In fish, increasing the crude lipid level of feed can save protein and improve feed utilization. Mirror carp (Cyprinus carpio) is one of the most widely farmed fish species in the world. In this study, mirror carp larvae were fed isonitrogenous diets with different lipid levels (3%, 5%, 7%, 9%, 11%, and 13%). The rearing trial lasted for eight weeks. The results revealed that when the fat content was 9%, the AWGR, WGR, and FCR were highest, whereas FCR was lowest. The AWGR was correlated with the dietary lipid level, and the regression equation was y = −2.312x² + 45.01x + 214.49. Compared with those in the control group, the T-CHO and TG contents were significantly greater in the 13% lipid content groups and significantly lower in the 9% lipid content groups (p < 0.05). In terms of muscle quality, the contents of MUFAs, PUFAs, and DHA + EPA were significantly greater than those in the other experimental groups (p < 0.05). Oil red O staining revealed a lipid content of 13% with severe fat deposition. In addition, the results of the analysis of antioxidant enzyme activity revealed that the activities of GSH, CAT and T-AOC were significantly greater at the 9% lipid content, and that the MDA content was significantly greater at the 13% lipid content (p < 0.05). Similarly, the mRNA levels of GH, IGF-I, FAS, and LPL were significantly highest at a lipid level of 9% (p < 0.05). The above results revealed that the optimal dietary lipid requirement for the fast growth of mirror carp (6.86 ± 0.95 g) was 9.74% on the basis of nonlinear regression analysis of the AWGR. The dietary lipid level (9%) improved the growth, stress resistance, and lipid utilization of mirror carp to a certain extent.

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 (∑SFA), monounsaturated fatty acids (∑MUFA), polyunsaturated fatty acids (∑PUFA), and fatty acids (∑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 (∑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.

Common carp (Cyprinus carpio) is an economically important fish worldwide, with many of its species bred for consumption. However, there are few reports on the comprehensive comparative analysis of the muscle nutritional quality and stress resistance characteristics of different common carp species. In this study, after 15 months of feeding in the same environment, the nutritional components, serum biochemical indices, liver antioxidant and intestinal digestive enzyme activities, and muscle growth-related gene expression were determined in Songpu mirror carp (SPM; Cyprinus carpio Songpumirror), Heilongjiang wild carp (HLJ; Cyprinus carpio haematopirus), cold-resistant strain of purse red carp (CPR; Cyprinus carpio ‘Red purse cold-resistant’), Songhe carp (SH; Cyprinus carpio ‘Songhe’), and Songpu carp (SP; Cyprinus carpio Songpu). Muscle nutrient composition showed that HLJ had a significantly lower crude fat content and higher docosahexaenoic acid (DHA) + eicosapentaenoic acid (EPA) proportion than the other four common carp species (p < 0.05). The contents of lysine (Lys) and aspargine (Asp) were significantly higher in the CPR than in other species (p < 0.05). Serum biochemical parameters showed that total protein (TP), total cholesterol (T-CHO), triglycerides (TG), and low-density lipoprotein (LDL) were significantly lower in SPM than in the other species (p < 0.05). The results of tissue enzyme activity showed that the activities of superoxide dismutase (SOD) and catalase from Micrococcus lysodeikticus (CAT) in the liver were significantly higher, while the activities of lipase (LPS), trypsin (TRS), and α-amylase (α-AMS) in the intestine were significantly the lower in HLJ than in the other species (p < 0.05). In addition, the relative expression levels of growth hormone (GH), growth hormone receptor (GHR), insulin-like growth factor 1 (IGF1), insulin-like growth factor receptor (IGF1R), and myoblast determination factor (MyoD) in SP and SH were significantly higher than those in the other species, while the relative expression of myostatin (MSTN) in HLJ was significantly higher (p < 0.05). Therefore, there were significant differences in muscle nutritional quality, serum biochemical indices, liver, and intestinal enzyme activities, and muscle growth potential among the five species of common carp. This study could provide a theoretical basis for the germplasm evaluation and variety improvement of common carp.

Jingpo Lake is the largest mountain barrier lake in China and plays a key role in breeding, power generation, and providing a source of drinking water. Microbes are important participants in the formation of lake resources and energy cycles. However, the ecological protection of Jingpo Lake has faced serious challenges in recent years. In this study, we investigate the responses of the microbial community’s composition of sediments at five locations to an environmental gradient representing water quality and water-depth changes using a metagenomic sequence. We found that the diversity and composition of the microbiota sediments were altered spatially and correlated with the physicochemical factors of water samples. In the microbial community, relatively lower Chao1, alternating conditional expectations, and Shannon and Simpson indices were found at the shallowest location with higher total phosphorus and chlorophyll a. Furthermore, the Kyoto Encyclopedia of Genes and Genomes analysis revealed that the metabolism function was the most abundant functional classification in Jingpo Lake. The levels of total phosphorus, chlorophyll a and pH were positively correlated with the abundance of Flavobacterium and the bacterial functions of the carbohydrate metabolism and amino acid metabolism. In conclusion, our results reveal the physical and chemical characteristics, as well as the microbial community characteristics, of Jingpo Lake, which provides new insights for studying the relationship between environmental factors and the bacterial community distribution of freshwater ecosystems, in addition to also providing a theoretical basis for the environmental monitoring and protection of the lake.

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 (p < 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 (p < 0.05). In addition, the activities of α-AMS, LPS, TPS, SOD, CAT, and GSH-Px were significantly greater in the 34% protein group (p < 0.05). Similarly, the relative expression levels of GH, IGF-I, TOR, 4EBP2, Rhag, Rhbg, and Rhcg1 were significantly greater in the 34% protein group compared to the other protein groups (p < 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%.

Big-head Schizothoracin (Aspiorhynchus laticeps) and Tarim Schizothoracin (Schizothorax biddulphi) are locally sympatric in the Tarim River Basin. Although another Schizothoracin (Schizothorax esocinus) in Tarim River basin has been speculated to be hybrid offspring of Big-head Schizothoracin and Tarim Schizothoracin, there was no genetic evidence. Previous studies on the genetics and evolution of Schizothoracins in Xinjiang Province were mostly based on mitochondrial DNA (mtDNA), whose characteristics of maternal inheritance made it hard to answer the question of whether there was hybridization and introgression between Big-head Schizothoracin and Tarim Schizothoracin. In this study, cytochrome b (cytb) gene of mtDNA and internal transcribed spacer 2 (ITS2) that is encoded by the nuclear genome were genotyped within the entire samples at the same time. Our results confirmed that Schizothorax esocinus was the hybrid offspring of Big-head Schizothoracin and Tarim Schizothoracin. The heterozygous ITS2 genotypes and/or Aspiorhynchus laticeps-like mtDNA were also detected in a subset of samples that should have been identified as pure Schizothorax biddulphi based on morphology. The ITS2 is characterized by multi-copy, concert evolution, and biparental inheritance. Thus, by comparing with mtDNA data, broad-scale bidirectional hybridization and introgression between Big-head Schizothoracin and Tarim Schizothoracin were revealed. Although interspecific hybridization may play a positive role in ecology and evolution, interspecific hybrids could threaten their parental species by the swamping of genetics and demography. As both parents of hybridization are critically endangered fishes, in this case, it is urgently necessary to strengthen the scientific assessment of the risks of the hybrids and the control of the hybridization and introgression between Aspiorhynchus laticeps and Schizothorax biddulphi in the Tarim River Basin.