Growth, feed utilization and body composition in two size groups of Atlantic halibut (Hippoglossus hippoglossus) fed diets differing in protein and carbohydrate content

Two size groups (60 and 800 g) of Atlantic halibut were fed four different experimental diets varying in content of protein (41-56% of dry matter) in substitution to starch (22-6%). The response to dietary treatments differed between the two sizes of fish. In 60 g halibut growth, feed efficiency and percentage energy retention were 20%, 25% and 23% greater, respectively, in the groups fed the highest protein level compared with those fed the lowest level (P < 0.05). In contrast, no effect of diet was seen on these variables in the 800 g fish. Further, the 60 g fish fed the diets with highest protein content became leaner than those fed the lowest protein levels, whereas no response was seen in whole body composition of 800 g fish. In the 60 g halibut, liver glycogen content increased with dietary starch level and ended more than three times higher in fish fed the diet with the highest starch level than in those fed the lowest. Liver lipid, protein and ash levels showed marked and significant trends in the opposite direction. In conclusion, 800 g halibut seem to have a lower optimal dietary protein level and to be less sensitive to dietary carbohydrates than 60 g halibut.