Study On The Topography And Hydrodynamic Characteristics Of The Four Major Chinese Carps' Typical Spawning Grounds In The Middle Reaches Of The Yangtze River

Four major Chinese carp,namely black carp?Mylopharyngodon piceus?,grass carp?Ctenopharyngodon idellus?,silver carp?Hypophthalmichthys molitrix?and bighead carp?Hypophthalmichthys nobilis?,are important economic freshwater fish varieties in China.They primarily inhabit and breed in the Yangtze River.Historically,there were36 spawning grounds distributed in the main stream of the Yangtze River,which were located from Chongqing to Pengze within a range of 1695km.The annual total egg production in these spawning grounds was about 118 billion.However,after the construction of the Gezhouba Water Control Project in the 1980s,six spawning areas were flooded and annual egg production was reduced to approximately 20 billion.The middle Yangtze River,which is about 898km long?from Yichang to Hukou?,is the main reproduction area for the four major Chinese carp.The 380km reach from Yichang to Chenglingji is one of the most important habitats.In this section,there were11 spawning grounds recorded for the four carps,and these spawning grounds together shared 42.7%of the total fish larvae production in the Yangtze River.In resent years,due to the natural and human factors,especially the completion of the Three Gorges Dam,the underwater topography and hydrodynamic condition between Yichang and Chenglingji sections had been continuously altered and egg abundance of these four species of carp was less than 0.1 billion in Jianli section in 2009.Egg abundance has shown a distinctive decreasing tendency.In order to increase the stock abundance of the four major Chinese carps,the former Ministry of Agriculture had carried out the activities of fishery resources releasing and enhancement since 2010.In an attempt to alleviate the impacts of river flow alterations caused by dams,creating hydrological processes similar to the natural state had been suggested as an effective measure to stimulate the four carp species to spawn.The Three Gorges Reservoir had created artificial flood processes by ecological operation experiments to facilitate these carps'spawning activities since 2011.The spawning grounds and their corresponding topography and hydrodynamic conditions are important for the reproduction of the four major Chinese carps.However,due to the limitation of traditional detection tools,the research on the spawning ground characteristics has been rarely carried out and the corresponding spawning-trigger mechanism is still unknown.For this point,this study took the four major Chinese carp's typical spawning grounds form Yichang to Chenglingji as study areas.We adopted the advanced detection techniques and conducted the research in the following fields.First,we combined the fish egg collection and the ultrasonic telemetry technology to locate the precise spawning ground and to investigate the fish migration routes in the breeding season and the corresponding temporal and spatial distribution.Second,we observed the water flow field in real-time in typical spawning ground and record the explicit underwater topography with acoustic doppler current profiler?ADCP?.Finally,we analyzed the previous works with three dimensional numerical simulation,mathematical statistics and data mining method,and then illustrated the efficient underwater topography and hydrodynamic characteristics of the four major Chinese carps spawning ground.Moreover,the proposed solutions and results would provide theoretical and technological support for the spawning ground evaluation and recovery as well as fish resource protection.The main results of this research are as follows:1.Locating spawning groundsAccording to the fish egg collection monitoring results,the main spawning grounds from Yichang to Jingzhou were in Yichang?under Gezhouba Dam?,Yidu,Zhicheng and Jiangkou section.The egg production in these typical spawning grounds was 0.74 billion,which accounted for 66.61%of the total egg production in the Yangtze River.Movement characteristics and distribution status of the four major Chinese carps during the spawning period were studied by ultrasonic telemetry technology.The results showed that,the releasing of the studied carps showed anadromous migratory characteristics,and they conducted repeated migrations among the Zhicheng,Yidu and Yichang spawning grounds during the spawning season.Meanwhile,the migration behavior was affected by the flow and the increasing flow would help trigger the upstream migratory of these four carps.After the breeding period?from August to September?,they swam downstream,leaving the spawning ground.The four major Chinese carps were not randomly or uniformly distributed in the spawning ground,but selective.They were mainly distributed in A1 section in the upper region of Zhicheng spawning ground and in C1,C2 sections in the middle region of Yidu spawning ground.These regions were considered to be the core of spawning areas.Combining the results of fish egg collection and the ultrasonic telemetry technology,the existence of spawning grounds was proved.2.Topographic feature of the spawning groundsThe spawning grounds of the four major Chinese carps were mainly located in the river sections with complex topography.In this research,three river types,namely straight type?Yidu spawning ground?,“s”type?Zhicheng spawning ground?and curved type?Chenglingji spawning ground?were selected as the studied area.River cross sectional bathymetry was measured using ADCP and hua test T5 GNSS GPS positioning system,as well as the marine surveying navigation software.The average slope of the spawning ground in Chenglingji was the largest,which was 2.71 degrees.Then was in Zhicheng,which was 2.05 degrees.The value in Yidu spawning ground was the lowest,which was 1.91 degrees.The average relief amplitude of the spawning ground in Chenglingji was the largest,which was 4.66E^-2m.Then in Zhicheng spawning ground,which was 3.80E^-2m.The value in Yidu spawning ground was the lowest,which was 3.44E^-2m.The average elevation variation coefficient of the spawning ground in Chenglingji was the largest,which was 7.18E^-4.Then in Zhicheng spawning ground,which was 4.43E^-4.The value in Yidu spawning ground was the lowest,which was3.25E^-4.On the whole,the topographic indexes were higher near the bend.During the breeding period,the four major Chinese carps were mainly distributed in the first corner of the Zhicheng spawning ground and in the central position of the Yidu spawning ground.The river way in both positions became more and more narrow and had a certain degree of slope and relief amplitude.3.Hydrodynamic characteristics of the spawning groundA three–dimensional hydrodynamic model,which was a module of the EFDC?The Environmental Fluid Dynamics Code?,was established according to the flow,water levels and bathymetry to simulate the hydrologic processes during the two ecological operations and in seven gradients of flow:6000m³/s,10000m³/s,15000m³/s,20000m³/s,25000m³/s,30000m³/s and 35000m³/s.In the setting flow range,the four indicators-velocity,water depth,Froude number and vorticity-were increased with the increase of flow.The increase rate of water depth is equal in the whole section while the velocity increased faster in the mainstream direction than that in other place and Froude number and vorticity did not have clear trend.The four major Chinese carps would actively choose a larger vorticity area as their spawning ground.However,they did not frequently move in that area,but chose the upstream region where the vorticity was smaller.4.Suitable ecological hydraulic indexAccording to Bai Haixia's preliminary research,velocity and Froude number had a strong positive correlation with flow.Combined ultrasonic telemetry technology and A three–dimensional hydrodynamic model,we extracted of fish distribution sites during the reservoir ecological operation and analyzed the ecological hydraulic indexes in these sites.By statistics of 106 monitoring data,it was concluded that the proper range of velocity for the four major Chinese carps was 0.63-1.53m/s,and the suitable range of velocity was 0.78-1.38m/s;the proper range of water depth was 12.77-23.51m,and the suitable range of water depth was 12.77-19.93m;the proper range of Froude number was 0.049-0.119,and the suitable range of Froude number was 0.069-0.109.5.Ecological flow requirement during breeding seasonThe habitat suitability model was built based on hydrodynamic suitability curves to assess spawning habitat quantity under different flow conditions.The Weighted Usable Area?WUA?in Zhicheng and Yidu spawning grounds was first increased and then decreased with the increase of flow.When discharges in Zhicheng spawning ground ranged from 15000m³/s to 25000m³/s,WUA values maintained at a high level.WUA values peaked?6.40km²?at a discharge of 22500m³/s,indicating that a discharge of22500m³/s was the most suitable for the four major Chinese carp to spawn in Zhicheng spawning ground.When the discharges in Yidu spawning ground ranged from15000m³/s to 21300m³/s,WUA values maintained a high level.WUA value reached a peak?8.39km²?at a discharge rate of 15000m³/s,indicating that a discharge of15000m³/s was the most suitable for the four major Chinese carp to spawn in the Yidu spawning ground.Because of the special river morphology,when the suitable area in the mainstream region decreased,the suitable area in the bend increased more,which lead to the WUA in Chenglingji spawning ground increased with the flow within the set range.Most of four major Chinese carps'spawning activities occurred at a high level of WUA,and the daily spawning egg numbers increased with the WUA value.The simulation results showed that,when the flow was between 15000m³/s and 21300m³/s,the WUA values in both Yidu and Zhicheng spawning ground would keep at a high level.Therefore,the discharge between 15000m³/s and 21300m³/s could be recommended as ecological operation target flow.