tool for rapid, specific, and sensitive identification of pathogens in field applications. RPA-LF assays were designed and validated for detection of C. forsteri and C. orientalis. For each assay, no cross-species amplification was seen and detection as low as 30-50 gene copy equivalents was achieved. Reactions can be completed in 10 minutes. Similar specificity and sensitivity were demonstrated for SBT samples when compared to qPCR analysis, and use of equipment-free incubation using body heat outside of laboratory settings was demonstrated. By developing rapid, ready-to-use diagnostics, the SBT industry can identify risks relating to blood flukes far quicker than is currently possible.
Aporocotylid blood flukes Cardicola forsteri and C. orientalis are an ongoing health concern for Southern Bluefin Tuna (SBT), Thunnus maccoyii , ranched in Australia. Therapeutic application of praziquantel (PZQ) has reduced SBT mortalities, however PZQ is not a residual treatment therefore reinfection can occur after the single treatment application. This study documents the epidemiology of Cardicola spp. infection in ranched SBT post treatment over three ranching seasons (2018, 2019 and 2021). Infection prevalence (percentage of SBT affected) and intensity (parasite load) was determined by adult fluke counts from heart, egg counts from gill filaments and the use of specific quantitative polymerase chain reaction (qPCR) for detection of C. forsteri and C. orientalis ITS-2 DNA in SBT hearts and gills. SBT Condition Index decreased as intensity of Cardicola spp. DNA in SBT gills increased, suggesting blood fluke infection had a negative effect on SBT growth (Spearman’s r = −0.2426, d.f. = 138, p = 0.0041). Prevalence and intensity of infection indicated PZQ remained highly effective at controlling Cardicola spp. infection in ranched SBT, 10 years after PZQ administration began in this industry. Company A had the highest prevalence and intensity of Cardicola spp. infection in 2018, and Company G had the highest in 2019. No consistent pattern was seen in 2021. Overall, intensity of infection did not increase as ranching duration increased post treatment. Results from this study improve our knowledge of the biology of blood flukes and helps the SBT industry to modify or design new blood fluke management strategies to reduce health risks and improve performance of SBT.
Aporocotylid blood flukes Cardicola forsteri and C. orientalis are an ongoing health concern for the Southern Bluefin Tuna (Thunnus maccoyii, 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 C. forsteri in SBT heart and copy numbers of C. forsteri and C. orientalis 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. Cardicola orientalis was detected at transfer and no longer detected after treatment with praziquantel. Cardicola 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 Cardicola spp. infection. Based on C. forsteri ITS-2 DNA, infection intensity at harvest was significantly greater for both companies in 2021 when compared to 2018 and 2019. Continued monitoring of Cardicola spp. in SBT and improvements in diagnostics contribute to our understanding of Cardicola spp. epizootiology and the detection of changes in treatment efficacy.
