Feeding experiments were conducted to examine the availability to rainbow trout of manganese (Mn) and magnesium (Mg) contained in white fish meal and to determine the minimum supplemental Mn and Mg levels to white fish meal based diet for the normal growth of rainbow trout. The lowest growth was observed in fish receiving a diet without supplemental Mn or Mg and the growth was effectively improved by the addition of Mn and Mg at levels of more than 15μg/g and 0.5mg/g, respectively. The Mn content of vertebrae was lowest in fish fed a diet without supplemental Mn and was proportional to dietary Mn levels, reaching a plataeu by supplementing more than 15μg/g diet. The ratio of Ca to Mg in the vertebrae was highest in fish fed a diet without supplemental Mg and effectively decreased to lower levels by Mg supplementation at levels more than 0.5mg/g diet.
The results of these experiments have demonstrated that supplementation of Mn and Mg at more than 15μg/g and 0.5mg/g, respectively, to white fish meal diet is necessary to sustain the normal growth of rainbow trout. The results also suggested that the availability of Mn in white fish meal was very low and that the utilization of Mg absorbed was distorted by a certain substance in white fish meal.