Availability to Rainbow Trout of Zinc in White Fish Meal and of Various Zinc Compounds

Abstract

A feeding experiment was conducted to examine availability to rainbow trout of Zn contained in white fish meal and that of various Zn compounds (ZnSO₄, ZnNO₃, ZnCl₂, 5ZnO•2CO₃) and to determine a minumum supplementary Zn level to white fish meal based diet for normal growth of rainbow trout. Fingerlings weighing 0.3g in average were fed with the test diets for 40 weeks at water temperature of 5-20°C. The lowest growth and the highest rate of occurrence of dwarfism together with the lens cataract were observed in fish receiving a diet without supplementary Zn (No-Zn) and these symptoms were effectively improved by addition of Zn at a level more than 40μg/g with ZnSO₄ or ZnNO₃. Zn content of vertebrae was lowest in fish fed No-Zn diet and was pro-portional to dietary Zn levels, reaching a plateau by adding more than 40μg/g of Zn.
The results of this experiment have demonstrated that supplementation of Zn at mare than 40μg/g to white fish meal diet is necessary to obtain normal growth of rainbow trout without the appearance of dwarfism and cataract and that ZnSO₄ or ZnNO₃ are found to be suitable as a Zn source for this purpose. The results have also indicated that the availability of Zn in white fish meal to rainbow trout was very low, judging from the appearance of deficiency symptoms obser-ved in the fish fed with the diet without supplementary Zn which contained enough amount of Zn to satisfy apparent Zn requirement of rainbow trout.