Effects of dietary zinc levels on growth performance, digestive enzyme activities, plasma physiological response, hepatic antioxidant responses and metallothionein gene expression in juvenile spotted sea bass ( Lateolabrax maculatus )

Abstract

The current study was conducted to investigate the effects of dietary zinc on growth performance, body composition, feed utilization, digestive enzyme activities, plasma physiological response, antioxidant responses and metallothionein (MT) gene expression in juvenile spotted sea bass, Lateolabrax maculatus. Six experimental diets (D1-6) were formulated with increasing levels of Zn (0, 20, 40, 80, 160 and 320 mg/kg, respectively) supplied as ZnSO4·H2O. The concentrations of Zn in experimental diets were 36, 55, 74, 110, 175 and 327 mg/kg diet determined by analysis. Each experimental diet was assigned to triplicate groups of 25 L. maculatus for a period of 56 days. The weight gain (WG) and specific growth rate (SGR) in the D4, D5 and D6 groups were significantly increased than that of the control group (p < 0.05), while the feed conversion ratio (FCR) significantly decreased (p < 0.05), respectively. The zinc contents of plasma and vertebra were significantly affected by dietary zinc levels, with the highest zinc content of plasma and vertebra occurring in D6 group (p < 0.05). The levels of plasma alkaline phosphatase (AKP), glucose and catalase (CAT) in the D5 and D6 groups were significantly increased compared with those in the control group (p < 0.05). The content of aspartate aminotransferase (AST) and cortisol (COR) in the D3 and D4 groups was significantly decreased compared with that in the control group (p < 0.05). The intestinal Na+, K+-ATP and CK activities in D3 group were significantly higher than those in the control group (p < 0.05). Compared with the control, the D3 group significantly increased the activities of SOD and T-AOC (p < 0.05), and the D5 group significantly decreased the content of MDA (p < 0.05). The expression of intestinal metallothionein (MT) mRNA in D5 and D6 groups was significantly higher than that of fish in control, D2 and D3 groups (p < 0.05), respectively. Based on the broken-line regression analysis of SGR and AKP, the optimal dietary zinc levels for maximal growth of juvenile L. maculatus were estimated to be 92 and 107 mg/kg diet, respectively.