Dietary zinc requirement of Catla (Catla catla) juveniles

Zinc (Zn) is an essential inorganic micronutrient in fish, as it plays important roles in many biological functions. This study was performed to evaluate the dietary Zn requirements of Catla catla and its impacts on growth, proximate composition, nutrient utilization and antioxidant enzyme activities. Six experimental diets were prepared from the basal diet supplemented with 0, 20, 40, 60, 80 and 100 mg/kg Zn from ZnSO_4, and the Zn contents were 2.15, 22.11, 42.14, 62.06, 81.89 and 101.63 mg/kg, respectively. Each experimental diet was fed in triplicate for 60 days, and each replicate contained 15 fish (initial weight 12.581 ± 0.1293 g). At the end of the trial, increasing the dietary Zn concentration to 62.06 mg/kg significantly (p < 0.05) improved the weight gain (WG), specific growth rate and feed conversion ratio of the fish; however, higher dietary Zn concentration had no effect. A dietary Zn level of up to 62.06 mg/kg significantly improved (p < 0.05) the whole-body crude protein (CP) and crude fat (CF) contents and plateaued thereafter. Furthermore, Zn absorption significantly (p < 0.05) increased with 81.89 mg/kg Zn in the diet and plateaued thereafter. The Zn content in the whole body, kidney, muscles, scales and bones gradually increased (p < 0.05) to 62.06 mg/kg of Zn in the diet and remained stable afterwards. The somatic indices and antioxidant enzymatic activity of C. catla juveniles significantly (p < 0.05) increased to 62.06 mg/kg and plateaued above this level. The thiobarbituric acid reactive substances (TBARS) decreased (p < 0.05) with increasing Zn supplementation to 62.06 mg/kg and then stabilized. The alkaline phosphatase (ALP) activity increased (p < 0.05) with increasing Zn concentration to 62.06 mg/kg and then remained stable with further increases. The optimal level of Zn for C. catla juveniles in this study was estimated to be 52.43, 62.12 and 58.65 mg/kg on the basis of WG%, Zn in bone and TBARS, respectively. In conclusion, dietary Zn supplementation significantly (p < 0.05) improved growth performance, feed utilization, body composition, antioxidant status, Zn contents in tissues and ALP activity in C. catla. Future research should investigate the long-term effects of dietary zinc on immune function, stress tolerance, and overall fish health, as well as the bioavailability of different zinc sources and their interactions with other micronutrients.