Peaks in epizootics of sea lice (mostly Caligus chiastos) and blood flukes (Cardicola forsteri) among Southern Bluefin Tuna (Thunnus maccoyii) appear to coincide with the onset of an increased mortality. The mortality event occurs 6-12 weeks after T. maccoyii have been transferred into static ranching pontoons from the wild. This research gathered epizootiological data from weeks 4 to 13 post-transfer.
Counts of both parasites in the research pontoons reached levels far heavier than previously documented in ranched T. maccoyii. For sea lice, the prevalence in most pontoons was 100%; the highest intensity reached 495 individuals, and mean counts at the peak of the infection exceeded 265 lice per fish. Almost all of the 5407 individual lice counted were identified as adult C. chiastos. Lice counts were correlated positively with increased eye pathology, negatively correlated with condition index (weight to length ratio), and positively correlated with plasma cortisol (stress hormone). For the blood fluke, prevalences were less uniform than those of sea lice, with lower rates of infection at the beginning (ranging from 10% to 40%), reaching 100% mid-study, and declining again (40% in one pontoon). The highest intensity reached 441 individual flukes. Fluke counts were correlated with sea lice counts. Peaks in these epizootics occurred near the onset of elevated mortalities, which started after 7 weeks of ranching.
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