Seasonal epizootics of sea lice, Caligus spp., on southern bluefin tuna, Thunnus maccoyii (Castelnau), in a long-term farming trial

Within the typical 2–8 month (January to August inclusive) farming cycle for southern bluefin tuna, Thunnus maccoyii (Castelnau), in Spencer Gulf, South Australia, counts of a sea louse, Caligus chiastos Lin et Ho, 2003, were strongly statistically associated with both fish condition and severity of eye damage. During a trial examining the feasibility of maintaining T. maccoyii in farms for more than 1 year, including over the summer season when temperatures may exceed 24 °C, we collected additional epidemiological data on burdens of sea lice over a 17-month period (April 2005 to August 2006 inclusive), on a total of 200 T. maccoyii and 40 'control' T. maccoyii farmed and harvested within 2006. In the first farming season, an epizootic of C. chiastos was characterized by a significant increase in prevalence from 0% to 55% in the first 6 weeks after transfer to farms from the wild, which was followed by a significant decline to zero over the next 12 weeks. A single specimen of a second species of Caligus was also detected within this 4.5-month period. In the second farming season, we recorded a third species of sea louse, C. amblygenitalis Tripathi, 1961. In March 2006, a second epizootic peak occurred, this time with mixed infections of C. chiastos and C. amblygenitalis , with a combined prevalence of 100%. The prevalence of both sea lice species then declined significantly over the second winter period (June to August inclusive). On all but one date that sea lice were detected, sea lice counts were significantly associated with the severity of gross eye damage. Because both peaks in infection occurred in summer months (December to February inclusive), we conclude that infections of sea lice pose a risk to the farming of T. maccoyii under certain summer conditions within Spencer Gulf.