Effects of digestible carbohydrate levels on growth performance, feed utilization, body composition, and biochemical indices of juvenile spotted knifejaw, Oplegnathus punctatus

The effects of digestible carbohydrate levels (0–25 %) on growth, body composition, feed efficiency, and biochemical parameters of juvenile Oplegnathus punctatus (IBW, 12.51 ± 0.02 g) were studied. By adjusting the ratio of corn starch to cellulose, six iso-nitrogenous and iso-lipid diets were formulated. Each diet was divided into three groups (18 fish per replication) for 8 weeks. The results showed that specific growth rate (SGR) and weight gain (WG) were significantly improved (P < 0.05) as digestible carbohydrate levels increased from 0 % to 20 %, but this trend was reversed when the level exceeded 20 %. Correspondingly, feed conversion rate (FCR), protein efficiency ratio (PER), and nitrogen retention (NR) values tended to improved. As digestible carbohydrate levels increased from 0 % to 25 %, daily energy intake (DEI) increased while energy retention (ER) decreased (P < 0.05). Additionally, whole-body crude protein and crude lipid content tended to increase with rising digestible carbohydrate levels. The glycogen content in the liver and muscle increased as digestible carbohydrate levels rose from 0 % to 15 %, while plasma triacylglycerol (TG) concentration decreased as digestible carbohydrate levels increased from 0 % to 20 % (P < 0.05). Compared with other groups, fish fed the C25 diet displayed lower (P < 0.05) amylase activity. Based on the quadratic regression analysis of SGR and WG, dietary digestible carbohydrate levels of 18.31–19.00 % can provide maximum growth for juvenile O. punctatus.