Effects of Dietary Protein and Lipid Levels on Growth, Nutrient Utilization, and the Whole-body Composition of Turbot, Scophthalmus maximus, Linnaeus 1758, at Different Growth Stages

Three 9-wk feeding trials were performed to assess the effects of dietary protein and lipid levels on the growth, feed utilization, and body composition of turbot, Scophthalmus maximus Linnaeus, at three different growth stages with initial average weight 4.5 +/- 0.01, 59.1 +/- 0.24, and 209.1 +/- 0.21 g, respectively. Six practical test diets were formulated to contain three protein levels (45, 50, and 55%), each of which was supplemented with two lipid levels (12.0 and 16.0%), to produce a range of P : E ratios (from 87.4 to 110.0 mg protein/kcal). The results of three experiments suggested that growth performance generally improved with increasing dietary protein irrespective of dietary lipid. Fish fed diets with 55% protein (12 and 16% lipid, P : E ratio of 110.9 and 107.1 mg protein/kcal) had the highest specific growth rates (SGRs), feed efficiency ratios (FERs), and energy retention. At the same protein level, FER and protein efficiency ratio (PER) increased significantly with the increase of lipid except in large fish (initial average weight 209.1 +/- 0.21 g). SGR of medium turbot (initial average weight 59.1 +/- 0.24 g) was also improved by the lipid increase, indicating a protein-sparing effect of lipid.