Growth, feed utilization and body composition of juvenile Atlantic halibut ( Hippoglossus hippoglossus) fed diets differing in the ratio between the macronutrients

A 10-week trial was conducted to study the effect of replacing dietary protein with fat or carbohydrate on growth, feed utilization and body composition of Atlantic halibut with a starting weight of about 140 g. Six diets containing 51–60% protein, 20–27% fat and 7 or 10% starch were used. The performance of the fish was good in this experiment, and there were no significant effects of dietary treatment on specific growth rate (0.86±0.01%/d) and feed efficiency ratio (1.41±0.02 g gain/g dry feed). Increasing the dietary fat level resulted in increased dry matter, fat and energy contents in the carcass but no changes were found in the composition of the viscera. Nitrogen retention in the whole body was not influenced by dietary treatment (41±1%). A significant protein sparing effect of fat was seen. Energy retention was not affected by increasing the dietary fat content from 20 to 24%. It increased however, from 47±1 to 58±2% when, in addition to an increase in the starch content, the dietary fat content was increased from 24 to 27%. For good growth of this size of halibut, these results indicate that in a diet containing 24 MJ gross energy, it is not necessary to include more than 51% protein.