Effects of Dietary Protein Levels on the Growth, Physiological, and Biochemical Indices of Juvenile Yellow River Carp (Cyprinus carpio haematopterus)

An appropriate protein content in the diet can effectively increase the growth rate, muscle quality, and environmental stress resistance of fish. In this study, juvenile Yellow River carp (51.56 ± 0.17 g) were fed isofat diets with different protein concentrations (22%, 25%, 28%, 31%, 34%, and 37%). The results showed that, compared with other protein content groups, when the protein content was 34%, the WGR, SGR, and FCR were significantly higher, while the FCR was significantly lower (<i>p</i> < 0.05). Among them, the SGR and FCR were positively correlated with the dietary protein content, with the regression equations being y₁ = -32.208x² + 21.897x - 1.4001 (R² = 0.8622) and y₂ = 97.027x² - 68.428x + 13.269 (R² = 0.9663), respectively. When x was 33.99% and 35.26%, the SGR and FCR had extreme values. The contents of CP, Lys, EAA, and EAA/TAA were significantly greater in the 34% protein group, reflecting muscle quality (<i>p</i> < 0.05). In addition, the activities of α-AMS, LPS, TPS, SOD, CAT, and GSH-Px were significantly greater in the 34% protein group (<i>p</i> < 0.05). Similarly, the relative expression levels of <i>GH</i>, <i>IGF-I</i>, <i>TOR</i>, <i>4EBP2</i>, <i>Rhag</i>, <i>Rhbg</i>, and <i>Rhcg1</i> were significantly greater in the 34% protein group compared to the other protein groups (<i>p</i> < 0.05). The above results indicated that when the protein content in the diet was 34%, it significantly improved the growth and stress resistance of juvenile Yellow River carp. Based on the nonlinear regression equations for the SGR and FCR, the optimal dietary protein content of juvenile Yellow River carp (51.56 ± 0.17 g) was determined to be 33.99-35.26%.