Chinese sturgeon, Acipenser sinensis Gray, an anadromous fish, mainly distributes in the mainstream of Yangtze River, the continental shelf of East China Sea and Yellow Sea. Historical spawning grounds of A. sinensis were in the upper Yangtze and the lower Jingsha River, covering a stretch of at least 600 km of river length. After the damming of Yangtze River by the Gezhouba Dam in January 1981, the spawning grounds for A. sinensis were no longer accessible and fish alternatively spawned within a 30 km section below the Gezhouba Dam. Moreover, only one regular spawning ground has been found in the downstream area of the Dam. It is in a narrow portion within a 7 km length, just below the dam. A minor spawning ground was also found at Huyatan, around 15 km below the dam. The present attempts to estimate stock size of A. sinensis suggest that the major source of recruitment from a complementary colony is still obtained through natural reproduction. A. sinensis requires a certain water temperature, moderate bottom velocity, and special riverbed topography to spawn successfully. Due to the restriction of experimental instruments and methods, the research on the topography of spawning grounds is a first attempt to tentatively characterize the area in terms of fine-scale bottom profile and structure.
Based on measurements taken during five surveys between 1999 and 2006, we analyzed the river bed morphology by assessing the elevation, slopes, and the bottom “roughness” of the spawning grounds by ArcGIS 9.0. The terrain of the riverbed was simulated by Inverse Distance Weighted scheme with an 8.0 m of grid size. The results showed that the roughness in regularly used alternative spawning site directly at the dam and the minor spawning ground further downstream are 1.74×10-3 and 0.36×10^-3, respectively. Moreover, the slope of area between Gezhouba Dam and Moji Hill is much bigger than that between the Linjiang Stream and Huyatan. The average and standard deviation of length of elevations at the upstream and downstream spawning sites are 32.35±2.18 m and 30.14±3.85 m, respectively. Taking the average water level for spawning between 1983 and 2002 as a standard, the average water depth on the upstream and downstream spawning sites were 11.66 m and 13.87 m, respectively. These depths may be the optimal water depth for A. sinensis to spawn under the given hydrodynamic regime in the Yangtze River.
Four views are considered in this article. (1) The topographical characteristics of the spawning ground which locates in the reaches between the Gezhouba Dam and the Shilihong are very similar to those on the historical spawning grounds. The natural reproduction of A. sinensis rarely occurs in the reach between the Linjiang Stream and Huyatan, and it is postulated that the major reason is to be found in the unsuitable terrain, which makes the hydrological regime and hydraulic conditions not suitable to trigger spawning activity. (2) The topography of the spawning ground below the Gezhouba Dam has been changed drastically during recent decades because of the River Regime Regulating Work. The increased complexity of the topography may yield some benefit for A. sinensis to attract fish to inhabit before the natural reproduction stage is reached, but the destruction of slopes on the spawning grounds perhaps goes against the dispersion of fertilized eggs and embryonic development. (3) The results provide further support for the hypothesis of “Dividing Function of the Spawning Ground of A. sinensis” in the area below the Dam at the present time. The turning of riverway and adverse slope of riverbed may be the absolutely necessary requirement for a suitable spawning ground of A. sinensis. Although the data are limted for such a working hypothesis, further studies should combine the accurate terrain data from historical spawning grounds of A. sinensis with thos from the new grounds to verify this potentially valid hypothesis. (4) The substrate, hydrological and hydraulis conditions are affected by the topography of the riverbed to a certain extent. It is necessary to make further investigation on the influence of riverbed terrain on the natural reproduction of A. sinensis, especially when considering the changes that may be anticipated with the full functioning of the Three Gorges Dam which must have also an effect on the hydrodynamics downstream even beyond the Gezhouba Dam.