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The ecosystem and Pleistocene glaciations play important roles in population demography. The freshwater gudgeon, Gobiobotia filifer, is an endemic benthic fish in the Yangtze River and is a good model for ecological and evolutionary studies. This study aimed to decode the population structure of G. filifer in the Yangtze River and reveal whether divergence occurred before or after population radiation. A total of 292 specimens from eight locations in the upper and middle reaches of the Yangtze River were collected from 2014 to 2016 and analyzed via mitochondrial DNA Cyt b gene sequencing. A moderately high level of genetic diversity was found without structures among the population. However, phylogenetic and network topology showed two distinct haplotype groups, and each group contained a similar proportion of individuals from all sampled sites. This suggested the existence of two genetically divergent source populations in G. filifer. We deduced that a secondary contact of distinct glacial refugia was the main factor creating sympatric populations of G. filifer, and climate improvement promoted population expansion and colonization.

Gobiobotia filifer is a small benthic fish distributed in Yangtze River Basin. The abundance of G. filifer increased after impoundment of Xiluodu Dam and Xiangjiaba Dam. The state of population structure and changes of genetic diversity before and after impoundment of Xiluodu Dam and Xiangjiaba Dam were interesting issues. However, efficient molecular markers were rare, which will limit us to solve above problems. Twenty‐eight expressed sequence tag SSRs (EST‐SSRs) were successfully identified and verified as stable amplification and polymorphic loci by polyacrylamide gel electrophoresis (PAGE) and capillary electrophoresis. The number of alleles at these EST‐SSR loci ranged from 3 to 14, the polymorphism information content values were 0.125–0.897, and the observed and expected heterozygosities were 0.0–0.857 and 0.132–0.928, respectively. Cross‐species amplification of the 28 loci developed in this study was examined in seven individuals of each of the 7 taxa. The amplification efficiency of 28 EST‐SSRs primer pairs is related to the distance of genetic relationship between cross‐species with G. filifer, and same subfamily species (Xenophysogobio boulengeri and Xenophysogobio nudicorpa) showed the highest (50%) amplification efficiency. These EST‐SSR markers could be used to analyse genetic diversity and population structure of G. filifer and related species.

Length–weight (LWR) and length–length (LLR) relationships were estimated for 20 species of fish from the upper Yangtze River, southwest China. Samples were collected using multiple types of gear (2–6 cm mesh trammel nets, 2 cm mesh trap nets, and baited hooks) in Jiangjin and Yibin sections from 2007 to 2009. New maximum lengths for 16 species were recorded for FishBase. LWRs were significant for all species. Standard length–total length equations for converting size measurements for these 20 species are also presented. The results provide primary data for future fish research and conservation.

With the consideration of Orowan strengthening effect, a new constitutive model of particulate-reinforced magnesium matrix nanocomposites for thixotropic plastic forming was derived by analyzing relationships among parameters (stress, strain, strain rate, liquid phase rate f L , volume fraction of nano-reinforcement f p , temperature T ). The multiple nonlinear regression method was used to determine the parameters in the constitutive model. A specific constitutive model of SiCp/AZ61 nanocomposites in thixotropic plastic forming was proposed. The calculated values obtained from the constitutive model are in good agreement with the experimental results. Therefore, the constitutive model provides references for the thixotropic forming theory, simulation and fabrication of nanocomposites.

Rhinogobio ventralis, an endemic species in the upper Yangtze River, has been threatened by overfishing and dam construction. To understand its population dynamics after the Three Gorges Reservoir was built and before the Xiangjiaba and Xiluodu reservoirs were built, we employed body length-frequency data to estimate growth and mortality parameters, population abundance and utilization of the fish based on surveys in the Jiangjin and Yibin sections of the upper Yangtze River conducted from 2007-2009. Fish ranged from 52 to 250 mm in length and 2.7 to 307.2 g in weight, with an average length of 150.8 ± 40.7 mm and an average weight of 72.3 ± 49.7 g. The average size of the fish in the Jiangjin section of the Yangtze River (168.6 ± 29.5 mm) was significantly larger than that in Yibin section (125.6 ± 41.2 mm). The length-weight relationship of R. ventralis was well-fit with a power function, W = 6.06 × 10 P < 0.01, n = 436). Asymptotic length (L∞) and growth constant (k) were estimated using length frequency data as 338 mm and 0.24/yr, respectively. Average natural mortality was comprehensively estimated as 0.43 using three empirical formulas proposed by Pauly, Gunderson & Dygert, and Jensen, respectively. The total mortalities were estimated using a length-converted catch curve analysis as 2.26 in the Jiangjin section and 2.09 in the Yibin section of the Yangtze River. The observed exploitation rates were 0.81 in the Jiangjin sec- tion and 0.79 in the Yibin section, which were higher than the maximum exploitation rates of 0.57 in the Ji- ·研究报告·

Summary The aim of this study was to quantify the fish fauna in the Three Gorges Dam (TGD) and assess the tendency for change as well as for any spatial and temporal distribution. Two types of echo-sounder equipment (HTI Model 241 and Simrad EY60) were used to detect the fish density and fish spatio-temporal distribution. Eight surveys were conducted between 2006 and 2010, with the results showing that fish abundance in the Three Gorges Reservoir (TGR) increased yearly from 15.5 to 123 ind./1000 m3, a change apparently correlated with the corresponding increase in the reservoir water level. Horizontally, fish density increased from the lower reaches to the upper reaches. Fish preferred inhabiting the area around the Zhongxian section during winter and the Chongqing section in spring. Vertically, fish density was highest in the lower layer, followed by the middle layer. The difference in fish density in different water layers observed in spring was not as evident as in the winter. Fish target strength throughout the eight investigations showed a skewed distribution with respect to fish size. The overall increasing tendency of fish density and the spatio-temporal distribution of fish species were closely related to the water level of the TGR during the impounding period. Other influencing factors included water flow rate, water depth, reservoir shape, habitat type, and protection policy.