A long-term feeding experiment (60-62 weeks) was conducted to examine the effects of the deletion of manganese (Mn) or of the total deletion of trace elements from the mineral mixture in white fish meal diets on growth and chemical compositions of rainbow trout.
The total deletion of trace elements from the diet results in reduced growth, lens cataract, and short body dwarfism as the most noticeable gross external signs. In addition the contents of Zn in vertebrae, testis, and ovary were also found to be very low, although a substantial amount of the Zn in vertebrae, testis, and ovary were also found to be very low, although a substantial amount of the Zn required by the fish was contained in the diet derived from the white fish meal used. These results suggest that the availability ot rainbow trout of Zn contained in white fish meal is very low.
The fish fed on the Mn-deficient diet developed lens cataracts and short body dwarfism, but the growth was not much affected by Mn deficiency. The contents of Mn and Zn vertebrae, testis, and ovary of this group were significantly lower than those in the fish receiving the control diet. On the other hand, feeding of the fish with a white fish meal diet without a supplement of riboflavin caused a deficiency symptom.
These results have shown that even though white fish meal containing various kinds of minerals and riboflavin is used as a protein source, a supplement of dietary Mn and Zn or of riboflavin is essential for rainbow trout.