Sea lice infections of wild fishes near ranched southern bluefin tuna (Thunnus maccoyii) in South Australia

In contrast with sea lice infestations of other farmed fishes, attached larval stages of sea lice on ranched southern bluefin tuna (Thunnus maccoyii) are rarely detected. In this study, we monitored sea lice on ranched T. maccoyii and surveyed wild fishes adjacent to ranching sea cages over a 3-month period in early 2009. Prevalence of the adult Caligus chiastos on tuna within a day of arrival at the ranching site was 10%; prevalence then increased significantly and peaked almost 25 days later to 75%; by harvest (after a further 18–28 days), prevalence decreased significantly to 0%. We collected and examined a total of 502 wild fishes outside T. maccoyii sea cages, comprising 307 Degen's leatherjackets (Thamnaconus degeni), 136 yellowtail horse mackerel (Trachurus novaezelandiae), 31 sand trevally (Pseudocaranx wrighti), 10 West Australian salmon (Arripis truttacea), 6 Port Jackson sharks (Heterodontus portusjacksoni), and a single blue mackerel (Scomber australasicus) and pilchard (Sardinops sagax); we also examined an additional 10 pilchards that were collected from the centre of Spencer Gulf and stored fresh in a T. maccoyii feed bin. Of these potential hosts, we identified adult C. chiastos only from Degen's leatherjackets; of the many larvae also occurring on this host, molecular comparison of five specimens analysing cytochrome C oxidase I region of mitochondrial DNA and five specimens analysing partial D1–D2 domains of 28S rDNA confirmed that these were C. chiastos. In contrast with the decline in infections of C. chiastos on ranched T. maccoyii near the end of March, on Degen's leatherjackets there was a significant increase in prevalence and abundance over the study period, with a peak prevalence of 97.14% and a mean abundance reaching 11.17 lice per fish near the end of April. The percentage of chalimus larvae on Degen's leatherjackets increased over the study period, ranging from 0% near the start of sampling to over 93% on the final sample date. We also recorded additional copepod infestations, including Orbitacolax williamsi on Degen's leatherjackets, Caligus sp. on sand trevally, and Dissonus nudiventris on Port Jackson sharks. We conclude that Degen's leatherjacket, which is a major scavenger of excess tuna feed, is likely to contribute to sea lice infestations of T. maccoyii. ©