Temporal Growth Patterns of Farmed Juvenile Southern Bluefin Tuna, Thunnus maccoyii (Castelnau) Fed Moist Pellets

The growth, condition, and feed utilization patterns of juvenile southern bluefin tuna (SBT) fed moist-pellets were examined over a 19-wk period from March to July 1999. The SBT had significant weight gain over the course of the study, increasing on average from about 27 to 34 kg (dependent on size class). No significant weight gain by the SBT occured in the first 5 wk of the study. Following this initial period of slow growth, the rate of weight gain increased, ranging between 40 to 90 g/d. Weight gain peaked after 11 wk, with no further gain occurring after this time point. Weight gain was strongly related to average daily feed intake (AFI) which was predominantly influenced by water temperature. Weight gain was minimal following the decrease of water temperatures below 15 C, consistent with a decrease in feed intake from peak values at the beginning of the study to basal levels by week 13. Basal feeding levels were maintained for the remainder of the study through to week 19. Although water temperature and AFI were strongly related, other time related effects also appeared to be significant. A relationship between condition index and feed intake was also identified. Condition index of the SBT increased from about 19 kg/m3 to 22 kg/m3 over the 19-wk period. Similar to the patterns observed in growth, there was also an initial delay in increases of condition index. The results from this study support that the majority of weight gain by juvenile SBT occurs during the early part of the production season and that this is most likely influenced indirectly through responses to water temperature. The results of this study also suggest that there is little value in conducting growth trials beyond the point where water temperatures decrease below 15 C. Although there was minimal weight gain or improvement in condition beyond the 11-wk time point, these parameters would need to be considered in conjunction with flesh quality characteristics to identify optimum harvesting regimes.