Effects of dietary protein at two lipid levels on growth, gonadal development, body composition and liver metabolic enzymes of brown trout (Salmo trutta fario) broodstock

This study was conducted to estimate the effects of dietary protein and lipid levels on the growth, body composition, gonadal development and activity of liver metabolic enzymes of the brown trout Salmo trutta fario broodstock. Ten diets were formulated containing five different protein levels (360, 390, 420, 450 and 480 g/kg) and two different lipid levels (90 and 180 g/kg). The experiment was a completely randomized 5 × 2 factorial design. The fish with an initial body weight of 462.53 ± 45.40 g were cultured in a spring water flow‐through system for 77 days. The growth performance was affected by dietary protein and lipid levels. Increase in the dietary protein and lipid resulted in increase in the body lipid and protein contents. The male gonadosomatic index decreased in the groups treated with relatively high levels of protein (390–480 g/kg). The activities of the hepatic amino acid‐catabolizing enzymes—alanine aminotransferase and aspartate aminotransferase—increased significantly with the increase in the dietary protein level. The activities of the lipogenic enzymes (fatty acid synthetase and glucose‐6‐phosphate dehydrogenase) and gluconeogenic enzyme (fructose‐1,6‐bisphosphatase) increased with the increase in the dietary protein level. Further, an increase in the activities of glycolytic enzyme in liver with an increase in the dietary lipid was observed. The variation in some intermediary metabolizing enzymes due to dietary components supports the high‐metabolic adaptability of this species to dietary protein and lipid levels. A diet with 450 g/kg protein and 90 g/kg lipid with 24.56 mg/kJ protein/energy ratio maximizes the growth, feed efficiency and gonadal development.