Studies On The Influences Of The Operation Of The Three Gorges Dam On Reproduction Of The Chinese Sturgeon, Acipenser Sinensis: The Ecohydrological Mechanism And Conservation Strategy

Chinese sturgeon (Acipensor sinensis) mainly lives in the continent shelf of the Yellow China Sea and the East China Sea and spawns in the Yangtze and Pearl River. Its spawning ground in the upper Yangtze River was restricted in a narrow reach downstream close to the Gezhouba Dam after the dam was established 1981. According to its operation protocol, the impounding period of the Three Gorges Reservoir (TGR) starts from early October, which would be overlapped by the spawning activity of Chinese sturgeon. It could be estimated that the reduction of discharge caused by the impoundment of the reservoir would result in a decrease in spawning scale of the sturgeon. In this thesis, adopting ecohydrological theories and methods, on the basis of investigations on historical records and surveys on current status of the spawning ground below the Gezhouba Dam,the ecohydrological pattern of the spawning ground of the sturgeon was initially established and was used for reviewing the possible impacts of the operation of the Three Gorges Project (TGP) on the natural spawning of Chinese sturgeon, and working out strategies of re-operation of the dam for the improvement of ecohydrological conditions for the Chinese sturgeon to spawn naturally. The main results are as followings:(1) Due to the blockage of the Gezhouba Dam, the reach in which natural spawning grounds of Chinese sturgeon distributed was shortened from about 800 km to 5 km and moved from the upper reach of the Yangtze River to downstream close to the dam. Additional impacts of the operation of TGP on the natural spawning of Chinese sturgeon would be mainly induced by the change of discharge in the spawning season of the sturgeon. For instance, due to the impoundment of the reservoir in October, a less discharge after impoundment than before would result in lower water level and insufficient water flow for the natural spawning of Chinese sturgeon.(2) Before the construction of the Gezhouba Dam, the natural spawning grounds of Chinese sturgeon were mainly distributed in the reach between Pingshan section in lower Jinsha River and Modong section at the end of the Three Gorges Reservoir. Based on the investigations on the natural distribution of its spawning grounds, relationships between hydrological factors (i.e. water level and flow) and the natural propagation of Chinese sturgeon were analyzed according to the hydrological records from 1961 to 1980. Natural spawning activities of Chinese sturgeon occurred mainly between October and November in Yibin and Pingshan sections when the water level dropped. Typical flow during spawning period was between 6700m3/s and 3500m3/s in Pingshan section, and between 7050m3/s and 3700m3/s in Yibin section. Typical water level was between 285.93m and 281.96m in Pingshan section, and between 265.33m and 262.17m in Yibin section.(3) Through investigations on relationship between natural spawning activities and hydrological conditions during a period of 22 years from 1983 to 2004 after the construction of the Gezhouba Dam, variation of hydrodynamic factors such as water flow and water level in the spawning season of Chinese sturgeon was analyzed. During spawning period in the reach below the dam, typical flow was between 17000 and 10000m3/s, and reducing speed of water flow was between 9.72 m3/s/h and 3.47m3/s/h; typical water level was between 45.35 and 43.63m, and dropping speed of the water level was between 0.209m/d and 0.034m/d.(4) According to empirical and theoretical frequency distribution curves of typical water flow and water level of the spawning ground below the Gezhouba Dam during the spawning period, the spawning probability of Chinese sturgeon was estimated as approximately between 79.5% and 80.1% in October, and between 25.9% and 28.4% in November. In comparison with statistical data, two-thirds of the natural spawning occurred in October and one-third in November, and this was close to the theoretical estimation.(5) According to the surveys and historical data on the hydrological and hydraulic conditions of spawning grounds, it could be concluded that the Chinese sturgeons prefer to stay in the areas with a water flow of 0.12m/s-0.86m/s in pre- and post-breeding periods while they usually mate in water areas with a depth of 6.10-15.00m and a flow of 0.62m/s-1.16m/s.(6) Based on the analysis on impacts of operation patterns of TGP in different hydrological years on the hydrological conditions of natural spawning of Chinese sturgeon, it is concluded that the designed operation pattern of TGP will be able to satisfy the ecohydrological requirements of Chinese sturgeon. The related strategies and measures for preventing the expected and unexpected inflow of the reservoir which could damage the natural spawning of Chinese sturgeon could be considered as followings:1) To impound the reservoir prior to the spawning seasons to improve water flow conditions for Chinese sturgeon in dry years;2) To make further researches on impacts of water flow regime on the spawning activities of Chinese sturgeon to guarantee it's spawning in the normal hydrological years;3) To study impacts of the operation of cascading reservoirs upstream on water release of TGR and adjust its operation protocol to reduce the adverse effects on the ecological requirements of spawning field.