Relationship between growth and biochemical indices in laboratory-reared juvenile Japanese flounder (Paralichthys olivaceus), and its application to wild fish

In order to estimate growth rates based on biochemical indices of the liver of wild Japanese flounder (Paralichthys olivaceus), juveniles were reared at six ration levels (0, 0.5, 2, 4, 6 and 8% body weight day(-1)) in the la boratory for 14 days, and the relationship between their growth rates and b iochemical indices (RNA/DNA, protein/DNA, triglyceride/DNA, phospholipid/DN A and cathepsin D activities) were determined. Positive and approximately l inear relationships were seen between growth rates and the indices of RNA/D NA, protein/ DNA and phospholipid/DNA. The triglyceride/DNA ratio decreased with increasing growth rates up to approximately 1% body weight day(-1), t hen increased linearly with increasing growth rates. There was no significa nt correlation between growth rates and cathepsin D activity, and the highe st values were obtained in the starved fish. Compared with laboratory-reare d specimens, wild specimens of similar sizes were found to have significant ly larger livers. The RNA/DNA, protein/DNA and phospholipid/DNA ratios of w ild specimens fell in a broad range between ration groups of reared juvenil es. The protein/DNA ratios of wild specimens were low and outside the range of the reared juveniles at six ration levels. In contrast, the levels of c athepsin D activity of wild fish were highest compared to the reared fish. Estimated growth rates of wild fish from the RNA/DNA, protein/DNA and phosp holipid/DNA regressions obtained from the rearing experiment were 1.66, -1. 74 and 0.10% day(-1), respectively. Based on our results, theRNA/DNA index may be regarded as the most valid and reliable growth estimator. It is note d that the larger liver size, the lower liver protein/DNA ratio and the une xpectedly high level of cathepsin D activities of wild specimens found in t his study may reflect the different metabolic conditions of fish reared in the laboratory compared to those collected in the field.