Effects of dietary protein levels on growth performance, muscle composition and fiber recruitment of juvenile small yellow croaker (Larimichthys polyactis)

This study investigated the effect of protein levels (PLs) in dietary feed on the growth performance, muscle composition, and fiber recruitment in juvenile small yellow croaker (Larimichthys polyactis). A total of 900 fish weighing 10.67 +/- 0.07 g were randomly allocated into six groups that were fed dietary PLs of 40%, 42.5%, 45%, 47.5%, 50%, and 52.5%, respectively. Each group was sorted into 1.0 x 1.0 x 1.0 m3 cages, with 50 individuals per cage. The fish were fed for 50 days; thereafter, all of live fish were analyzed for weight and length, 15 in-dividuals per group were randomly selected for sampling and analysis. The weight gain rate and specific growth rate significantly increased when the dietary PL increased from 40% to 47.5% (P < 0.05). The lowest feed conversion rate was 1.32 when the dietary PL was 46.40%. The crude protein content in the muscle of juvenile L. polyactis was increased when dietary PL increased from 40% to 45%. Compared to dietary PL of 40%, dietary PL of 45% significantly improved accumulation of glutamic acid, valine, threonine, isoleucine, leucine, phenylalanine, serine, arginine, and lysine in the juvenile L. polyactis muscle. The muscle morphology revealed that a dietary PL of 47.5% could significantly promote the generation of small-sized fibers (semidiameter < 20 mu m). Furthermore, changes in relative mRNA expression of paired box-7, myogenic differentiation factor D, and myogenin influenced the muscle fiber tendencies. Our results indicate that the optimum dietary PL range for both body and muscle growth of juvenile L. polyactis is between 47.33% and 47.50%.