Optimal dietary carbohydrate‐to‐lipid ratios for silvery‐black porgy ( Sparidentex hasta ) juveniles

Four isonitrogenous diets containing different carbohydrate:lipid ( CHO :L) ratios (0.3, 0.6, 1.1 and 1.8) were tested in triplicate groups (16 fish per replicate) of silvery‐black porgy juveniles for 8 weeks. Growth performance was not affected by different dietary CHO :L ratios ( P  > 0.05); however, the viscerosomatic index, the intraperitoneal fat, whole‐body lipid, energy and n‐3 long chain polyunsaturated fatty acids levels increased with decreasing dietary CHO :L ratios ( P <  0.05). Fish fed with 1.8 CHO :L diet had the lowest apparent digestibility coefficients of protein and lipid, as well as the lowest plasma haemolytic and lysozyme activities ( P  < 0.05). Red blood cell counts and plasma glucose levels were higher in fish fed with 1.1 and 1.8 CHO :L ratio diets than in the other groups ( P  < 0.05). Plasma alkaline phosphatase and alanine aminotransferase, as well as the level of lipid peroxidation and total antioxidant capacity in the liver and plasma increased as dietary CHO :L ratios decreased ( P  < 0.05). The results of the current study indicated that the diets with CHO :L ratios between 0.6 and 1.1 are optimal for silvery‐black porgy, whereas higher ratios may result in hyperglycaemia and immune suppression, and lower CHO :L ratios may lead to oxidative stress and liver dysfunction.