Dietary zinc requirement of juvenile largemouth bass (Micropterus salmoides)

To determine the dietary zinc requirement of juvenile largemouth bass (Micropterus salmoides), casein, gelatin and fish meal were used as the main protein sources and ZnSO₄·H₂O as zinc source to produce a semi-purified diet. Then, zinc was added into the basal diet at levels of 0, 25, 50, 100 and 200 mg/kg, to obtain five diets containing zinc of 24.8, 48.8, 78.9, 126.1 and 223.6 mg/kg dry matter, respectively. Largemouth bass with initial body mass of (10.99±0.07) g were fed one of the five diets for 8 weeks. The results showed that the supplementation of 25 mg/kg Zn significantly increased the weight gain rate (WGR), and reduced feed coefficient ratio (FCR) of largemouth bass, which tended to stabilize when higher levels of Zn were added. When dietary Zn reached 25-49 mg/kg, the serum activity of CuZn-SOD and T-SOD remained stable, while Zn-79 group showed the highest AKP activity. The zinc content in whole fish and vertebrae increased with the increasing dietary Zn, and showed no significant increase when dietary Zn content was higher than 126 mg/kg (Zn-126 group). The iron content in whole body, iron and manganese contents in vertebra, and the retention of iron and zinc in whole body decreased with the increasing dietary Zn. In conclusion, the supplementation of Zn in diet improved the weight gain and feed utilization, promoted serum immunity and Zn deposit in whole body and vertebra. Based on the WGR, FCR, whole body zinc and vertebra zinc, the broken-line analysis indicated that the dietary requirement of Zn was 45.5, 44.6, 121.8 and 130.5 mg/kg dry matter, respectively.