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Diseases of Aquatic Organisms

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DAO 79:57-63 (2008)  -  DOI:

An epizootic of Caligus chiastos on farmed southern bluefin tuna Thunnus maccoyii off South Australia

Craig J. Hayward1,2,*, Hamish M. Aiken1, Barbara F. Nowak1

1School of Aquaculture and Aquafin Cooperative Research Centre, University of Tasmania, Locked Bag 1-370, Launceston, Tasmania 7250, Australia
2Present address: South Australian Research and Development Institute, Lincoln Marine Science Centre, PO Box 1511, Port Lincoln, South Australia 5606, Australia

ABSTRACT: In some years, large numbers of Caligus chiastos have been observed on the external surfaces of southern bluefin tuna, particularly on the head and eyes, in some sea cages in Spencer Gulf, Australia. As no epidemiological data were available, we monitored sea lice on tuna (N = 130) in 4 research cages sampled at 6 wk intervals during the 2005 farming season. No lice were observed on a sample of 10 wild-caught tuna when the cohort was transferred to cages in early April. By late May more than half the sampled tuna (22 of 40) were infected, with up to 42 parasites; we also recorded one unidentified Caligus sp. at this time. In early July the number of tuna infected with lice declined to 10%; in the final sample in late August none were detected. Prevalence in May was significantly higher than on other dates (p ≤ 0.001), whereas mean abundances did not differ significantly (p > 0.05). The decline in prevalence corresponded with a seasonal fall in temperature, from ca. 17°C in May to 14°C in August. Counts of lice at the peak of infection were associated with the severity of eye damage (Spearman’s rank correlation coefficient, rS,38df = 0.654, p < 0.001); this may be because lice graze on the cornea or because tuna injure their eyes when flashing (rubbing against objects). Counts at this time were also strongly and inversely correlated with the condition index (rS,38df = –0.707, p < 0.001). It appears that tuna become infested with adult sea lice via wild teleosts and elasmobranchs attracted to sea cages.

KEY WORDS: Sea lice · Caligids · Copepoda · Parasite · Southern bluefin tuna · Thunnus maccoyii · Epidemiology

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Cite this article as: Hayward CJ, Nowak BF, Aiken HM (2008) An epizootic of Caligus chiastos on farmed southern bluefin tuna Thunnus maccoyii off South Australia. Dis Aquat Org 79:57-63.

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