Riboflavin supplementation of diets for rainbow trout

The present study was designed to determine whether a modern practical trout grower diet requires riboflavin supplementation and whether the levels of this vitamin currently included in trout diets may be within a toxic range. Six diets differing only in the level of riboflavin supplementation (0, 4, 10, 25, 50 and 100 mg/kg diet) were formulated from a basal mix, which supplied 8.2 mg/kg. The diets were fed in a randomized complete block design to 18 tanks of 100 fish, and the experiment continued for 5 periods of 28 days. The fish fed a supplement of 4 mg/kg diet achieved a greater final liveweight than those fed supplements of 0, 50 or 100 mg/kg. Other growth parameters (condition factor and carcass composition) as well as food consumption and feed-conversion efficiency were unaffected by the level of dietary riboflavin. The unsupplemented basal diet supported flavin saturation of liver and heart but not of spleen, head kidney or posterior kidney, a finding consistent with the inability of this diet to promote maximal liveweight gain. The results suggest that, although riboflavin supplementation of practical trout diets is advisable, an optimal level of supplementation exists. The rainbow trout may exhibit sensitivity to high levels of dietary riboflavin.