The measurement of visceral temperature patterns and implications for feeding practices in ranched southern bluefin tuna Thunnus maccoyii
Creators: name: Ellis, DC
Publisher: name: University Of Tasmania
Year: 2023

description: Southern bluefin tuna (Thunnus maccoyii) warm their viscera when digesting food. Through surgical implantation of archival tags, this thesis explores visceral warming patterns in southern bluefin tuna (SBT) with the aim of identifying relationships between visceral heat, nutrient supply, feed frequency and efficiency in SBT. Based on six trials with different but related objectives, it was found that dietary energy influences visceral warming, time taken to reach peak visceral temperature and duration of visceral warming when SBT receive one meal per day. When SBT are fed more than one meal per day, feed intake may be measured when dietary energy is known and water temperatures are cool. Different industry feeding practices were shown to have no impact on visceral warming patterns when SBT were regularly fed two times per day with a high energy diet compared with six times per day using a low energy diet emphasising the importance of providing an appropriate protein and lipid balanced diet. SBT visceral warming patterns in this trial altered when regular meals were missed. An 18 week trial involving four baitfish feeding treatments with different protein to lipid ratios fed in 3 x 6 week time periods demonstrated that maintaining a consistent feed profile of approximately 7 % lipid especially in the first 6 weeks of culture will optimise SBT performance in respect to growth, food conversion and body condition. Specific growth rates from all treatments were significantly better than Atlantic bluefin (Thunnus thynnus) of the same size and age. An analysis of dietary energy with regard to visceral warming showed that dietary energy is not a reliable measure of feed intake and that visceral warming is more influenced by water temperature and feeding behaviour. In cooler water temperatures SBT feed less, increase visceral warming and conserve heat. In warmer water temperatures SBT feed more and expend visceral heat suggesting that SBT have a physiology response to body temperature that is not directly related to dietary energy intake. A trial investigating visceral and tissue temperature profiles showed that at water temperatures of 20oC or less, SBT maintain basal and maximum visceral temperatures between 4oC and 10oC above ambient water temperature and that visceral temperatures can be predicted with confidence. At water temperatures above 20oC the relationship between basal and maximum visceral temperatures and water temperature may be predicted with less certainty. SBT maintain red muscle temperature at approximately 30oC irrespective of feeding regime or water temperature, white muscle temperature at approximately 6oC above water temperature irrespective of water temperature and feeding regime, but visceral cavity temperature of SBT is influenced by both water temperature and feeding regimes in water temperatures up to approximately 20oC. Temperature profiles developed through this research suggest that water temperatures above 20oC lead to heat stress in SBT.