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BACKGROUND It is of great significance to study the breeding environment of the naked carp in Qinghai Lake for the conservation and restoration of natural habitat. The naked carp is the only economic species in Qinghai Lake, colloquially called “Huang Yu”, and plays a core role in the lake ecosystem. As an anadromous species, the naked cap migrates between Qinghai Lake and major rivers to spawn such as the Buha, Shaliu, Quanji, Heima, and Hargai rivers from April to August every year. Due to intensive fishing, arid climate, and the lack of spawning rivers, the amount of the naked carp’s resources has dropped sharply, from 28000 tons in the 1960s to 2263 tons in the 1990s. In view of its important ecological status, it is urgent to restore and protect the resources of naked carp in Qinghai Lake. Since 1982, Qinghai Province has forbidden fishing to restore and protect the fish resources. At the same time, it has established fish conservation bases and artificial breeding and releasing stations around Qinghai Lake. By 2022, the fish resources of the naked carp had increased to 108500 tons, 42 times as many as in the early stages of protection. The breeding environment is the key factor for the continuation of the fish population, and different fish require different parameters of the spawning ground, such as river, light, sediment and water temperature. It is also very important to obtain the reproductive environmental parameters and determine the optimal spawning ground for the protection and expansion of the fish resources. At present, only Zhou et al.[3] have discussed the environmental requirements of natural reproduction of the naked carp by indoor artificial simulation. Therefore, it is of great significance to study the breeding environment of the naked carp for the conservation and restoration of natural habitat. Otolith δ18O provides a method for fish population identification, migration and environment exploration of spawning ground. Otolith, a calcium carbonate mineral in bony fish, has both auditory and balancing functions and is known as the “recorder” of time and water environment. Microchemical analysis of otolith can reveal the life history and water environment changes of fish. Especially, the core area of otolith is formed at the early stage of incubation, and its microchemical composition can reflect the hydrochemical characteristics of the habitat and solve the problem of fish identification. The otolith δ18O is closely related to water temperature. Under normal salinity conditions, the δ18O change of 1‰ corresponds to the seawater temperature change of about 5℃. For example, the higher otolith δ18O values of Pacific bluefin tuna, which migrated across the Pacific from the Western Pacific (WPO) to the Eastern Pacific (EPO), reflected the cold-water temperatures encountered during migration. Similarly, the lower δ18O value in otolith core of Oncorhynchus keta, which migrated between freshwater and marine environments, indicated that Chinese salmon were also anadrotic spawning fish, returned to the ocean after hatching. Yuan et al.[12] analyzed the δ13C and δ18O values in the otolith of small yellow croaker in the Yellow Sea and the Bohai Sea, the population of southern Yellow Sea was subdivided into offshore and coastal populations for the first time, and no station crossing between the two populations was recorded. The overall successful rate of discrimination was recorded to be 82.6% by cluster analysis. Tatsuya et al.[13] conducted otolith δ18O and microstructure analyses to investigate nursery habitat temperatures and early life growth rates. In conclusion, the otolith δ18O provides a method for fish population identification, migration, and environment exploration of spawning ground. In this study, the SHRIMP Ⅱ ion probe was used to determine the otolith δ18O of the naked carp in Qinghai Lake. Combined with the hydrochemical composition of lake water and river water in Qinghai Lake, the optimal migration and spawning sites and environmental conditions of the naked carp were investigated to provide effective information for further revealing its life history. RESULTS The SHRIMP Ⅱ ion probe was used to determine the in-situ δ18O composition of the otolith microregion of five naked carp in Qinghai Lake. The in-situ oxygen isotope analysis of otolith was performed by SHRIMP Ⅱ ion probe at the School of Earth Sciences, Australian National University. Sectioned otoliths were prepared for SHRIMP analysis by casting them in epoxy resin, with NBS 18 and 19 reference calcites, to form a 35 mm diameter mount. After being documented by optical photomicroscopy, the samples were coated with high purity Al and transferred to the ANU SHRIMP Ⅱ for analysis using procedures based on those described in detail by Long et al.[21-22]. In brief, the SHRIMP Ⅱ was operated in multi-collector, negative ion mode. A 15kV, ~3nA Cs+ primary ion beam was focused to a 25mm diameter spot on the Al-coated target, producing 200−250pA of secondary 16O−. 16O− and 18O− were measured simultaneously on Faraday cups using Keithley 642 electrometers. Each analysis consisted of a pre-burn of about 3 min to allow the secondary ion isotopic composition to stabilise, followed by 10 or 14 times (10s/time) estimates of the 18O/16O ratio. The accuracy was 0.1‰−0.2‰, and the standard deviation was about 0.3‰. The reference material was measured once every 5 samples completed during analysis. The spot starting position of the ion probe was the core of the otolith, along the longest growth axis to the edge. The spot diameter of the ion probe was about 25μm, and the spot depth was about 2−3μm. The spot test was shown in Fig.E.1A. (2) The δ18O composition along the growth axis of the otolith. The δ18O ratios along the growth axis of 5 naked carp otoliths were shown in Fig.E.1B. The mean values of otolith δ18O from No.1 to No.5 were 1.29‰, 1.79‰, 0.99‰, 0.93‰, −7.41‰ respectively, and their interval range were −4.88‰−3.46‰, −0.28‰−3.91‰, −1.43‰−2.94‰, −1.81‰−3.35‰, −9.36‰−5.21‰, respectively. The otolith δ18O in individual juvenile plaice ranged from 0.2‰ to 1.9‰, when they were reared at two temperatures (11℃ and 17℃), which indicated that the otolith δ18O ratios were significantly affected by water temperature, but not by feeding level, and there were no significant synergistic effects. The δ18O ratios of plaice are close to the higher δ18O ratios of samples 1 to 4 in this study, which is the result of the fact that naked carp lived in the lake with high salinity and low water temperature for part of the time. The Schizothorax kozlovi and the naked carp belong to the subfamily Schizostomus. The average otolith δ18O ratios of the Schizothorax kozlovi cultured in fresh water is −9.4‰, which is very close to the δ18O ratios of the naked carp No.5, indicating that sample No.5 has been living in fresh water. The gray bands are the incremental zones of otolith, and the interval zones between the incremental zones are discontinuous zones. The δ18O ratios of the naked carp in samples 1 to 5 show a trend of lower incremental zones and higher discontinuous zones, because the δ18O ratios of otolith is inversely correlated with water temperature. The incremental zones of otolith are formed in spring and summer, while the discontinuous zones in autumn and winter, and the water temperature in spring and summer is significantly higher than that in autumn and winter. The otolith δ18O ratios of the five samples were analyzed by one-way variance analysis. The δ18O ratios of the No.1, No.3 and No.4 naked carp were not significantly different (P>0.05), while theδ18O ratios of the No.2 and No.5 naked carp were significantly different from the other three samples (P

Crisp grass carp is a type of grass carp (Ctenopharyngodon idella) that has been fed faba beans. Flesh quality and expression of muscle quality-related genes in muscle of grass carp were evaluated. Compared with ordinary grass carp, 1901 differentially expressed genes (DEGs) were up-regulated and 1628 DEGs were down-regulated in crisp grass carp. The GO analysis results showed that the DEGs were enriched in ribosomes and proteins. Kyoto Encyclopaedia of genes and genomes (KEGG) pathway analysis results showed that amino acid metabolism was closely related to meat quality. COX3, ACADS, ACAT1, ECHS1, EHHADH, HADHAA, HADH, HMGCL, MCCC1 and MCCC2 were the top 10 hub genes most closely associated with other nodes and showed a strong positive correlation. Hardness was positively correlated with the COX3, ECHS1, ACADS, ACAT1, HADHA, HADH, HMGCL and MCCC1. These results have been demonstrated to play important roles in clarifying the molecular mechanism of muscle firmness increase in crisp grass carp. Highlights: Feeding faba bean meal improved texture parameters of grass carp. A more comprehensive crisp grass carp transcriptome database was established. Several amino acid metabolism pathway that were closely related to meat quality were identified. [ABSTRACT FROM AUTHOR]