Effect of dietary carbohydrate level on growth, feed utilization, energy retention, body composition, and digestive and metabolic enzyme activities of juvenile cobia, Rachycentron canadum

Carnivorous fish, such as cobia Rachycentron canadum, tend not to have a specific requirement for dietary carbohydrates (CBH). However, if CBH is not provided in the diet, other energy-yielding nutrients, such as protein and lipids, are catabolized to produce energy. Therefore, knowing the optimum level of dietary CBH would be useful for cobia aquaculture. In the present study, four isonitrogenous and isolipidic diets were prepared to investigate the effects of dietary CBH level (4.8–22.5 %) on growth performance and body composition of, and feed utilization, by juvenile cobia (initially weighing 22.2 ± 0.27 g/fish). Each diet was given to triplicate groups of cobia for 8 weeks. Both the final body weight and specific growth rate increased with increasing dietary CBH levels. Feed intake decreased with increasing dietary CBH levels (P < 0.05), whereas, feed efficiency and the protein efficiency ratio increased (P < 0.05). As the level of dietary CBH increased, the condition factor, hepatosomatic index, viscerosomatic index, and intraperitoneal fat ratio generally increased (P < 0.05). The whole-body lipid content generally increased with increasing dietary CBH levels, with the lipid content of fish fed a 22.5 % dietary CBH diet being significantly higher than other treatments. Moisture content showed the reverse response to that of lipids. Body protein and ash contents were not significantly related to dietary CBH (P > 0.05). Activities of hepatic glucokinase increased with increasing dietary CBH levels, being significantly higher in fish fed a 22.5 % CBH diet compared with other groups (P < 0.05). In contrast, the activities of hepatic hexokinase and pyruvate kinase showed the opposite trend. The activities of intestinal amylase and trypsin were negatively related to dietary CBH levels. Thus, dietary CBH promoted the growth and feed utilization of cobia, although it also had negative effects, such as higher lipid deposition and higher body indices. Based on these results, we recommend that inclusion of CBH in the diet of cobia is beneficial, whereas, the optimal level of CBH should be carefully evaluated.