Biochemical correlates of growth in New Zealand snapper, Pagrus auratus

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.