Effects of dietary lipid levels on growth performance and feed utilization in juvenile Black Sea turbot (Psetta maxima) with reference to nitrogen excretion

Abstract

In the present study, iso-nitrogenous test diets were formulated to contain two different levels of lipid (9% and 12%) with dietary energy ranging from 19 kJ/g to 20 kJ/g diet. White fish meal was the main protein source of the diet and the protein level was 55% in all diets offered to triplicate groups of juvenile Black Sea turbot (mean initial weight of 24.5±0.3 g) for a period of 70 days. Specific growth rates were significantly better (P < 0.05) in high lipid diet group compared to the groups fed low lipid diets. Feed conversion rates (FCR) were influenced by the lipid levels and the best FCR was obtained in fish given the diet with high lipid content. Similarly, protein efficiency was also significantly better (P < 0.05) in fish fed diets with high lipid level. The better utilization of protein in the diet with high lipid content resulted in a significantly higher (P < 0.05) nitrogen retention as percent of intake when compared to the other group with lower lipid level. The amount of nitrogen loss into the water environment was significantly reduced (P < 0.05) to the same extent in the group fed diets with high lipid level. The lipid retention values calculated based on the intake rates were also better in the high lipid diets compared to the other group with low lipid level, however the difference was not significant (P > 0.05) among the experimental groups. The results in the present study indicate that the use of high lipid level with a protein to energy ratio of 27 mg/kJ (12% lipid, 55% protein, 20 kJ/g gross energy) performed best. Increasing the dietary lipid to 20 kJ/g diet has reduced the total nitrogen excretion from Black Sea turbot by 23%.