Optimal diet composition for European whitefish (Coregonus lavaretus): analysis of growth and nutrient utilisation in mixture model trials

Two feed mixture trials were conducted on the European whitefish. For both trials, nine diets with varying levels of fish meal (FM), fish oil (FO) and cornstarch (CS), and fixed levels of wheat meal and vitamin¡mineral premix (12%), were formulated according to D-optimality criteria assuming a quadratic response surface. In the two trials, the proportions of the ingredients were, respectively: 38-86% and 43-85% for FM, 2-22% and 0-30% for FO, and 0-33% and 0-15% for CS. The extruded diets were fed to satiation for 72 and 85 days in a flow-through, freshwater system at 15 °C. Optimal allocation of replicates among 25 and 29 tanks was based on D-optimality criteria. The response surfaces were modelled as quadratic and super-cubic polynomials specific for mixture designs. The data from the trials were combined for the final analysis, which included the experiment as a random effect. Fish growth increased with increasing FM content up to 66.3% FM (16.7% FO and 5.0% CS). Protein growth peaked at a somewhat higher FM content of 72.0% (10.2% FO, 5.9% CS), and lipid deposition rate at the highest studied FO content (53.1% FM, 30.0% FO and 4.9% CS). Feed efficiency improved with decreasing CS content and the highest value was obtained with the mixture containing 58.0% FM, 30.0% FO and 0.0% CS. Protein retention efficiency was improved with decreasing FM content and peaked at a mixture of 50.2% FM, 30.0% FO and 7.8% CS. The diet inducing 95% of the maximum growth with the minimum FM content comprised of 51.2% FM, 30.0% FO and 6.8% CS. Based on the data from these two experiments, an area of optimal diet formulation can be assessed by graphically superimposing isobars of 95% growth, feed efficiency and dietary protein utilisation responses. This optimal composition for the European whitefish appears to be in the range of 53-61% FM, 24-30% FO and 0-7% CS together with the 12% wheat meal and vitamin-mineral premix. However, this optimisation is based on round weight and because high dietary lipid levels induced high lipid deposition rates, the practical optimum for dressed weight would be at a somewhat lower FO level. Mixture experimentation proved to be an efficient method to rapidly screen European whitefish diets for nutritional optimisation.