Ontogenetic changes in RNA, DNA and protein contents of laboratory-reared Pacific bluefin tuna Thunnus orientalis

The ontogenetic changes in the growth potential of larval and juvenile laboratory-reared Pacific bluefin tuna were examined based on RNA-DNA and protein-DNA ratios. Experimental fish were reared at the Ohshima Experiment Station of Kinki University Fisheries Laboratory in August 2002. Samples were taken from 13 to 35 days after hatching (DAH). Metamorphosis from larva to the juvenile stage was observed around 23 DAH. Somatic growth of Pacific bluefin tuna was accelerated after metamorphosis. The value of the RNA-DNA ratio from 13 to 19 DAH increased slightly from 3.77 +/- 0.58 (mean +/- SD) to 7.28 +/- 2.23. After that, the ratio markedly increased from 13.89 +/- 3.71 on 21 DAH to 19.11 +/- 4.27 on 23 DAH, which was the end of the metamorphic period. After 25 DAH, the ratio remained at a high level of 15-20. The protein-DNA ratio showed a similar tendency to the RNA-DNA ratio. These results suggest that the rapid increase in the RNA-DNA ratio in the metamorphic period supports the consequent rapid somatic growth in the juvenile stage. The high ratio after the metamorphic period could be because of the species-specific traits large prey exhibit for their survival and because of the tuna's fast-growth after the juvenile stage.