| According to the report of the natural reproduction ecological survey of Chinese sturgeon(Acipenser sinensis)conducted during 2017-2020,no natural reproduction activity of Chinese sturgeon has been observed in the lower near-dam river section of Gezhouba for several years,which may be more related to the abiotic environmental changes in the area.Topography is an important factor in the formation of abiotic environmental factors such as flow velocity,and substrate is a key environmental factor for Chinese sturgeon to conduct natural reproductive activities.Therefore,this study combines hydroacoustic methods and underwater video imaging technology to investigate the correlation between hydroacoustic features and substrate phenology,and to describe the evolution of topography(1999-2020)and substrate(2004-2020)of the Chinese sturgeon spawning grounds under the Gezhouba Dam,to reveal the change pattern in conjunction with hydrological events in the Yangtze River basin,and to analyze the correlation between topographic and substrate changes and the natural reproduction status of Chinese sturgeon The study also analyzed the topography and substrate of the spawning area in spawning years,and analyzed the suitable topography and substrate for natural reproduction,which eventually provided scientific basis for the construction of artificial simulated spawning grounds for Chinese sturgeon,the screening of potential spawning grounds for Chinese sturgeon,and the restoration and reconstruction of spawning grounds for Chinese sturgeon under the Gezhouba Dam.The main research results are as follows:The analysis results of the elevation data from 1999 to 2020 obtained after related processing show that the topography of the river section about 8.5 km below the Gezhouba Dam changed to some extent during 1999 to 2020,and its topographic changes were mainly concentrated in 1999 to 2007,and the area with topographic changes greater than 1 m per year accounted for more than 30%,and the area with topographic changes greater than 1 m after 2007 accounted for 12～26%.After 2007,the percentage of topographic changes greater than 1 m was 12～26%,and the changes increased from 2013 to 2015 when the river section was flushed,and from 2015 to 2020 when the flushing and siltation alternated.The correlation between substrate hydroacoustic characteristics and substrate phenological characteristics was reconstructed when the correlation proposed by some previous scholars was not available.The results of combining the hydroacoustic method and underwater video image technology showed that the relationship between congestion(E)and hardness(H)could be fitted with the Boltzmann model with the equation: E = 3+ 95/(1 + exp((H-0.00679)/(0.00232))(R2 = 0.74),the relationship between mean particle size(D)and roughness(R)can be fitted with a logistic model with the equation expressed as: D = 285-285/(1 + R/0.000824)(R2 = 0.87),and the fractal dimension is related to the main composition of the substrate.The substrate evolution of the Chinese sturgeon spawning grounds under the Gezhouba Dam was analyzed by combining the correlations between substrate characteristics.The results showed that three hydroacoustic parameters,roughness,hardness,and fractal dimension,showed consistent trends,increasing from 2004 to 2007,decreasing from 2008 to 2012,continuing to increase from 2013 to 2019,and beginning to decrease in 2020,but with different magnitudes of change.By converting the roughness of the study area to grain size through the roughness-grain size correlation equation,it was found that the main changes in the substrate of the lower river section of Gezhouba were as follows: the proportion of substrate containing sand decreased by 33.08%between 2004 and 2008,increased by 12.69% between 2008 and 2012,decreased by33.83% between 2012 and 2015,decreased in 2019 compared to 2015 The substrate substrate experienced a process from 2004 to 2020,from a large amount of sand,to a moderate amount of sand,to a very small amount of sand,and finally a rebound of sand.The elevation and hydroacoustic parameters of the spawning area were extracted and analyzed during spawning years,and the results showed that the suitable spawning terrain for Chinese sturgeon may be gentle and may be negatively sloped.The range of hardness in the spawning area was concentrated,and the roughness and fractal dimension fluctuated,and the suitable substrate filling degree was judged to be between 20 and 80%,the mean substrate size was between 88 and 220 mm,and the substrate type was mainly composed of small and medium-sized pebbles based on the respective correlations.The hydrological events in the Yangtze River basin can reveal the topographic and substrate variation patterns.The topographic changes of the Chinese sturgeon spawning grounds under the Gezhou Dam were mainly due to the Three Gorges Dam impoundment operation,flooding and human activities before 2007,and the topographic changes afterwards were mainly due to the natural siltation of the boundary area,and the interruption of the natural reproduction of Chinese sturgeon may not be due to the topographic changes between 1999 and 2020.The construction and operation of hydraulic projects,the occurrence of floods in the Yangtze River basin,and human activities all affect the evolution of substrate: the discharge of clear water caused by the impoundment operation of the Three Gorges Dam will lead to a continuous reduction of the nudging quality downstream of it;the occurrence of floods in the Yangtze River basin facilitates the transfer of sediment from upstream to midstream and downstream;and human activities such as river potential adjustment projects and sediment scheduling will directly affect the change of sediment content.Among them,the continuous substrate scouring during the period of 2013 to 2019 greatly affected Chinese sturgeon,and the natural reproduction activities of Chinese sturgeon ranged from discontinuous occurrence to complete interruption,and the substrate changes were significantly correlated with the natural reproduction status of Chinese sturgeon.This study combined hydroacoustic methods and underwater video technology to construct correlations between hydroacoustic and phenological characteristics of substrate,and to elaborate the topographic and substrate evolution of Chinese sturgeon spawning grounds under the Gezhouba Dam through spatial distribution analysis,to reveal the change pattern with the help of hydrological events in the Yangtze River basin,to analyze the correlation between Chinese sturgeon natural reproduction status and topographic and substrate changes,and finally to infer the suitable topography and substrate for Chinese sturgeon natural reproduction.The results of this study provide basic data for future research on the artificially simulated spawning environment of Chinese sturgeon and the restoration measures of natural reproduction of Chinese sturgeon,and provide references for the reconstruction of Chinese sturgeon populations in the field. |
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