Nucleic acid concentrations and enzyme activities as correlates of growth rate of the saithe Pollachius virens: growth-rate estimates of open-sea fish

The aim of this study was to estimate the growth rate of saithe, Pollachius virens L., in wild populations collected from around the Beryl Alpha oil platform in October 1988 and from Loch Ewe on the west coast of Scotland in August 1989, by comparing the concentrations of various growth-rate indicators in the white muscle with those of laboratory-maintained individuals of known growth rates. There were significant correlations between the individual growth rates of laboratory-maintained saithe and concentrations of white muscle RNA expressed as g RNA mg^-1 protein, mg RNA mg^-1 DNA and as mg RNA g^-1 muscle. Growth rate was also correlated with the activities of the enzymes citrate synthase, cytochrome oxidase and lactate dehydrogenase. RNA concentrations decreased with increasing body size, and weight-corrected estimates of RNA concentrations for a standard-sized individual were determined from the scaling relationships. RNA concentrations expressed in three different ways gave similar estimates of the growth rate for two samples of wild saithe, one from around an oil platform and the other from the west coast of Scotland. RNA concentrations were correlated with aerobic enzyme levels in oil-platform fish, although the growth rate estimates of the same wild fish were lower when expressed as a function of aerobic enzyme levels than when expressed as a function of lactate dehydrogenase concentration. It is concluded that the capacity for protein synthesis of white muscle expressed as RNA concentrations in relation to wet or dry weight may be used as an estimate of instantaneous growth rate and that additional confidence in such estimates may be gained from measurement of the levels of aerobic enzymes.