Nutritional
condition, as evaluated by RNA/DNA ratios, of hatchery-reared Japanese
flounder from hatch to release
W.S.
Gwak, T. Tsusaki, M. Tanaka-2003
Aquaculture, 219(4): 503-514
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.
(Division
of Applied Biosciences, Graduate School of Agriculture, Kyoto University,
Kyoto 606-8502, Japan, e-mail: wsgwak@kais.kyoto-u.ac.jp)