| The Chinese sturgeon(Acipenser sinensis Gray,1835),a large anadromous species, mainly distributes in the mainstem of Yangtze River,the Pearl River and the continental shelf of East Asian.The migration routeway of A.sinensis was cut down by the construction of Gezhouba Dam(GD) in 1981.All spawning areas of A.sinensis located in the lower Jinsha and the upper Yangtze River were no longer accessible.It was proved that A.sinensis was able to accomplish the natural reproduction below GD.But it spawned only within a 30 km river reach from the downstream of GD to Gulaobei. Futhermore,the natural reproduction of A.sinensis took place every year only within a 4 km river reach from GD to Miaozui.Meanwhile,a drastic decline in the wild population of A.sinensis went with due to the disappearance of original spawning areas, environment degeneration,navigation and historical overfishing.The species was listed as a First Class Protected Animal by the national government in 1989 and Endangered species in the IUCN red list in 1996.The restoration of A.sinensis natural population was extremely difficult because of the late sexual maturity and long reproductive cycle. Present studies indicated that most recruitment individuals of A.sinensis juveniles were originated from the natural breeding in spite of the scarcity of the mature population.The contribution of artificial enhancement and releasing was less than 10%even though the enhancement has a certain scale.In this case,the appropriate abiotic environmnet for the A.sinensis reproduction in nature was essential to ensure and enhance the scale and efficiency of the species natural reproduction,which was closely related to the conservation and restoration of natural population of A.sinensis.Based on this point,five factors of the abiotic environment (topography of spawning area,riverbed substrate,velocity field,hydrological conditions and meterological conditions during the peroid of natural reproduction) strongly associated with natural reproduction of A.sinensis and these relationships with spawning activities were studied thoroughly in this paper.Our purpose was to provide scientific bases for the melioration of spawning area and spawning conditions,and even building of artificial spawning areas of A.sinensis.The main study methods and results in this study are showed as follows. 1.The bedform morphology data of five historic and two present spawning areas were acquired by on-site surveys and utilization of large-scale relief map and navigation channel chart related to the spawning reaches.ArcGIS 9.2 software was used to analyze the bedform morphology of the seven spawning areas.It was showed that the main river pattern in the spawning reaches was straight with a little turning angle(the average value of turning angle in the five historic spawning areas was 31.2°).Large variations were occurred in the river width and water depth(the average values in the five historic spawning areas were 167.6 m and 16.8 m,respectively).There was a negative relationship between the length and gradient of the adverse slope in the lower part of spawning areas(y=-1892.6×Ln(x)+1986.4,R~2=0.625,n=7).Moreover,the reproduction efficiency was higher in the spawning area with adverse slope having moderate slope length and gradient.The river width of two present spawning areas was significantly larger than five historic spawning areas.At the spawning area below GD(the only one at present where the natural reproduction of A.sinensis took place every year),the topographic structure of the upstream mating site and the downstream mating site were very similar.Moreover,the topographic structure of the downstream mating site was more similar to that of the historic spawning areas.Combined with the natural reproduction scale and efficiency of A.sinensis(based on the reproduction frequency,population of brood fish,embryo numbers and emergent larvae in the spawning area),the perfect spawning area should have topographic characteristics as follows.The length of spawning area(S) was 3 013 m.The largest river width(B) was 369 m.The turning angle of fiver channel(α) was 33.2°.The meander ratio(S/L) was 1.06.The value of B/S was 0.14.The difference between B and the smallest river width was 161.2 m.The altitude difference of adverse slope was 18.1 m. The length and gradient of the adverse slope were 1 337.3 m and 1.47%,respectively. The riverbed topography of Huyatan spawning area was very different from the perfect spawning area.Maybe it was one of important reasons why A.sinensis did not spawn there every year.The topography of the present spawning area below GD had been changed because of the River Regime Regulating Project under the Dam.In a short time, it was advantageous to the inhabitation of A.sinensis before natural reproduction. However,that was adverse to the dispersal and incubation of fertilized eggs.And the long-term effects need further study.2.The evolution of riverbed substrate of Yichang reach(below GD) since the dam closure(the year of 1998-2008) was thoroughly investigated by several methods as follows.It included analysis and summary of related literatures with regard to riverbed substrate in the study area,detection of the Digital Scientific Echosounders(the year of 2004-2008),underwater video observation(the year of 2007-2008) and on-site samplings(the year of 2007-2008).The ultrasonic echo characteristics of riverbed substrate were analyzed by the method of First Echo Division from the special software of Visual Bottom Typer 1.9.The hardness and roughness values of the echo were expressed as El' and El,respectively.The result showed that the riverbed substrate below GD coarsended quickly because of the scour process at the initial operation stage of the Dam reservoir.In the year of 1984,the permanent backwater region of the reservior achieved fluvial equilibrium and the coarsening process in the downstream area basically completed.In the year of 1992, the fluctuating backwater area achieved fluvial equilibrium and the riverbed substrate below GD fined or coarsened with the incoming water and sediment.However,on the whole,because the bed load over the Dam reduced obviously,the sandy riverbed had been turned into sand and gravel riverbed.In A.sinensis spawning season(October-November) of 2007,in the river reach between GD and Lujiahe riffle(80 km),the range of the riverbed substrate El' and El values were 0-0.08 and 0-0.12,respectively.As a whole,the El' value of mainstream channal was large,and the El' value of coastal channel was small.The spatial distribution of El value was similar to El'.Based on the analysis in the year of 2004-2006 and 2008,no obvious varaiations were found in the spatial distribution of El' and El value when considered in large spatial scale.The on-site samplings showed that the average diameter of gravel in the main channel of Erjiang(95.6 mm) was obviously larger than that in Miaozui(22.3 mm) and the ship way of Dajiang lock(65.6 mm).The elliptical gravels,oblate gravels and sand were the major components of riverbed substrate in the river reach between GD and Miaozui.The gravels with long diameter 20-50 cm occupied about 50%.The gravels with long diameter 10-20 cm occupied about 30%.And about 20%of gravels with long diameter less than 10 cm.The spatial structure of the gravels was commonly cube or tetrahedron,and among them much interstice,rift and crack.The main sites for the distribution of A.sinensis fertilized eggs were located in the area where the rift was scoured cleanly.The fertilized eggs distributed in the gravel interstice in the form of agglomerate or granule.In the A.sinensis spawning season of 2004-2008,the spatial distribution of riverbed El' and El values changed by the years in the river reach between GD and Miaozui.As a whole,in the year of 2004-2007,the El' and El values in high value areas raised by the year,and the distribution ranges of El' and El values enlarged.But the trend of variation held down in 2008 than that in 2007.According to the distribution characteristics of El' and El values,the gravel-sand riverbed substrate between GD and Miaozui can be divided into nineteen types.It showed that the river substrate of type 1,2,3,5 and 6 were in the majority.Moreover,in A.sinensis spawning season of 2006-2008,the spatial distribution of river substrate types did not change obviously,except in the tail area of the current dike.3.By the model calculation of River 2D(the finite element hydrodynamic model) and the on-stie surveys of Acoustic Doppler Current Profiler(ADCP),we analyzed the characteristics of velocity field in A.sinensis spawning seasons of 1996-2008.In addition,eight metrics for quantifying spatial velocity gradient along horizontal or vertical directions were put forward to quantitate the characteristics of 285 transects in the river reach from GD to Honghuatao during December 2-10,2005.For the 14 natural reproductive activities from the year 1996 to 2003,we calculated the velocity field in the typical flow conditions(minimum,intermediate and maximum discharge).The result showed that variation range of current velocities was very large while the spawning action happened.Especially the current velocities changed obviously in the downstream mating site.The on-site surveys on the initial spawning day or 2-3 days after spawning were conducted by the ADCP in the year of 2004-2008.It indicated that the structure of velocity fields were similar in different years in large spatial scale. However,it showed some differences in small spatial scale.Compared to the period before the River Regime Regulating Project(before the year of 2004),the velocity field strcuture had been changed greatly in the year of 2005-2008.It mainly introduced a slow velocity area around the current dike.The velocity in central area V decreased and velocity in two-sides increased.According to the on-site observation by ADCP,the spatial distribution of velocity in transect was closely correlated with the shape of transect,that was to say,with the riverbed topography in streamwise.However,as a whole,the surface velocity was higher than bottom velocity,and the velocity in the central channel was higher than that near the side.According to the three on-site surveys on velocity field during the spawning activities in 2004-2005,the variation range of mean transet velocity of 20 transects was 72.99-175.23 cm/s,and the average value was 128.89±26.28 cm/s.The vertical velocity component of transect 3,4 and 11 in mating sites were 2.10 cm/s higher than other transects.From transect 1 to 30,the average flow direction decreased from 225.91°to 164.39°.The variation ranges of flow direction in transect 3,4 and 11 were large, which the coefficient of variations were 18.4%higher than others.On A.sinensis initial spawning days of the year 2004-2006,the range of bottom velocity in the downstream mating site was 108.74 cm/s-129.30 cm/s.Maybe it was the optimal current velocity for A.sinensis spawning.According to the on-site surveys on vertical velocity distributon,the velocity gradient of vertical lines from surface to bottom was negatively related with the water depths of the lines.The exponential function and logarithmic function both can be used to describe the time-averaged vertical velocity distribution.On the whole,the velocity turbulence intensity and relative turbulence intensity both decreased at first and then increased from the surface to the bottom.The variation of turbulence intensity of vertical velocity was similar to that of velocity.However,the variation of relative turbulence intensity of vertical velocity did not show obvious law,the values in different water depth did not indicate significant difference.And the relative turbulence intensity of vertical velocity was much larger than that of velocity.A transect in Miaozui were measured repeatedly for nine times to test the eight proposed velocity gradient metrics.It showed that the variation ranges of all metrics were less than 15%.That was to say,these velocity gradient metrics were suitable to describe the water flow with turbulance charateristics.K-Means algorithm was conducted based on transect-averaged velocity and eight metrics,the results showed that No.7,No.8 and No.2 predominate over the other,together taking 88.66%area in the whole reach.The reach between GD and Miaozui has the most diversified hydraulic environment.R-type cluster showed that the metrics have no close pertinence with the average velocity magnitude of transect.It means that the eight metrics are useful to differentiate areas with similar current magnitudes but different spatial gradients,it necessary to use these metrics to portray the natural of current velocity.4.In the spawning area of A.sinensis below GD,the hydrological conditions in spawning seasons(October-November) before and after(in the year of 1983-2002 and 2003-2005) the operation of the Three Gorges Dam(TGD) were analyzed.It showed that the hydrological conditions during the spawning seasons changed obviously since the operation of TGD reservoir in the year of 2003.The average water temperature in the spawning season increased by 1.48℃.And the silt content decreased to 0.032 kg/m~3 (only for 8.2%before operation).A suitability model based on hydrological conditions was proposed to evaluate the suitability of daily hydrological condition for spawning during the spawning seasons before(1983-1986) and after(2003-2005) the operation of the TGD.The average suitablity model showed that spawning suitablity for spawning increased at first and then decreased in the whole spawning season.However,the spawning suitablity was comparatively high in the whole spawning season.The spawning activities of A.sinensis did not always take place in the situation when the suitablity of hydrological condition was the highest.That was to say,the hydrological condition was not the only factor to determine the spawning time.The variation trend of silt content before spawning used to steady and decrease before the TGD reservoir operation,however,the trend was steady and increase after the reservoir operation.It suggested that sharply decrease of silt content may bring on complex effects on the natural reproduction of A.sinensis.5.A total of 7 meteorological elements(including wind,rainfall,temperature,air pressure,sunshine hours,humidity and cloud) in the initial spawning day of 55 spawning activities within the years(1963-1965,1970-1975,historic;1983-2006,present) in respective spawning areas were studied.The results show that the commencement of spawning activities of A.sinensis has some selectivity for the rainy day,but shies away from the cloudy day.The dramatic variation of weather type around the initial spawning day may be an inducement to the commencement of spawning activities,and consecutive sunny days in the present spawning area also have the same effect to the commencement of spawning activities.While the average temperature on the initial spawning day in the first and second spawning periods within the present spawning area shows a significant variation,other meteorological elements on the initial spawning day within both historic and present spawning areas do not differ significantly among the first spawning period, the second spawning period and the variation range(extent of date between the early and the late initial spawning days) of the initial spawning days.The variation trend of average air pressure on the initial spawning days in the first and second spawning periods is in the other diection to a certain extent.The sunshine hours,water level and silt content in the present spawning area do not have mutual relationship between each other as those in the historic spawning areas.Meteorological conditions may be an important factor for the commencement of the natural spawning behavior of A.sinensis.Based on above results,it can be concluded that topography of spawning area, riverbed substrate,velocity field,hydrological conditions and meterological conditions were associated with the natural reproduction of A.sinensis.The spatial distribution characteristics of riverbed substrate and velocity field were determined by the bedform morphology in a great extent,and the former two had direct influence on the spawning activities.However,the bedform morphology,riverbed substrate and velocity field influenced each other.They determined the special space for natural reproduction of A. sinensis together.The commencement of spawning activities showed a certain relationship with the variation of hydrological condition,however,it did not obviously. The appropriate hydrological condition only determined the approximate time range of spawning.In addition,the meteorological conditon also had a certain influence on the commencement of the spawning activities.To offer enough space and create advantageous spawning conditions for natural reproduction of A.sinensis is very important especially when the natural population of broodfish decreased gradually.Good measures will ensure and enhance the scale and efficiency of A.sinensis natural reproduction.It is very important when the recruitment data are considered.It is no doubt that the increase of recruitment is very important for conserving and rehabilitating the wild population of A.sinensis.According to the results concluded in this paper,an enhancement project can be conducted in present spawning area below GD,or carry on some ecological regulations on operation of TGD and GD so as to provide better spawning conditions.It is feasible at present.Furthermore,the conclusions in this paper also provided a theoretical basis for building the artifical spawning areas of A.sinensis in the future.
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