Abstract:
Total ribonucleic acid (RNA) and protein, and the activity of the metabolic enzymes citrate synthase
(CS) and lactate dehydrogenase (LDH) were determined in the white muscle tissue of snapper,
Pagrus auratus, in order to examine their usefulness as indices of somatic growth. Measurements of
these biochemical indices in addition to water content and morphological indices derived from the
length-weight relationship from a population of wild fish were monitored on a monthly basis during
the period February 1998 to January 1999, in the Hauraki Gulf, on the north-eastern coast of New
Zealand.
RNA and protein contents and CS activity declined progressively with increasing body size, whereas
LDH activity increased as body size increased. In contrast, water content showed no correlation with
body size. The significant decline in the concentrations of RNA and protein, and the activity of the
aerobic enzyme CS with increasing body size may indicate a reduction in protein synthesis and
energy production with increasing body size that seems likely to influence growth rate. However, an
increase in the activity of the enzyme LDH with increasing body size may reflect energy demand for
anaerobic burst swimming.
Expressions of biochemical indices per unit body mass and in relation to body size showed seasonal
variation coinciding with the seasonal cycle of snapper somatic growth, and a positive relationship
was observed between RNA, protein content, and CS activity. Concentrations of RNA and protein
and the activity of CS were low in winter when snapper growth decreased and high in spring and late
summer-early autumn when the growth increased.
A significant advance made during this study was the expression of the biochemical correlates of fish
growth in relation to body size that permits an investigation of correlation between two biological
variables (e.g. RNA and body size) with a third environmental variable (e.g. month or temperature).
The slopes of RNA-body size, protein-body size, and CS-body size relationships showed seasonal
variations and indicated interrelated effects of temperature and month on the metabolic process in
snapper. RNA and protein were higher in 1998 than in 1997 indicating higher snapper growth in
1998. However, the slope of the LDH-body size relationship displayed no significant monthly
variation. LDH activity, expressed as units per gram wet weight white muscle tissue, displayed
significant seasonal variation but did not relate to somatic growth.
In contrast, the length-weight relationship indicated growth differences between 1997 and 1998, but
showed no consistent seasonal cycle between years. However, K showed slight differences among
months, but showed inconclusive differences between years. The variability in the length-weight
relationship and K values made them weak growth correlates comparing with the concentrations of
RNA and protein or the activity of CS, which indicated growth variation due to body size and
seasonal effects.