Feeding experiments were conducted to examine the effect of dietary tricalcium phosphate on the availability of Zn to rainbow trout and to determine a minimum supplementary Zn level to a 7% tricalcium phosphate diet. Supplement of 7% tricalium phosphate, the same level as that in white fish meal based diet, to a semipurified diet greatly reduced growth rate and feed efficiency which were not improved by addition of 40μg of Zn. Supplement of 80μg of Zn was necessary for the diet to obtain the same growth rate and feed efficiency together with mineral compositions of whole body as those in fish fed with a tricalcium phosphate-free diet of 40μg of Zn. In addition, availability of Zn was found to be reduced as elevation of dietary tricalcium phosphate level.
Thus, tricalcium phosphate in diet was found to be one of the inhibitors against Zn bioavailability to fish. The fact that supplement of Zn above 40μg/g to white fish meal diets is necessary for normal growth of rainbow trout (without appearance of dwarfism and cataract), is also supported by these results; since the 7% tricalcium phosphate diet with 80μg of Zn/g is the same as in the Zn concentration of the white fish meal based diet with a minimum level of Zn (40μg/g) which can sustain normal growth of rainbow trout.
