Effects of Different Dietary Protein Levels on the Growth Performance, Physicochemical Indexes, Quality, and Molecular Expression of Yellow River Carp (Cyprinus carpio haematopterus)

Simple Summary: Yellow River Carp (Cyprinus carpio haematopterus) is a major commercial farmed species belonging to Cyprinus carpio, which has been cultivated widely, especially in Northern China. However, no relative literature focused on appropriate dietary protein level for large-sized Cyprinus carpio haematopterus. The present study investigated the effects of different dietary protein levels on the growth performance, physicochemical indexes, quality, and molecular expression of Cyprinus carpio haematopterus and aimed to determine which dietary protein level was most appropriate for Cyprinus carpio haematopterus. It was found that the optimal dietary protein level for Cyprinus carpio haematopterus (160.24 +/- 15.56 g) is 250-280 g/kg based on different aspect analysis. This study is also helpful in laying the foundations for the breeding of new carp varieties with low dietary protein levels. A 12-week rearing trial was carried out to estimate effects on the growth performance, physicochemical indexes, quality, and the molecular expression of Yellow River Carp (Cyprinus carpio haematopterus) using five practical diets, including dietary protein levels of 220, 250, 280, 310, and 340 g/kg. The results illustrated that the fish's weight gain (WG) and specific growth rate (SGR) were significantly influenced, with an ascending dietary protein level of up to 250 g/kg (p < 0.05). The carp muscle contents of total saturated fatty acids (Sigma SFA), monounsaturated fatty acids (Sigma MUFA), polyunsaturated fatty acids (Sigma PUFA), and fatty acids (Sigma FA) decreased significantly with the ascending dietary protein levels, except for the 250 g/kg protein diet (p < 0.05). Only the glutamic acid and total essential amino acid (Sigma EAA) contents were significantly influenced by the ascending dietary protein levels (p < 0.05). The relative GH expression of the carp muscle significantly decreased with the increase in the dietary protein level up to 310 g/kg, and then it significantly increased (p < 0.05). In the intestines, the peak relative TOR expression was observed on the 220 g/kg protein diet, while the relative 4EBP1 expression was significantly influenced by the dietary protein level up to 250 g/kg (p < 0.05). In the muscle, the peak relative TOR and 4EBP1 expression levels were observed on the 250 g/kg protein diet. In gills, the lowest relative Rhag, Rhbg, and Rhcg1 expression levels were observed on the 250 g/kg protein diet. Based on all of the aforementioned results, the optimal dietary protein level for Cyprinus carpio haematopterus (160.24 +/- 15.56 g) is 250-280 g/kg.