Effect of eight diets comparing three different lipid levels (15, 22 and 27%) and two fish meal qualities were studied on growth and liver histology. Fish meal quality was judged by the content of biogenic amines and temperature processing techniques. The experiment included a comparison of pelleted feed with extruded feed for the 22% lipid diet. A total of 1140 gilthead seabream of 70 g average initial body weight were randomly stocked in 500-1 fiberglass tanks in duplicate groups of 60 fish. After 2 months of experiment, the fish were transferred to 1-m3tanks. Fish were fed twice a day to apparent satiation for 6 months until they reached about 400 g (commercial size). Fish fed diets containing high quality fish meal showed, in general, a higher growth than those fish fed with low quality fish meal. For diets containing high quality fish meal, the fish fed 22 and 27% dietary lipid had significantly higher growth than those fish fed 15% dietary lipid. On the contrary, in diets containing low quality fish meal, only fish fed 27% dietary lipid showed significantly the higher growth rate. Fish fed the pelletized diets showed a lower growth than those fish fed extruded diets. Livers from fish fed diets containing high quality fish meal and 27% lipid showed foci of swelling hepatocytes that were not found for low quality fish meal at the same dietary lipid content. Ultrastructurally, these foci were characterized to present irregular nuclei displaced to periphery of hepatocytes and large lipid droplets in the cytoplasm. Livers from fish fed high and low fish meal qualities with 22% lipid showed similar morphological characters of hepatocytes to those that fed 15% lipid, but the difference was observed in the nuclei displacement.
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