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


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Hayward, CJ, Aiken, H and Nowak, BF 2008 , 'An epizootic of Caligus chiastos on farmed southern bluefin tuna Thunnus maccoyii off South Australia' , Diseases of Aquatic Organisms, vol. 79, no. 1 , pp. 57-63 , doi: 10.3354/dao01890.

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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 ·

Item Type: Article
Authors/Creators:Hayward, CJ and Aiken, H and Nowak, BF
Journal or Publication Title: Diseases of Aquatic Organisms
ISSN: 0177-5103
DOI / ID Number: 10.3354/dao01890
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Copyright © 2008 Inter Research

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