Correlates of growth in farmed juvenile southern bluefin tuna Thunnus maccoyii (Castelnau)

In developing the mariculture of the southern bluefin tuna, there is a need to reduce handling of large tuna and to be able to assess their recent growth in research and commercial situations. The potential of various indices as correlates of growth and nutritional status was examined in PIT tagged juvenile tuna grown for 133 days. The study focused on the correlations between specific growth rate for wet weight (SGR) or fork length (SGR(L)) and morphological (condition factor and tissue-somatic indices), physical (muscle temperature) and biochemical (tissue protein, RNA and DNA concentrations) indices. Tuna held in cages were fed twice per day to satiation on one of four feeds (defrosted pilchards, research and two commercial prototypes). Final wet weight, change in wet weight, SGR, SGR(L), condition factor and white muscle concentrations of protein, RNA and DNA were all higher for tagged tuna fed pilchards. Irrespective of diet, the white muscle protein, RNA and DNA concentrations and the ratios between them were all significantly and positively related to SGR. Liver protein, RNA and DNA concentration and the protein:DNA ratio were significantly and positively related to SGR. SGR was predicted by condition factor and by muscle temperature and this was reflected by the positive and significant correlations between muscle temperature and biochemical indices from muscle tissue. Regression models best predicted SGR from condition, pyloric caecal somatic index and white muscle RNA concentration and were tested by predicting (known) SGR of tuna grown in the previous year.