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Yadong trout (Salmo trutta fario) is an important and characteristic aquaculture fish species in Yadong, Xizang, China. Farmed Yadong trout has been produced on a large scale. Usually, two different-colored eggs (yellow and orange) are produced during Yadong trout farming and show differential reproductive performance. There has been very limited information on the nutrient composition of eggs with different colors, and relevant results have only been reported for rainbow trout. The present study aimed to analyze the nutrient compositions in different-colored Yadong trout eggs to elucidate the factors responsible for the different reproductive performances of the two egg types. The experimental samples were collected from Yadong Industry Park, where the broodstocks were reared with artificial compound feeds. Each egg type was analyzed in 3 replicates. Yellow eggs were produced by 2–3 year-old broodstocks, and orange eggs were produced from 3–5 year-old broodstocks. After spawning, the eggs were cryopreserved and transported to the laboratory by plane. The samples were lyophilized and used for the determination of proximate composition, fatty acids, amino acids, and other reproduction-related nutrients. The crude protein and crude lipid contents were analyzed using the Kjeldahl and Soxhlet extraction methods, respectively. The fatty acid composition (expressed as % total fatty acids) and amino acid composition (expressed as % dry matter) were assayed using gas chromatography and an automatic amino acid analyzer, respectively. The protein carbonyl and malondialdehyde concentration were assayed using commercial kits supplied by Nanjing Jiancheng Bioengineering Institute. The α-vitamin E and astaxanthin were assayed using national standard methods, and the α-vitamin E was measured from wet samples. The statistics of proximate composition, fatty acids, and amino acids data were determined using a t-test for independent samples. The results were expressed as mean ± standard error, and showed that yellow eggs had lower contents of crude protein and ash contents, but higher contents of crude lipid and moisture than orange eggs. There was no significant difference in carbohydrate and energy content between yellow and orange eggs. The saturated fatty acid (SFA), n-3 polyunsaturated fatty acid (n-3 PUFA), and docosahexaenoic acid (DHA) contents of yellow eggs were significantly lower than those of orange eggs, while the n-6 polyunsaturated fatty acids (n-6 PUFA) and EPA contents did not differ significantly between the two egg types. The C16:0 and C20:0 contents of yellow eggs were significantly lower than those of orange eggs, while the contents of monounsaturated fatty acids (MUFA), including C16:1n-7, C18:1n-9, C20:1n-9, and C22:1n-9, of yellow eggs were significantly higher than those of orange eggs. The contents of essential amino acids, non-essential amino acids, and total amino acids were slightly lower in yellow eggs compared to orange eggs. Among the essential amino acids, the contents of valine, methionine, leucine, lysine, and histidine in yellow eggs were significantly lower than those in orange eggs. Among the non-essential amino acids, except for glycine and alanine which were significantly lower in yellow eggs than in orange eggs, the contents of other amino acids did not differ significantly between the two egg types. The contents of protein carbonyl and malondialdehyde in yellow eggs were higher than those in orange eggs, while the contents of α-vitamin E and astaxanthin in yellow eggs were lower than those in orange eggs. In conclusion, orange eggs had higher contents of protein, some essential amino acids, DHA, α-vitamin E, and astaxanthin, but lower contents of MUFA, crude lipid, and peroxidation products (protein carbonyl and malondialdehyde) than yellow eggs. These differences in nutrient composition may contribute to the superior reproductive performance of orange eggs over yellow eggs.

Fish meal and fish oil are important sources of protein and lipid in feeds. The lack of fish meal and fish oil supply has become an important limiting factor for the aquaculture industry. The proportion of fish oil and fish meal used in commercial feeds for farmed fish and crustaceans is continually decreasing, while the carbohydrate content is increasing. Carbohydrates are one of the three major nutrients that provide energy for the body. They have a much lower production price than protein and fat. Starch is a polymeric carbohydrate made from glucose molecules; it is the most commonly used in aquatic feeds and can replace fish meal and fish oil to reduce feed cost. Therefore, it is important to fully explore the nutritional function of carbohydrates for aquaculture. Hypoxia is a common stress in aquaculture, and acute hypoxia may lead to massive mortality of cultured fish in a short period of time resulting in serious economic losses to aquaculture. Therefore, it is crucial to find ways to improve the acute hypoxia tolerance of fish. Fish mainly use glucose for energy under acute hypoxia. Hypoxia inhibits oxidative phosphorylation in mitochondria and activates the anaerobic glycolysis pathway. Glucose degrades to produce lactate and ATP. Carnivorous fishes have limited absorption and utilization capacities to feed carbohydrates, unlike omnivorous fishes. Dextrin is an intermediate starch hydrolysis product, with a molecular weight between starch and glucose; it has good adhesion properties and is more easily digested and absorbed than starch. Therefore, we hypothesized that the use of easily digestible carbohydrate in the feed is an effective way to improve the acute hypoxia tolerance of fish. Takifugu rubripes is loved by Japanese and Korean consumers owing to its delicious taste and high nutritional value. It is a characteristic species of Chinese mariculture fish. It is mainly cultured in a high-density factory, and the dissolved oxygen in the water often relies on water exchange and an oxygenation pump; its gill cover is degraded and is at risk of acute hypoxia. This study determined the effect of the addition of corn starch or dextrin (corn starch hydrolysate) to the feed on growth performance, acute hypoxia survival rate, metabolite content, and the hypoxia inducible factor (HIF) signaling pathway of T. rubripes after 8 weeks of feeding. There were no significant differences in weight gain, feed conversion ratio, hepatosomatic index, viscerosomatic index, condition factor, and body composition between the starch group and the dextrin group (P > 0.05). However, the survival rate during acute hypoxia of the dextrin group was significantly higher than that of the starch group (P < 0.05). There were no significant differences in the liver glycogen and lactate contents, and lactate dehydrogenase A4 (ldha) gene expression between the starch group and dextrin group (P > 0.05) during normoxia. However, the lactate content and ldha gene expression were significantly higher in the liver of the dextrin group than those in the starch group (P < 0.05) during hypoxia. This indicated that feeding dextrin strongly activated anaerobic glycolysis to provide more energy under hypoxia. The serum triglyceride (TG) content significantly increased in the dextrin group compared with the normoxia groups, although the TG content in the serum and liver significantly decreased in the starch group after acute hypoxia (P < 0.05). This suggested that feeding starch promoted lipid catabolism and oxygen consumption. There was no significant difference in the total soluble protein content of the muscle between the starch group and the dextrin group (P > 0.05) during normoxia. However, the total soluble protein content of the muscle decreased in the dextrin group compared with the starch group (P < 0.05) after acute hypoxia. Meanwhile, the total muscle soluble protein content, and the gene expression of liver v-akt murine thymoma viral oncogene homolog 1 (akt1), and mechanistic target of rapamycin kinase (mtor) significantly decreased in the dextrin group after hypoxia compared with the normoxia group. However, the liver mtor gene expression significantly increased in the starch group after hypoxia (P < 0.05). This data demonstrated that protein synthesis was inhibited in the dextrin group under hypoxia. Hypoxia inducible factor (HIF) is the most critical transcription factor in cellular response to hypoxic stress. The dextrin group had higher hypoxia inducible factor 1 subunit alpha like (hif-3α) gene expression in the liver and lower hypoxia inducible factor 1 subunit alpha a (hif-1α) gene expression in the muscle (P < 0.05) compared with the starch group during normoxia. The gene expression of liver hif-1α and hif-3α and muscle hif-1α and vascular endothelial growth factor A (vegfa) significantly increased in the dextrin group compared with the starch group (P < 0.05) during hypoxia. This proved that feeding dextrin strongly activated the HIF signaling pathway under hypoxia. In summary, replacing starch with easily digestible dextrin in the feed did not affect the growth performance. Instead, it activated the HIF signaling pathway and anaerobic glycolysis to provide more energy for fish. Meanwhile, feeding dextrin inhibited lipid catabolism and protein synthesis, and reduced oxygen consumption to improve the acute hypoxia tolerance of T. rubripes. The study provides important guidance for the formulation design of hypoxia-tolerant feed and healthy development of aquaculture.

Fish are the main source of long-chain polyunsaturated fatty acids (LC-PUFA), in particular docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3), which are nutritionally valuable to humans. Moreover, lipid and fatty acid distribution in fish tissues are highly diverse across species. The lipid distribution pattern across tissues determines fatty acid profile, one of the product quality indicators of fish. However, this diversity of lipid distribution and fatty acid profile is not fully known, although several studies have been conducted to compare the nutritive composition of muscle in different fish species in both freshwater and marine environments. This study aimed to comprehensively investigate the lipid distribution and fatty acid profile, as well as the somatic indexes and approximate composition of five benthic marine fish species, namely yellow anglerfish (Lophius litulon), flathead (Platycephalus indicus), eelpout (Zoarces viviparus), East Asian flatfish (Pleuronichthys cornutus), and zebra sole (Zebrias zebrinus). Fishes were purchased from the 15th Street Seafood Market of Qingdao, which were stored on ice since captured in an inshore fishery in Qingdao (located at 119°30'–121°00'E and 35°35'–37°09'N), and then delivered to the market from the fishing harbor in the same morning. After purchase, fishes were kept on ice and immediately taken to the laboratory. The experimental design consisted of nine fish of each species bulked in groups of three, comprising three replicates. First, the total weight and length of each fish were measured, and the liver and viscera weight were recorded to calculate the viscerosomatic index (VSI), hepatosomatic index (HSI), and condition factor (CF). Subsequently, samples of muscle, liver, brain, eye, skin, subcutaneous adipose tissue around fins (only for East Asian flatfish and zebra sole), and intraperitoneal adipose tissue (only for flathead) were collected for lipid and fatty acid analysis. The dorsal muscle sampling position is behind the head on the right body side. The ventral muscle samples were collected under the dorsal muscle sampling spot. The proximate composition analysis of the dorsal muscle, ventral muscle, and liver (three individual samples per group) was performed according to the Association of Official Analytical Chemists standard methods. For the moisture assay, samples were oven-dried at 105 ℃ until they achieved a constant weight. The protein content was assayed by measuring nitrogen (N × 6.25) using the Soxhlet method, the lipids were analyzed using the Soxhlet method (petroleum ether extraction), and the ash by incineration at 550 ℃. The fatty acid compositions of all tissues were analyzed with gas chromatography (GC-2010, Shimadzu, Japan). The results showed that the flat body type of flatfish determines that the gut in these fish species cannot store high lipid contents in the liver or intraperitoneal adipose tissues. Like other flatfish, East Asian flatfish and zebra sole had lower HSI and VSI. In contrast, the HSI and VSI of yellow anglerfish and flathead were associated with high lipid contents in the gut. Additionally, the lipid distribution across tissues indicated that yellow anglerfish and flathead predominantly stored lipid in the liver and intraperitoneal adipose tissue, respectively. The muscle lipid content of all the five species was low, especially for yellow anglerfish and zebra sole (0.3%–0.4%). Yellow anglerfish, flathead, and East Asian flatfish had higher liver lipid content (19%–29%) than zebra sole and eelpout (4%–5%). Yellow anglerfish and flathead store lipid predominantly in the liver and intraperitoneal adipose tissue, respectively. The n-3 LC-PUFA content, especially DHA, in the muscle of yellow anglerfish was significantly higher than that of other species. The yellow anglerfish dorsal muscle also had the highest 18:1n-9 content. The relative abundances of different monounsaturated fatty acids (MUFA) and saturated fatty acids (SFA) in different fish species indicate a preference for energy storage and lipid mobilization. The most distinct fatty acid characteristics of the dorsal muscle of eelpout were low 16:0 content and high EPA content. For liver fatty acids, flathead and East Asian flatfish had significantly high MUFA contents, such as 18:1n-9, but their DHA contents were low. High DHA, 20:4n-6, and 22:5n-3 contents were consistently observed in nearly all yellow anglerfish, eelpout, and East Asian flatfish tissues, respectively, while low EPA content was observed for all of them. Among all five fish species, eelpout had a much higher ARA content than the other species in nearly all tissues. This could be due to the high ARA level in the food sources, namely algae. Yellow anglerfish had significantly high n-3 LC-PUFA content and thus high fatty acids nutritional value. This study revealed that even though these fishes are all benthic species, there are great differences in lipid and fatty acid composition among them.

Yadong trout (Salmo trutta fario), the only salmonid fish in the Tibetan Plateau, is a national agricultural product with geographical indications in Yadong. The development of Yadong trout aquaculture has significant implications for local farmers. However, no study has yet investigated the nutrient requirements of this fish. This study aimed to analyze the proximate amino acid, and fatty acid composition of various tissues of wild and farmed Yadong trout, in order to provide a reference for the evaluation of fish fillets and the formulation of optimal feed. Ten wild fish captured in the Yadong River, and 10 fish farmed with formulated feed in Yadong Industry Park were used in this study. The farmed fish were F1 generation wild broodstock captured in the Yadong River and fed formulated feeds throughout their lifetime. The body length of wild Yadong trout was in the range of 13.4~28.7 cm, and the body weight was in the range of 38.48~346.85 g. The body length and body weight of farmed Yadong trout were in the range of 16.4~24.5 cm and 78.39~275.29 g, respectively. Frozen samples were transported to the laboratory by flight. The proximate composition of whole fish, muscle, and liver; fatty acid composition of whole fish and various tissues including muscle, liver, intestine, brain, and eye; as well as amino acid composition of whole fish, muscle, and liver were analyzed. The fatty acid composition (expressed as % total fatty acids) and amino acid composition (expressed as % dry matter basis) were assayed using gas chromatography and an automatic amino acid analyzer, respectively. In addition, t-tests were performed for independent samples; results are expressed as the mean ± standard deviation. The results showed that wild fish had lower condition factors but higher moisture and ash content than farmed fish. Wild fish muscle had lower crude lipid content but higher moisture content than farmed fish. The whole-body protein and lipid contents, as well as the proximate composition of the liver, were not significantly different between wild and farmed fish (P > 0.05). The contents of saturated fatty acids (mainly C16:0 and C18:0) and n-3 polyunsaturated fatty acid (PUFA) in whole fish, muscle, and liver were significantly higher (P < 0.05), whereas contents of monounsaturated fatty acids (MUFA) (mainly C16:1n-7 and C18:1n-9) and n-6 PUFA (mainly C18:2n-6) were significantly lower in wild Yadong trout than in farmed Yadong trout (P < 0.05). EPA and C20:4n-6 in whole fish, muscle, and liver, as well as DHA in the muscle of wild Yadong trout were significantly higher than those in farmed Yadong trout (P < 0.05). In the intestine of wild Yadong trout, the contents of C14:0, C16:0, C18:0, C16:1n-7, C20:5n-3, and C22:5n-3 were significantly higher (P < 0.05), while those of C18:1n-9 and C18:2n-6 were significantly lower (P < 0.05) than in the farmed fish. In the brain of wild Yadong trout, the contents of C16:1n-7, C20:5n-3, and C22:5n-3 were significantly higher (P < 0.05), while the contents of C18:1n-9 and C20:2n-6 were significantly lower (P < 0.05) than those of farmed fish. In the eyes of wild Yadong trout, the contents of C14:0, C16:0, and C18:0, C16:1n-7, C20:1n-9, C20:2n-6, C20:4n-6, C20:5n-3, and C22:5n-3 were significantly higher (P < 0.05), while the contents of C18:1n-9, C22:1n-9, C24:1n-9, C18:2n-6, and C18:3n-3 were significantly lower (P < 0.05) than those of farmed Yadong trout. The total essential amino acid content in wild fish was significantly higher than that in farmed fish. The contents of threonine, valine, phenylalanine, lysine, and glycine in the muscle of wild fish were significantly higher (P < 0.05) than those in farmed fish, while no significant difference (P > 0.05) was observed in other amino acids. These results indicate that wild Yadong trout were leaner than farmed trout in terms of muscle lipid content. For trout, which are mainly consumed fresh, this trait may make farmed Yadong trout more acceptable than wild trout. However, in terms of fatty acid composition and amino acids, the wild Yadong trout seemed to be more acceptable than farmed trout. In particular, n-3 PUFA content is an important nutritional trait in fish fillets. The wild Yadong trout had higher n-3 PUFA contents than the farmed trout. Thus, the fish oil in the feeds of Yadong trout should not be omitted, considering that fish chemical composition generally reflects that of their diets. In addition, the price of alternative oils such as soybean oil is no longer low; the use of lipid sources in the diets of Yadong trout should be reviewed. Similarly, the essential amino acid content in wild Yadong trout was higher than that in the farmed trout, suggesting that the protein sources should be reviewed, and a certain level of fish meal should be guaranteed. In conclusion, the results of this study indicate that the formulated feeds of Yadong trout need to be further optimized.