Effects of different ratios of crude protein and non-fibrous carbohydrates on growth, metabolism, physiology, nutrient utilization and muscle cellularity of Lophiosilurus alexandri, a carnivorous freshwater fish

Abstract

The aim of this research was to evaluate the effects of different ratios of crude protein/non-fibrous carbohydrates (CP/NFC) on growth, metabolism, physiology, nutrient utilization efficiency and muscle cellularity of juvenile Lophiosilurus alexandri. Treatments consisted of isoenergetic diets (4500 kcal/kg) with CP/NFC ratios of 0.78; 1.02; 1.26 and 1.60, delivered for 75 days. Polynomial regression analysis showed that the different CP/NFC ratios affected weight gain and specific growth rate with estimated maximum values of 1.27 and 1.25, respectively. Feed conversion rate and the hepatosomatic index had minimum values with CP/NFC ratios of 1.33 and 1.02, respectively. Muscle and liver glycogen levels were maximized with CP/NFC content of 0.87 and 1.26, respectively. The CP/NFC ratio of 1.20 maximized the deposition of lipids in the muscle while that of 1.11 maximized the concentration of total soluble proteins in the same tissue. Plasma total cholesterol, non-HDL cholesterol and triglycerides increased linearly with increasing dietary CP/NFC ratio. Ratios estimated at 0.99, 0.94, 0.93 and 0.98 minimize the concentrations of HDL, total protein, alanine aminotransferase and creatinine in fish plasma, respectively. Nucleus diameter of hepatocytes increased linearly with increasing dietary CP/NFC ratio, while percentage of liver tissue nuclei decreased linearly. Hypertrophic muscle fibers were influenced by diets with the estimated ratio of 1.21 CP/NFC maximizing the occurrence of these fibers. Fish body crude protein increased linearly with increasing CP/NFC ratios in the diet. Fish body content of mineral matter, total phosphorus and visceral fat were minimized with the ratios of 1.34, 1.38 and 1.12, respectively. There were no effects of CP/NFC ratios on the viscerosomatic index, feed intake, carcass yield, the enzymes aspartate aminotransferase and gamma-glutamyl transferase, percentages of cytoplasmic and capillary sinusoids in liver tissue, and body dry matter, crude energy, total lipids, viscera dry matter. It is concluded that diets containing CP/NFC within 1.11–1.33 improve productive performance and nutrient use efficiency of fish. The different CP/NFC ratios also interfere with some plasma and metabolic variables that, associated with performance results, indicate a better physiological state of juvenile L. alexandri with an initial average weight of 30 g.