Effects of dietary protein to lipid ratio on growth,digestive enzyme activities and muscle composition of juvenile channel catfish(Ictalurus punctatus)

A 60 day feeding trial of 3 dietary protein levels (28%,32% and 36%) and 3 dietary lipid levels (5.0%,7.5% and 10.0%) designed with 4 replications was conducted to determine the effects of dietary protein and lipid levels on growth,digestive enzyme activities and muscle composition of juvenile channel catfish［initial body weight:(1.5±0.02)g］.Fish were fed respectively to apparent satiation by 9 experimental diets named P28L10,P32L10,P36L10,P28L7.5,P32L7.5,P36L7.5,P28L5,P32L5 and P36L5 with 4 replications.The results indicated that weight growth rate (WGR) and feed coefficient rate (FCR) were significantly affected by dietary protein and lipid levels or a dietary protein × lipid interaction (P<0.05).WGR and FCR of fish fed the P28L10 diet were the lowest among all the treatments (P<0.05).Growth performance of fish fed diets with 28% protein were lower than that of fish fed higher protein levels (P<0.05);the lowest growth performance was observed in fish fed the P28L10 diet.The highest WGR was obtained in fish fed the P36L7.5 diet;and no obvious differences of WGR were found in fish fed the P36L7.5,P36L10 and P32L7.5 diets (P>0.05).Activities of liver-lipase and stomach-protease changed significantly as dietary lipid levels increased (P<0.05).Muscle lipid levels decreased significantly with dietary lipid levels (P<0.05),and muscle protein levels could be influenced by dietary protein and lipid levels.Viscerasomatic index (VSI),mesenteric lipid-somatic index (MSI) and condition factor (CF) together may show that muscle protein and lipid levels could be affected by dietary protein × lipid interaction.In conclusion,this study suggests that growth performance of juvenile channel catfish is improved at a suitable protein/lipid ratio in diet.Fish fed diet containing 32% protein,7.5% lipid and 22.4 mg/kJ P/DE ratio got the best growth performance and the ideal body composition.