Infections by the gill fluke Zeuxapta seriolae are a serious concern for sea cage aquaculture of kingfish, Seriola lalandi. The present study aimed to determine the pathophysiological effects of a progressive infection with Z. seriolae and the effects of treatment with hydrogen peroxide. For the progression of infection study, infected fish were taken from a sea cage farm, treated to remove parasites and then infected by cohabitation with heavily infected fish. Samples were taken at 2-week intervals for 8 weeks. Infection intensity peaked at 4 weeks post-infection (mean intensity 565.9) and the number of mature worms (2 mm fixed length or larger) peaked at 6 weeks post-infection. Attachment of Z. seriolae appeared to cause little localized pathology; however, the occurrence of hyperplastic lamellae increased as the infection progressed. Haemoglobin concentrations were negatively correlated with Z. seriolae intensity and were lower than controls at 4 weeks (35.8% decrease) and 6 weeks (57.4% decrease) post-infection. Blood lactate concentration and plasma osmolality increased throughout the course of infection. For the effect of treatment experiment, groups of infected and non-infected fish were sampled either before or after treatment with hydrogen peroxide. Treated fish from both infected and uninfected groups had increased plasma lactate, osmolality and pH compared with pre-treatment groups. Treatment with hydrogen peroxide appeared to have acute effects on fish health but the magnitude (e.g. lactate, osmolality) and extent of the effects (e.g. haemoglobin) was much less than that caused by chronic infection with Z. seriolae.

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The hooking mortality of two teleosts, the yellow stripey Lutjanus carponotatus (Lutjanidae), and the wire netting cod Epinephelus quoyanus (Serranidae), was examined for fish captured with lures and bait from shallow waters (<2 m) on the Great Barrier Reef. Total mortality for both species (n = 340 fish) over the 48-h observation period was low (1.76%). Baitfishing with single hooks caused a significantly higher post-release mortality rate (5.1%) than did lure fishing with treble or single hooks (0.4%), and was the hooking method most likely to cause bleeding and damage to vital organs. Death of fish was observed only in instances where hooks penetrated the pericardium or body cavity. Handling time was significantly affected by fish size and hooking location, did not vary significantly between fish species, and was significantly reduced when barbless hooks were used in both lure and baitfishing. One specimen of each species, deeply hooked in the gut or oesophagus while baitfishing, was allowed to retain the hook; both fish survived and subsequently regurgitated the hook during the observation period. The relevance of these data to management of line fisheries on the Great Barrier Reef is discussed.

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