THE DIETARY REQUIREMENT OF YOUNG RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) FOR FOLIC-ACID

We sought to determine the dietary folic acid requirement of young rai nbow trout using growth indices supported by measurements of tissue fo late concentrations. The investigation was conducted with purified die ts that had, by assay, basal folic acid levels of 0.08 and 0.16 mg/kg in the first and second, respectively, of two experiments. Each experi ment was started with fry (initial mean weight, 1.4 and 2.8 g/fish in Experiments 1 and 2, respectively) and was conducted at a water temper ature of 15-degrees-C. Experiment 1 lasted 18 wk and Experiment 2 last ed 16 wk. Recovery tests (of 8 wk duration, performed on fish fed the unsupplemented diet) and pair-feeding showed that the unsupplemented d iet led to a folate-specific deficiency condition in which the main he matological abnormality was the appearance of misshapen nuclei in a sm all proportion (2.3%) of erythrocytes. Dietary requirements were shown not to exceed 0.3 and 0.6 mg folic acid/kg (17 and 33 mug/MJ digestib le energy) for optimizing survival and growth indices, respectively. W e conclude that the dietary folate requirement of the trout is compara ble to that of other vertebrates for the purpose of achieving maximal weight gain.