Health and performance of ranched southern bluefin tuna
Creators: name: Kirchhoff, NT
Publisher: name: University Of Tasmania
Year: 2023

description: The Australian southern bluefin tuna (SBT) industry is continually looking for ways to improve fish health within the ranching environment, both for increased profitability and concern for animal welfare. This thesis utilized current and newly acquired knowledge about southern bluefin tuna health to make educated manipulations of their ranching environment and/or husbandry practices. SBT health, parasites and performance where then monitored as a measurement of success. Five major projects were completed, all projects completed on the commercial scale. Three projects focused on environmental and/or husbandry manipulations: (chapter 2) dietary supplementation of immunostimulants and vitamins; (chapter 4) moving cages further offshore; (chapter 6) an assessment of two management strategies for blood fluke, Cardicola forsteri, infection. While two projects aimed to gain more information about ranched SBT health: (chapter 3) a detailed description of the first two months of ranching; (chapter 5) correlation of SBT humoral immune response with infection stage of C. forsteri. Greater knowledge was obtained relating to the effects of ranching. Over the first two months, weight, length, condition index, hemaoglobin concentration, and immune response were all found to change significantly. Ranched SBT were found to acclimate to ranching within one month post transfer and were relatively healthy prior to an acute mortality event from week five to seven of ranching, which resulted in a cumulative mortality of 8.5%. The mortality event was associated with decreased hemaoglobin concentrations and changes in immune response. Additional information was also gained on one of the most common and significant infections during ranching, blood fluke Cardicola forsteri. The timeline for C. forsteri infection was validated using natural infections in SBT. Humoral immune response (i.e. lysozyme, alternative complement and specific antibody activity) was correlated to infection and was found to develop concurrently with C. forsteri. The majority of physiological response coincided with commencing egg production, at approximately 5 to 7 weeks of ranching. New and previous knowledge regarding C. forsteri infection in SBT were merged, resulting in a inclusive infection, physiological response and diagnosis timeline. Further enhancement of ranched SBT health and performance was also obtained within this thesis utilizing dietary supplementation, adjustments in ranching location, and chemotherapeutics. Supplementing the diet of ranched SBT for the first twelve weeks with Vitamins E and C resulted in 1.5 times higher lysozyme activity at 8 weeks of ranching. Vitamin supplementation was also associated with tow specific improvements in performance, including enhanced survival, decreased Cardicola forsteri prevalence and intensity, and enhanced alternative complement activity. No changes in health, immune response, and performance were associated with immunostimulant supplementation. Examination of the feasibility of offshore ranching yielded significant effects on the health and performance of ranched SBT. Compared to SBT ranched in the traditional near shore environment, SBT ranched further offshore had enhanced survival, increased condition index at 6 weeks of ranching, and superior health. The offshore cohort had no C.forsteri and a 5% prevalence of Caligus spp. compared to a prevalence of 85% for C. forsteri and 55% prevalence for Caligus spp. near shore at 6 weeks of ranching. In addition, offshore ranched SBT had elevated hemaoglobin concentration and lysozyme activity. Finally, management of C. forsteri infection was examined utilizing two management strategies: (1) chemotherapeutic treatment with Praziquantel and (2) temporary offshore ranching. Both management strategies successfully reduced infection as well as mortality, yet evidence of reinfection and/or delayed infection suggest further research needs to be completed to optimize these strategies.