Net energy evaluation for fish feed

Samenvatting

The major aims of this thesis are: 1. To investigate the effect of dietary macronutrient composition on the relationships between retained energy (RE) and digestible energy (DE) for African catfish, barramundi, carp, snakehead and striped catfish, 2. To quantify the net energy equations for the five selected fish species, 3. To investigate whether the net energy equations are similar across the five fish species studied and 4. To assess the impact of dietary macronutrient composition on the proximate composition and location of fat deposits in the different body compartments of African catfish, striped catfish and snakehead. The minor aims of this thesis are: 1. To investigate differences in the digestibility of carbohydrates between the selected fish species and 2. To study the effect of dietary starch levels on plasma glucose and triglycerides values in snakehead and striped catfish. The major aims were achieved by performing experiments on African catfish, common carp, snakehead and striped catfish and by re-analysing data of a recent published study on barramundi. For every experiment a total of four diets were formulated following a 2×2 factorial design with 2 digested protein to digested fat ratios and 2 digested protein to digested carbohydrates ratios. For African catfish, carp, striped catfish and snakehead, 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, respectively resulting in a 2×2×3 factorial design. For each fish species, regression of retained energy and digestible energy intake was performed to assess if the effect of dietary macronutrient composition on the relationships between retained energy and digestible energy exists. In addition, for each species, multiple regression of retained energy as a function of digested protein, fat and carbohydrates intake (in g.kg-0.8.d-1) was applied to estimate the energy utilization efficiency of each digestible macronutrient and determine the net energy equation. The net energy equations were compared among fish species studied. This study found that the digestibility of starch is high in all the fish species studied and that of non-starch polysaccharides (NSP) is species related and only digested by striped catfish and common carp. The energy utilization efficiencies of digested protein and fat are comparable between the fish species studied. The energy utilization efficiency of digested carbohydrates is different across the fish species studied causing the net energy equations to be different between the fish species studied. This different contribution of carbohydrates is due to the difference in the ability to use carbohydrates, i.e., the ability to convert plasma glucose to plasma triglycerides. Striped catfish has a better ability to convert plasma glucose to plasma triglycerides than snakehead. Dietary fat supplementation increased the fat content of liver, viscera, fillet and the rest fraction in striped catfish, African catfish and snakehead. Finally, the impact of digestibility of nutrients on the energy evaluation, the motivations to move from a digestible energy system to a net energy system and next steps for the energy evaluation of fish feed, such as the energy utilisation efficiency of non-starch polysaccharides, the optimal balance between digestible protein and net energy for fillet and whole body growth and net energy equations for different production systems are discussed.