Stress and immune responses of ranched southern bluefin tuna (Thunnus maccoyii castleneau)

Aquatic organisms under culture conditions are faced with an assortment of potential stressors that challenge the homeostatic state. The stress and immune responses of ranched southern bluefin tuna (SBT), Thunnus maccoyii were investigated. Quota restrictions by the CCSBT progressively had reduced Australia's allocation of the SBT global TAC to 5,265 tonnes per annum for the period 1989 to 2009. In response to quota reductions, SBT ranching developed as a capture-based aquaculture operation dependent on wild sourced stock. Other capture-based aquaculture operations exist in Japan and Mexico for Pacific bluefin tuna (PBT), Thunnus orientalis, and in numerous Mediterranean countries for Atlantic bluefin tuna (ABT), Thunnus thynnus. Overseas operations initially used traditional catch methods, like set net traps and line hooking, to source wild bluefin, but in the mid 1990s adopted the SBT model of purse seine capture and tow cage transport to grow-out sites. SBT capture/transport mortalities (0.5% down to 0.08% in recent years) compared favourably to overseas experience (ABT up to 2%, PBT 1 to 3%) as did initial total grow-out mortalities (2 to 5% for SBT, 3 to 5% for ABT, 6% for PBT). Increasing SBT mortalities in the mid 2000s, with an annual spike from 6 to 11 weeks after transfer, have been attributed to the aporocotylid parasites Cardicola forsteri and C.orientalis. Following an analysis of industry tow practices, suggested optimal parameters for SBT operations were tow densities of less than 0.3 fish / m3, tow biomass less than 5 kg / m3 andaverage tow speeds of around 0.5 knots. Stress indicators including cortisol, glucose, lactate, pH, osmolality, haemoglobin, insulin-like growth factor (IGF) and immune responses lysozyme and immunoglobulin (IgM) were of varying utility in elucidating SBT health aspects under ranching conditions. Suggested base reference levels in wild SBT were cortisol 10 to 70 ng/mL, glucose 5 to 10 mmol/L, lactate 1 to 10 mmol/L, pH 7.5 to 8.5, osmolality 375 to 460 mOsmol/kg, haemoglobin 13 to 21 g/dL, IGF 12 to 55 ng/mL, lysozyme 5 to 40 ˜í¬¿g/mL and IgM 4 to 16 mg/mL. An acute primary stress response was evident at the completion of tows and elevated cortisol, lactate, osmolality and lysozyme levels suggested a modest chronic stress response throughout the ranching period. IGF levels mid season suggested production potential was not influenced greatly by chronic stress. Glucose, pH and IgM levels were not affected over ranching timeframes. Cortisol, lactate and osmolality levels under long-term holding (LTH) of SBT indicated the stress response was heightened and endured over the 18 month grow-out period. LTH glucose levels also indicated a stress response although lysozyme levels were less responsive within the 2005 LTH SBT cohort. SBT appear to have a greater surface area cover of melanomacrophages (MMCs) within their haemopoietic organs than many fish. Kidney MMC area (1.21 + 0.17 %) was unresponsive to the influences of ranching whereas splenic MMC area (1.25 + 0.13 %) increased in a significant and predictable manner up to 2.36 times the initial area. Liver MMCs were very seldom detected in ranched SBT. The pigments melanin, haemosiderin and lipofuscin were all present in SBT MMCs, with kidney MMCs having more lipofuscin than haemosiderin, and splenic MMCs having high haemosiderin and less melanin. It was suggested that the increased splenic deposition likely was due to an increased exposure to exogenous waste material associated with infection by parasites, Cardicola spp. This study recommended optimal tow practices for SBT capture-based operations and documented stress and immune responses as a result of tows, and of ranching over grow-out periods of 6 to 8 months. It provided collaborating evidence for the extent and timing of the onset of substantive adverse impacts from the infection of ranched SBT by Cardicola spp., around 2008. It documented the heightened and enduring stress response of SBT under an extended grow-out period and the associated immunosuppression from reduced complement activity pathways. The MMC characteristics of the kidney and spleen of SBT were documented for the first time and suggested a high capacity to process antigens and other foreign material to which aquaculture species can be exposed. Splenic MMCs were suggested to be a reliable, post-hoc measure for comparison of SBT health performance under ranching conditions.