Effects of dietary carbohydrate level on growth performance, glucose metabolism, antioxidant capacity, immune response, and digestive function in coho salmon Oncorhynchus kisutch (Walbaum, 1792) fry

A 10-week feeding trial was conducted to assess the effects of graded dietary carbohydrate levels on growth performance and antioxidant capacity in coho salmon ( Oncorhynchus kisutch ) fry. Six isonitrogenous (48.75 % crude protein) and isolipidic (12.98 % crude lipid) diets containing 0.00 %, 4.50 %, 9.00 %, 13.50 %, 18.00 %, and 22.50 % carbohydrate were formulated. A total of 900 fish (0.27 ± 0.01 g) were randomly distributed into 18 experimental tanks (240 L water volume), with triplicate groups per diet. Results showed that the final body weight (FBW), specific growth rate (SGR) and protein efficiency ratio (PER) increased significantly with dietary carbohydrate up to 13.5 % ( p < 0.05), while feed conversion ratio (FCR) decreased ( p < 0.05). No significant differences ( p > 0.05) were observed in intestosomatic index (ISI), hepatosomatic index (HSI) and condition factor (CF). Whole-body moisture and ash content remained unchanged, whereas crude lipid and crude protein content were significantly higher in groups fed 13.5 %, 18.0 % and 22.5 % carbohydrate compared to 0.0 % and 4.5 % group ( p < 0.05). Hepatic superoxide dismutase (SOD) and catalase (CAT), along with hexokinase(HK), pyruvate kinase (PK), fructose-1,6-biphosphatase (FBPase) activity, and glycogen content increased significantly up to 13.5 % carbohydrate ( p < 0.05). Hepatic malondialdehyde (MDA) content was lowest at 13.5 % carbohydrate levels. The activities of hepatic acid phosphatase (ACP), alkaline phosphatase (AKP) and lysozyme (LZM)( p < 0.05) increased with carbohydrate levels ( p < 0.05). Intestinal amylase, lipase and protease activities peaked at 13.5 % carbohydrate levels ( p < 0.05). Broken-line regression analysis of SGR indicated an optimal dietary carbohydrate requirement of 14.15 % for coho salmon fry. This study provides concrete targets for optimizing dietary carbohydrate levels in coho salmon feed, while offering valuable insights for developing sustainable feeding strategies. • Optimal dietary carbohydrates enhanced coho salmon fry growth and feed utilization. • Optimal dietary carbohydrates improving digestive enzyme activity and glycogen metabolism. • Optimal dietary carbohydrates positively influence antioxidant capacity and immune function. • Based on SGR, the optimal dietary carbohydrate level was estimated to be 14.15 %.