Dietary requirement of Mn for juvenile largemouth bass (Micropterus salmoides)

To determine the dietary manganese (Mn) requirement of juvenile largemouth bass, Micropterus salmoides, 0, 4, 8, 16, 32 and 64 mg/kg Mn were added as the form of MnSO₄·H₂O into the semi-purified diet containing 5.2 mg/kg Mn, to obtain six diets with graded Mn level of 5.2, 9.6, 13.5, 20.7, 38.3 and 67.9 mg/kg, respectively. M. salmoides with initial body weight of (16.00±0.07) g were fed one of the six diets for 10 weeks. The results showed that adding Mn to the basal diet significantly increased the weight gain rate (WGR), and reduced feed conversion ratio (FCR) of M. salmoides, which tended to stabilize when dietary Mn reached 13.5 mg/kg. With the increase of dietary Mn, the serum activity of T-SOD and Mn-SOD, and Mn content in whole fish and vertebrae increased, and then kept relatively stable when dietary Mn reached 20.7 mg/kg, but the serum MDA content showed a decreasing trend. In addition, the iron content in whole body, iron and zinc contents in vertebra, and the Mn retention in whole body decreased with the increasing dietary Mn. In conclusion, the supplementation of Mn in semi-purified diet improved the weight gain and feed utilization, promoted serum antioxidant capacity and Mn retention in whole body and vertebra. Based on the WGR, FCR, Mn in whole body and vertebra, the broken-line analysis indicated that the dietary requirement of Mn for M. salmoides was 18.3, 20.5, 21.1 and 23.4 mg/kg dry matter, respectively.