Differences in energy utilisation efficiencies of digestible macronutrients in common carp (Cyprinus carpio) and barramundi (Lates calcarifer)

This study aimed to assess macronutrient-specific energy utilisation efficiency (i.e., protein, lipid and carbohydrate) for growth in common carp (an omnivorous species) and barramundi (a carnivorous species) and to assess if species-specific differences exist in energy efficiency of digestible protein (dCP), digestible fat (dFat) and digestible carbohydrates (dCarb). This was achieved by conducting a feeding trial experiment on common carp and by re-analysing data of a recent study on barramundi. A total of four diets were formulated following a 2 × 2 factorial design with 2 dCP-to-dFat ratios and 2 dCP-to-dCarb ratios. For carp, 2 feeding levels were applied such that the overall experimental design was a 2 × 2 × 2 factorial design, however for barramundi, three feeding levels were applied (satiation, 80% initial satiation and 60% initial satiation), resulting in a 2 × 2 × 3 factorial design. For each fish species, multiple regression of retained energy (RE) as a function of dCP, dFat and dCarb (in g.kg-0.8.d−1) was applied to estimate the energy utilisation efficiency of each digestible macronutrient. For carp, dCP, dFat and dCarb show linear relationships to RE, however for barramundi, dCP and dFat were linearly related to RE, but dCarb was curvilinearly related to RE. The estimated energy efficiencies of dCP, dFat and dCarb (respectively, kNE;dCP, kNE;dFat, and kNE;dCarb) for energy retention were 47, 86 and 60%, respectively, showing large degree of similarity with Nile tilapia and pigs. Carp and barramundi had similar kNE;dFat (86 vs. 94%), but different kNE;dCP (47 vs. 64%) and kNE;dCarb (60 vs. 18%). The net energy equations were NE = 11.2 x dCP + 34.1 x dFat +10.4 x dCarb for carp, and NE = 15.9 x dCP + 35.2 x dFat +9.4 x dCarb – 1.9 x (dCarb)2 for barramundi