| Chinese sturgeon (Acipenser sinensis Gray,1835) is a large anadromous sturgeon with the sole sturgeon species crosses the northern temperate zone. Historically, the species ranged widely occurring in the offshore of China (including Yellow Sea, East China Sea, Taiwan Strait, Korean Peninsula and Japan Sea) and the large rivers which flow into these seas (involving Yangtze River, Pearl River, Minjiang River, Qiantangjiang River and Yellow River). Currently, the populations of Minjiang River, Pearl River, Qiantangjiang River and Yellow River have been vanished, the Yangtze River population is the only remaining population of this species. The construction of Gezhouba Dam (GZD) blocked the migratory routes, and the spawning grounds accompanying with pre-spawning holding habitats up the GZD were disappeared in1981. Fortunately, spawning event was found from the GZD to Gulaobei reach with the length of30km in the following year, of which the stable spawning ground was located about3.85km long in the reach from GZD to Miaozui. The scale of spawning ground was only as5%large as the historical. In2003, the impounding of Three Gorges Dam (TGD) changed the natural hydrological rhythm of river reach below the GZD. In2006, the River Adjust Project below in the spawning ground altered the river bedform, which all affected the normal distributions and spawning of adult Chinese sturgeon. In addition, due to the habitat degradation, water pollution, navigation and bycatch, the stock of Chinese sturgeon have been markedly decreasing. This trend has not been reversed although a series of protective measurements were made to protect and restore the population, such as listing it as the first class of protected animals in China, prohibiting fishing, establishing the Nature Reserve and implementing artificial propagation from1983. Finally, the species was classified as Critically Endangered (CR) on the IUCN Red List in2010.The late sexual mature and long reproductive cycle of Chinese sturgeon are adverse to restore the population, but it would be compensated by the large fecundity. Consequently, these features reflect that it is critical to protect each adult. In order to protect and restore the population, it would be important to understand their complete migrations and habitats in the Yangtze River, and ensure the spawning success of each adult Chinese sturgeon. In this paper, the ultrasonic telemetry, echosounder, microhabitat measuring and Geographic Information System was employed to study the migration in the Yangtze River and habitat distributions in the key reach. Secondly, the relationship between the distributions of fish and some critical variables were analyzed, and habitat suitability models were established. Finally, the pre-spawning holding habitats in the key reach were determined and the potential pre-spawning holding habitats were searched based on the models. The results would supply the scientific reference for protecting the pre-spawning holding habitats and spawning ground, and exploring the potential habitats. The main methods and results in this study are as follows:1. From2006to2009, twenty seven ultrasonic-tagged wild adult Chinese sturgeon Acipenser sinensis [8males,19females; total length (LT) range=245～368cm] were captured on the spawning ground just downstream of the GZD in the Yangtze River. Twenty-six individuals were located for7～707days (mean number of relocations=859; range=3～4549). Sturgeon movements were divided into four stage-categories:1) Spawning Migration. Two tagged sturgeons (one female and one male) returned to the Yangtze River and migrated from the Yangtze Estuary (rkm0) to the spawning ground (rkm1678) between June and October. Their mean upstream ground speed was0.54km/h (0.26～1.77km/h) and2.20km/h (1.82～2.35km/h), and the speed of the male was faster than the female.2) Pre-spawning Holding. Four of five females tagged in November2008stayed within rkm1678.00～1674.15for about one year before the spawning period.3) Spawning Movements. All sturgeons swam mostly from the tailrace of the GZD (rkm1678) to the Miaozui (rkm1674.15) reach and some moved downstream about18.21rkm (range=3.93～24.64rkm), but then, returned upstream to the GZD. Most tagged sturgeons were on the spawning ground on the day when the spawning occurred.4) Post-spawning Migration. Males (N=6) and females (N=2) departed the spawning area on a different time schedule, females leaving before males. The mean seaward ground speed of six sturgeons was4.87km/h (range=0.68～7.60 km/h). There were no significant differences in ground speeds among reaches or between sexes (reaches=ANOVA, P=0.56>0.05; sexes=ANOVA, P=0.69>0.05). These broad spatiotemporal scale results will help establish an effective protection strategy for the species in the Yangtze River.2. Twenty-five ultrasonic-tagged wild adult Chinese sturgeons were used to study the distributions on the staging-spawning ground during the spawning seasons. The spatial distribution type of tagged adults was clumped (CE=0.332<1). Most all adult locations (91%) were in the USS, the east section (left side) of the dike, the head (upstream reach) of the DSS, and in the Shilihong reach. The distribution area of MCP was1.67km2, the5rkm from GZD (rkm1678) to Shilihong (rkm1673). Only9%of all locations were at Moji Hill (rkm1671), Yanzhiba Bar (rkm1665.5) and Huyatan (rkm1653). All data indicated that adults selected the main channel for moving upstream or within the spawning grounds. There was significant difference in distributions of fish before and after spawning (P<0.001), and the differences were also found within the different years and between sexes. The mean inhabiting depth of ripe fish was8.71m (95%CI8.58～8.85m), the depth ripe fish used decreased gradually before spawning, and then, swimming depth increased after spawning, with most shallow depths occurring on the spawning day (mean=5.12m). One-way ANOVA analysis showed that there was a significant difference (P<0.001) between sexes for depth use, with females using deeper depths than males. Also, depth use was significantly different between day and night (P<0.001), with adults using deeper water during the day than at night. The mean depth of non-ripe fish was14.59m (95%CI=13.75～15.43m) which was significant deeper than the mean depth of ripe females (ANOVA, P<0.001).3. The raster maps based on depth, bottom velocity, slope, and substrate embeddedness from GZD to Shilihong were established and the variable vaules were extracted from the maps. The mean depth of positions where tagged A. sinensis were located was10.10m and had a significant difference from the entire reach. The lagest value was38.32m. The mean velocity magnitude of inhabiting sites (Mean=1.18m/s,95%CI=1.16～1.19m/s) was larger than the entire reach (Mean=1.14m/s,95%CI=1.12～1.16m/s). The bottom velocity magnitude of locations was0.97m/s (95%CI=0.95～0.98m/s), also larger than the entire reach (Mean=0.95m/s,95%CI=0.94-0.96 m/s). For the topography, the average slope of fish locations was2.15°(95%CI1.96-2.33°) which was smaller than the total reach (mean=3.26°;95%CI=3.12-3.41°). The percentage of bottom sediments of fish locations was significantly larger than the total reach. ANOVA analysis also showed that the mean depth, mean velocity, bottom velocity and channel slope all had significant differences from the entire spawning ground. Pearson correlation analysis showed there was significant relationship between the environment variables. Finally, the suitability curves and habitat suitability index was modelled and the weighted usable areas for ripe and non-ripe fish was0.37km2and0.18km2, respectively.4. During the winter of2010and2011, hydroacoustics was used to detect the pre-spawning sturgeons in the key reach (GZD to Wuhan,620km) and the total of64signals of adult Chinese sturgeons was detected. The paths of2137.5km and610km were detected during2010and2011, correspondingly, the number of46and18individuals was found. The four clumped reaches were identified:the staging-spawning ground below the GZD (5individuals), Yanzhiba Bar (5individuals), Meizixi (8individuals) and Tiaoguan (23individuals), were similar as the historical distributions in the published paper and the results from our team. The mean depth of fish used, water depth of fish locations and the percentage apart from bottom were17.69m (95%CI=15.84～19.53m),21.63m (95%CI=19.00～24.26m) and85.66%(95%CI82.09～89.24%). These indicated that the fish were mostly close to the river bottom mostly. ANOVA analysis and the Non-parameters test was used to study the four reaches, which the results showed as follows:the depth of fish used in the staging-spawning ground below the GZD and Yanzhiba Bar reach was significant difference from the Meizixi and Tiaoguan, but no difference were found between later two reaches; the depth of fish locations was significant difference between each other except the results between the GZD and Yanzhiba Bar; However, there was no significant difference in the water layer of fish. In general, the stock was similar in the staging-spawning ground below the GZD and Yanzhiba Bar, but different from the Meizixi and Tiaoguan reaches.5. Depending on the measurement of aboitc environments and distributions of Chinese sturgeon, the key-factor suitability index curves of pre-spawning holding habitat for Chinese sturgeon were established. Also, the habitat suitability index model were built for the two main pre-spawning holding habitat i.e. Yanzhiba and Tiaoguan. Combined with the parameters of topographic and depth on the five natural reaches, the suitable pre-spawning holding habitat should be as follows:the type of reach was meandering or braided, the reach had sound bar, the length of habitat and deep water zones (>19m) were21.39km and13.88km, respectively. The turning angle of river channel was172°. The river width and the width of deep zones were874m and279m, respectively. The mean depth of thalweg and maximum depth were28.01m and40.55m. the value of S/L was2.56. Finally, nine potential pre-spawning holding habitats were extracted, which were involved four better suitable habitats and five suitable habitats.6. Based on the results of reproductive migrations and distributions of adult Chinese sturgeon in the Yangtze River, this study analyzed and discussed how the human activities, especially, the hydraulic projects, influence the normal migrating and selecting habitats of the adults. This paper provided specific frameworks for the protections and managements. To protect adult Chinese sturgeon in situ, a series of study work need be done in the future, such as studying sturgeon migratory mechanisms, improving the knowledge of sturgeon habitat, understanding the complete requirements of adults in the existing habitat, exploring potential spawning grounds and pre-spawning holding habitats, assessing human activities impact on the population, and updating in situ conserving and managing recommendations.
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