Optimal dietary requirements of zinc in marine medaka Oryzias melastigma : Importance of daily net flux

The optimal dietary Zn concentrations for a specific fish can vary greatly with different diets or life stages of fish. In the present study, we used the daily net Zn flux (i.e., the rate of flow of Zn per gram weight per day) to quantify the sufficiency of Zn to marine medaka at different Zn-supplement levels (i.e., 0-320 mg Zn/kg). A modified fractional model was also applied to estimate the essential daily net Zn flux in medaka. The feeding experiment results showed that retarded growth, increased mortality, enhanced Zn assimilation efficiency and anorexia appeared in medaka with a low daily net Zn flux (i.e., 0.64 and 1.2 μg/g/day). Once the daily net Zn flux reached the sufficient levels (i.e., 1.7-2.0 μg/g/day), enhanced growth and digestion enzymatic activities were observed. Excess daily net Zn flux (>2.9 μg/g/day) increased the body Zn concentration in fish, but had no beneficial effect on digestion enzymatic activities as well as Zn assimilation efficiency. The essential daily net Zn flux (1.8-2.0 μg/g/day for juvenile medaka) estimated from the modified fractional model was essentially the same as the optimal daily net Zn flux determined from the feeding experiments. Application of such modified fractional model in other fish species can explicitly provide the essential daily net Zn flux for a specific fish at any life stage. Statement of relevance. This study addressed the essential nutritional requirements of mineral in fish and proposed to use daily net flux to quantify the optimum requirements.