Groups of recently hatched fry of rainbow trout, Oncorhynchus mykiss were maintained in the laboratory in order to investigate the effects of age, ration level and temperature on whole body growth, nucleic acid concentrations, protein synthesis rates and enzyme activities. In fry of up to 30 days after hatching, which were feeding but still had some yolk sac, no significant change in mean RNA concentration was observed with ration level. In older fry of 50 days or more, when the yolk sac was completely absorbed and exogenous feeding fully established, the concentration of RNA was correlated with the rate of protein growth. RNA concentrations and activities of citrate synthase and lactate dehydrogenase were significantly different between fed and starved fry. As water temperature was raised (from 5 to 15° C), higher rates of protein growth were brought about by an increase in the rate of protein synthesis and also by increased efficiency of retention of synthesized protein (reduced protein turnover). In fed fry, no change in RNA concentration was found with increasing temperature, while the amount of RNA per cell (RNA: DNA) decreased, indicating that increased rates of protein synthesis were due to increased RNA efficiency.

Juvenile cod were acclimated to cold (5 °C) and warm (15 °C) water temperatures and fed sandeel at a similar ration size (3% body weight∙day−1) for at least 40 days. After this acclimation period, there were no significant differences in either weight-specific growth rate or weight-specific tissue protein synthesis rates (ventricle, gill, stomach, and intestine) between the cold- and warm-acclimated fish. However, every cold-acclimated tissue examined had a significantly higher RNA concentration (μg RNA∙g tissue−1) than the respective warm-acclimated tissue. Cold-acclimated ventricle and intestine had significantly reduced RNA activities (i.e., translational efficiency, g protein synthesized∙g RNA−1∙day−1) compared with the warm-acclimated tissues. In contrast, the mean RNA activities of cold-acclimated stomach and gill were not significantly different from those of the same tissues in the warm-acclimated fish. These alterations in RNA activity and RNA concentration with temperature acclimation probably represent a thermal compensatory mechanism for protein synthesis and growth in cod at 5 °C. Positive linear relationships were observed between tissue protein synthesis rates and tissue RNA concentrations (μg RNA∙g tissue−1). RNA/protein ratios (μg RNA∙mg protein−1) gave a positive (but statistically insignificant) trend with protein synthesis rates. In contrast, a negative trend (statistically insignificant) was observed between tissue protein synthesis rates and tissue RNA/DNA ratios (μg RNA∙μg DNA−1). The use of RNA measurements as biochemical correlates of growth rate in juvenile cod is discussed.

Herring Clupea harengus larvae were hatched and reared for 66 d in the laboratory to test hypotheses concerning correlations between nucleic acid concentrations and growth rates. A modified absorbance method is introduced which allowed RNA, DNA and protein to be measured on individual larvae. The amounts of RNA, DNA and protein increased per larva but no significant correlation was found between protein growth rates and RNA concentration, RNA:DNA or RNA:protein. After 7 d of starvation there was a significant loss of RNA, DNA and protein only in larvae 65 d old. It is concluded that nucleic acid ratios, except perhaps protein to DNA ratios, are not valuable for estimating growth rates of wild larvae.