Concurrent epizootic hyperinfections of sea lice (predominantly Caligus chiastos) and blood flukes ( Cardicola forsteri) in ranched Southern Bluefin tuna

Abstract

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. However, to date available data on parasite occurrence before commercial harvesting begins, are scant. This research gathered epizootiological data from weeks 4 to 13 post-transfer, for 153 T. maccoyii sampled from two research and four commercial pontoons. 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 (89.44% female, 10.14% male); adult females of two other species were also present, C. amblygenitalis (0.13%), in addition to an undescribed species, C. sp. (0.04%). Lice counts were correlated positively with gross eye pathology scores ( r s,151 df = 0.3394, p = 0.0000), negatively correlated with condition index ( r s,151 df = −0.5396, p = 0.0000), and positively correlated with plasma cortisol ( r s,131 df = 0.3906, p = 0.0000) and glucose ( r s,131 df = 0.2240, p = 0.0096). 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 negatively correlated with plasma haemoglobin ( r s,151 df = −0.2436, p = 0.0051) and positively with lysozyme ( r s,151 df = 0.3013, p = 0.0019). Fluke counts were also correlated with sea lice counts ( r s,150 df = 0.3143, p = 0.0000). Peaks in these epizootics occurred near the onset of elevated mortalities, which started after 7 weeks of ranching.