Analysis On Topographical Characteristics And Ecological Hydrodynamic Factors Of The Four Major Chinese Carps’ Yidu Spawning Ground In Yangtze River

Spawning ground is an important and sensitive place in the fish habitats, it has special topography and hydrodynamic characteristics. The characteristics have an important ecological significance for fish natural breeding. However, a variety of large-scale human activities on rivers, such as dams, construction of levees, river training and other projects, has changed the original spawning grounds, and also affected the quantity and quality of fish spawning grounds seriously, and then affected the reproduction and growth of fish. Therefore, investigation and analysis on fish spawning grounds and spawning requirements to environmental conditions is an urgent work for the restoration of spawning grounds, and developing watershed protection programs to protect natural fish resources.The four Major Chinese Carps (FMCC) including Mylopharyngodon piceus, Ctenopharyngodon idellus, Hypophthalmichthys molitrix and Aristichthys nobilis, are the main economic and special fish in China, located in the Yangtze River, Pearl River and Heilongjiang River. Furthermore, they are the main catching objects and the breeding cornerstone of freshwater fishing. The Yangtze River is the main supply bastion of FMCC with high-quality germplasm resources superior to other river systems in the history. As the genetic bank, the Yangtze River cannot be replaced by other rivers or other artificial methods. The FMCC has reproductive migratory characteristics, and the spawning grounds’conditions peculiar to other sections of the river cannot be replaced. However, the terrain and hydrodynamic conditions are important aspects forming special water environment. Therefore, it’s important to study and quantify the terrain characteristics and hydrodynamic conditions of spawning grounds for the spawning grounds’ ecological restoration and resource conservation.By comparing the historical change of the position and scale of FMCC’s spawning grounds in the Yangtze River, the distribution characteristic of FMCC’s spawning grounds along in the Yangtze River is analyzed. The topography and flow of Yidu spawning ground, a typical spawning ground of FMCC, are simultaneously measured by Acoustic Doppler Current Profilers (ADCP). And key terrain index system is set up preliminarily. According to the results of one-dimensional hydrodynamic model and statistical analysis methods, sections and topographic features in Yidu reach are analyzed, and then the terrain is descripted quantitatively. Furthermore, the three-dimensional flow in Yidu spawning ground is simulated through EFDC (The Environmental Fluid Dynamics Code) model based on the measured topography and hydrological boundary conditions. The hydrodynamic indexes— velocity, Froude number and vorticity are calculated and analyzed. Additionally, the correlation between these indexes and FMCC natural reproduction is recognized.These achievements will contribute to providing theoretical basis and technical support for revealing the flow trigger mechanism of FMCC spawning and repairing FMCC’s spawning grounds.The primary contents and results of this study are described as follows.1. Analysis on the historical evolution of FMCC’s spawning grounds in the Yangtze RiverThe survey to the FMCC’s spawning grounds in the Yangtze River by different scholars in different years is widely collected. The results indicate that the spatial distribution and position change of FMCC’s spawning grounds occurred mainly in the 1980s and after 2003. In addition to the disappearance and the newly formed, other spawning grounds remained stable.2. The topography index system is initially established to describe FMCC’s spawning grounds. And the difference between Yidu spawning ground and non-spawning reaches is analyzed.The indexes are selected to reflect topography changes along with the river and the flow process. According to the physical meaning, these indicators are assigned to the state indexes, the changing indexes and the spatial characteristic indexes, total of 18 indicators. The state indexes include river width, cross-sectional area, wetted perimeter, hydraulic radius and complex coefficient of cross section. The changing indexes include the changing rate of river width, wetted perimeter, hydraulic radius and complex factors of cross section along with the river and the flow process. The spatial characteristic indexes include roughness, relief amplitude and variance coefficient in elevation.According to the topography index system, the average value and standard deviation coefficient of each index are calculated through statistical methods. The results indicate that the average value of the complex coefficient and the discrete degree of the cross-sectional area along with the river in Yidu spawning ground have significant differences with non-spawning ground.Difference of the average value:In the regard of value, river width, wet perimeter, cross-sectional area and their changing rate along with the river and flow process in Yidu spawning ground are lower than non-spawning ground. However, the average value of hydraulic radius, complex coefficient of cross section and their changing rate along with the river and flow process are greater than the non-spawning ground. In the regard of trend, the complex coefficient is the quite different index between Yidu spawning ground and non-spawning ground. The complex coefficient and the changing rate with the way are increasing with the flow increasing in Yidu. And the average value of complex coefficient in Yidu reach is 1.016. By contrast, the complex coefficient and the changing rate with the way are decreasing with the flow increasing in non-spawning ground, and the average value of complex coefficient in in non-spawning ground is 1.011. The results above indicate that the complex degree of riverbed topography in Yidu spawning ground is higher than in non-spawning ground.Difference of discrete degree:whether it’s the spawning ground or not, the discrete degree of the indexes changing along with the river is higher relative to other indicators. The result means that the topography along with the river in Yidu or non-spawning reach is all more scattered. However, cross-sectional area is highest in the all indicators in Yidu spawning ground, and its value is-67.985, which is higher than non-spawning ground for -8.717.Difference of distribution:non-spawning grounds vary each other, resulting in great uncertainty to figure out clear representative different indicators between spawning grounds and non-spawning grounds. The method of Rank Test just can distinguish the different indicators between two reaches, so the result through this method has poor regularity.3. The situation of Yidu spawning grounds’topography changeAccording to the calculating result, the density of pool-riffles is 0.67 in 2014 higher than 0.50 in 2002, and significantly higher than 0.47 in non-spawning ground.The changes of Yidu spawning ground are compared from sectional shape and riverbed topography. In the regard of sectional shape, riparian sides collapse significantly in different degrees, especially in the corners. The concave banks are scoured with minimum lm, maximum more than 10 meters, but the convex banks have no significant deposition. Besides, the topography in the main channel of the river upstream does not change significantly, only with partial deposition and formation of new deep pool. However, the channel downstream is scoured obviously, and the watercourse is cut down significantly. Additionally, the riverbed morphology becomes more complicated, and the riverbed overall broadens resulting in the water area increased. In the regard of riverbed topography, the roughness, relief amplitude and variance coefficient in elevation are all increase significantly.4. The three-dimensional flow in Yidu spawning ground is simulated through EFDC model, and the relationship between hydraulic indicators and reproduction of fish is analyzed.The velocity, Froude number and vorticity are selected to describe flow characteristics. The three-dimensional hydrodynamic model based on measured topography and flow is adopted to simulate velocity and Froude number in different conditions. Meanwhile, the vorticity is calculated by Tecplot. It was found that velocity, Froude number and vorticity changes significantly with flow increased or decreased.The value of ecological hydraulic factor in different conditions and the amount of eggs in corresponding date are compared. And the result shows that:average velocity, Froude number and vorticity in Yidu spawning ground change significantly with the increase of flow. Besides, the amount of drawn eggs has a strong positive correlation with velocity, Froude number and vorticity by analyzing the relationship between ecological factors and the amount of eggs. Additionally, the spawning velocity distributes in the range of 0.97 m/s-1.31m/s when the FMCC’s spawning number is high in Yidu spawning ground, and the average velocity is 0.17m/s. The Froude number distributes in the range of 0.09-0.11, and the average Froude number is 0.10. The vorticity distributes in the range of 4.12× 10-3s-1-5.20 × 10-3s-1, and the average vorticity is 4.81 × 10-3s-1.