Effect of wheat bran replacement of wheat middlings in extrusion processed (floating) diets on the growth of juvenile rainbow trout ( Salmo gairdneri)

The effect of partial replacement of wheat middlings with wheat bran in extruded diets on the growth and physiological response of juvenile rainbow trout was determined in a 16-week study. Trout reared on an extruded diet containing a normally accepted level of wheat middlings showed a depressed growth rate, increased liver weight, increased liver glycogen content and elevated and prolonged plasma glucose levels compared with trout reared on extruded diets with wheat bran partially replacing the wheat middlings. The results suggest that the higher level of available carbohydrate in the diet containing the highest (but normally accepted) level of wheat middlings was, at least, partially responsible for the growth depression of the trout reared on this diet. The elevated and prolonged plasma glucose levels of these trout may have depressed appetite, feeding response and feed consumption which would appear to be the major cause of the growth depression.