Physiological Effects of Dietary Protein and Its Optimal Requirement for Early Juvenile Chinese Tapertail Anchovy (Coilia nasus)

To determine the optimal dietary protein level for early juvenile Chinese tapertail anchovy (Coilia nasus, initial body weight: 0.87 ± 0.01 g), five feeds containing graded protein levels (35.42%, 39.16%, 42.96%, 46.83%, and 50.65%) were used in an eight-week feeding experiment. Results showed that the growth performance and health status of C. nasus were significantly affected by dietary protein. Compared with the 35.42% group, the 42.96% and 46.83% groups’ WGR and SGR are notably higher, and FCR is notably lower. The 46.83% group showed higher crude protein and lower crude lipid contents. The activities of CAT and SOD, and the level of T-AOC, were significantly enhanced in the 42.96% and 46.83% groups. The 42.96–50.65% groups showed significantly higher GPx activities and lower MDA levels, and GR activity was increased in the 46.83% and 50.65% groups. In addition, the 39.16–50.65% groups upregulated the expressions of il-1β and tgf-β and downregulated tnf-α. For endoplasmic reticulum stress and apoptosis, the 42.96–50.65% groups significantly downregulated the expressions of atf4, chop, and apaf1, and the 39.16–50.65% groups upregulated cflar, bcl-2, and tradd, downregulated bax, casp9, and casp3. Quadratic regression analysis using WGR and FCR as indices showed that the optimal protein levels in feed for early juvenile C. nasus were 44.31% and 46.56%, respectively.