Investigatión of mortality causes in farmed southern bluefin tuna (thunnus maccoyii).

Abstract

The Australian southern bluefin tuna (SBT) (Thunnus maccovii) farming operation is currently considered the largest of this type in the world. The health status of SBT had been good until recent times. During the !ast few years an increase in mortalities of SBT a few weeks post-transfer to the Port Lincoln area has concemed members of this industry. This project aimed to identify possible causes of mortalities in SBT by comparing normal, moribund fish and fresh mortalities. Sampling of fish was camed out in the 2008 SBT farrning season from commercial cages from 5 different companies. Fish were allocated into two groups: normal and moribundldead, which were compared using a range of analyses. Intensity and prevalence were calculated for the counts of ectoparasites; cerebrospinal fluid (CSF) was analysed for the presence of Uronenza nigricans; blood samples were analysed for leveis of haemoglobin and plasma used for cortisol, lactate, pH, osmolality and lysozyme estimation; microbiological samples from spleen and kidney were obtained in TCBS/BA plates and isolated bacteria were identified. Histological samples from diverse organs were also collected and slides were examined at different magnifications for the presence of any histopathological changes or pathogens. Granulomatous reactions caused by Cardicola forsteri in the heart were categorized. Hearts were flushed in order to remove and count any adult C. forsieri prescnt and to calculate prevalence and intensity of infection.