Nutritional condition, as evaluated by RNA/DNA ratios, of hatchery-reared Japanese flounder from hatch to release

Abstract

To evaluate the changes in the nutritional condition of Japanese flounder Paralichthys olivaceus larvae and juveniles before release to the sea, and to investigate the species-specific changes of nucleic acid contents during metamorphosis and the following early juvenile period, biochemical analyses (RNA, DNA, protein content) were conducted. Analyses were performed at 2–3-day intervals during the period from 2 days after hatching (DAH) until 74 DAH, when the flounder juveniles with an average 52.1 mm total length (TL) were released. Special emphasis was placed on the metamorphic phase and all the analyses were carried out daily between 19 and 39 DAH. The average growth rate of fish from the day of hatching to the completion of metamorphosis was 0.34 mm day −1. Growth was retarded and the rate reached nearly 0 a few days after late-metamorphosing stage (H stage) coinciding with a decrease in RNA/DNA. Juvenile fish showed a higher daily growth rate of 0.93 mm day −1 beginning a few days after settlement. The contents of RNA and DNA per individual fish generally increased during development, except for a decrease a few days right after H stage. Four peaks of RNA/DNA ratios were observed at D stage just before onset of metamorphosis, H stage, 35 mm TL (60 DAH), and 4 days prior to release (70 DAH). The first two peaks corresponded to results obtained in previous studies. RNA/DNA ratios reached the third peak at 60 DAH (5.91±0.49). Protein/DNA ratios also increased from 43 to 60 DAH, suggesting that growth by hypertrophy occurred during this period. RNA/DNA ratios decreased for several days following the third peak and the changes in protein/DNA ratios were well correlated with this decrease. Significant decrease in RNA/DNA ratios from 5.21±0.58 on 70 DAH (fourth peak) to 4.43±0.16 on the day of release (74 DAH) is concurrent with a decrease in protein/DNA ratio, indicating the utilization of energy materials decreases in nutritional condition under pre-release treatment with no food supply. These findings suggest that Japanese flounder larvae and juveniles undergo marked changes in the nutritional condition during the early ontogeny even under the conditions of enough food availability, and RNA/DNA ratio is regarded as a useful tool for the evaluation of the nutritional condition of hatchery-reared juveniles to be released.