【作者】 张辉；

【导师】 危起伟；

【作者基本信息】 华中农业大学 ， 湿地资源与环境， 2009， 博士

【摘要】 中华鲟(Acipenser sinensis Gray,1835)是一种大型溯河洄游性鱼类,主要分布于长江干流、珠江及东亚大陆架水域。1981年葛洲坝水利工程截流阻断了其洄游通道,原来分布在金沙江下游和长江上游的所有产卵场都无法被中华鲟所利用。虽然后来证实中华鲟能够在葛洲坝坝下完成自然繁殖活动,但产卵位置却仅局限在葛洲坝坝下至古老背长约30 km的江段,并且每年都有自然繁殖活动发生的区域仅限于葛洲坝至庙咀长约4 km的江段。与此同时,由于原有产卵场的丧失、环境退化、航运和历史上的过度捕杀等诸多因素,中华鲟野生种群资源量也不断下降。1989年中华鲟被列为国家一级重点保护动物,1996年又被列为IUCN红色目录濒危物种。中华鲟性成熟晚,繁殖间隔长,资源被破坏以后自然恢复缓慢。虽然中华鲟繁殖群体数量稀少,但目前的研究却表明自然繁殖仍然是中华鲟幼鱼补充群体的主要来源,人工繁殖放流数量虽然具有一定规模,但贡献量仍不足10%。在此情形之下,能否为中华鲟自然繁殖活动提供适宜的非生物环境,确保和提高中华鲟自然繁殖规模和效果,就与中华鲟自然种群的保护和恢复密切相关。基于此,本文对与中华鲟自然繁殖活动密切相关的5个非生物因素(产卵场地形、河床质、流速场、自然繁殖活动发生期间水文状况和气象状况)进行了深入研究,并探讨了它们与自然繁殖活动之间的相互关系,以期能为中华鲟现有产卵场和产卵条件的改良或将来人工模拟建造中华鲟产卵场提供科学依据。本研究采用的主要研究方法及获得的主要结果如下:1.通过获取大比例尺河床地形图、航道图和原型观测的方法,获得了5处历史产卵场和2处现存产卵场详细的河床地形数据。采用ArcGIS 9.2软件对此7处产卵场河床形态的分析结果表明,产卵场江段以顺直微弯河型为主(5处历史产卵场平均转折角为31.2°),河宽和水深变化幅度较大(5处历史产卵场平均值分别为167.6 m和16.8 m),位于产卵场下段的负坡的长度和坡度呈现出一定程度的负相关关系(y=-1892.6×Ln(x)+1986.4,R~2=0.625,n=7),并且具有适宜坡长和坡度的产卵场往往具有较好的繁殖效果。葛洲坝下2处现存产卵场与5处历史产卵场相比,其最显著的差异是河宽要比历史产卵场的大得多。现存唯一已知每年都有自然繁殖活动发生的产卵场(葛洲坝坝下产卵场)内“上产卵区”和“下产卵区”的地形结构较为相似,并且“下产卵区”的地形结构与历史产卵场相比更为相似。结合中华鲟自然繁殖的规模和效果(基于产卵场亲鱼的尾数、胚胎和仔鱼数量以及繁殖活动发生频率等),理想状态下中华鲟产卵场的地形应该具备如下特征:产卵场长度(S)为3013m,最大河宽(B)为369m,河道转折角(α)为33.2°,曲折率(S/L)为1.06,B/S值为0.14,B与最小河宽之差为161.2 m,负坡高度差为18.1 m,负坡长度和坡度分别为1337.3 m和1.47%。虎牙滩产卵场的河床地形与理想产卵场的相似度较差,这可能就是中华鲟为何不能每年在此产卵的重要原因。葛洲坝下河势调整工程使得坝下现存产卵场的地形发生了较大改变,在短期内这可能会对中华鲟自然繁殖前期的栖息有利,而对受精卵的散播和孵化产生一定程度的不利影响,其长期影响还有待于进一步的研究。2.采用研究区域河床质相关文献调查、科学回声仪探测(2004～2008年)、水下视频观测(2007～2008年)和现场取样(2007～2008年)的方法对葛洲坝截流以来(1981～2008年期间)坝下宜昌江段河床质的变迁进行了深入调查,利用底质分析专用软件Visual Bottom Typer 1.9提供的首次回波划分(First Echo Division)的方法分析了河床质的超声波回波特性(硬度E1′和粗糙度E1)。文献分析结果表明,葛洲坝水库运行初期,坝下河床冲刷较明显,粗化较快,1984年常年回水区达到冲淤平衡,粗化基本完成,1992年变动回水区达到冲淤平衡,坝下河床质随来水来沙条件而变,发生粗化或细化。但由于推移质过坝量明显减小,坝下整体呈现粗化,原沙质河床转变为沙夹卵石或卵石夹沙河床。2007年中华鲟自然繁殖季节(10～11月)葛洲坝至卢家河浅滩江段(80 km)河床质E1′和E1值的分布范围为0～0.08和0～0.12。从总体上看,一般主流河槽E1′较大,沿岸侧河槽E1′较小,E1的分布也有相似的规律。结合2004～2006年和2008年的分析结果可知,E1′和E1值的空间分布在大范围江段内变化并不明显。现场取样调查表明,二江下槽卵石的平均粒径(95.6 mm)要明显大于庙咀(22.3mm)和大江船闸航道处(65.6 mm)。水下视频观察结果表明葛洲坝至庙咀江段表层河床质主要是由卵圆、扁圆形卵石及沙粒组成。其中卵石长径范围为20～50 cm的约占50%,卵石长径范围为10～20 cm的约占30%,卵石长径范围在10 cm以下的约占20%。卵石之间相互以立方体或四面体方式排列堆叠,形成了许多空隙、隙缝以及卵石夹缝等。卵石间隙冲刷洁净的河床区域是中华鲟受精卵的主要散播位置,受精卵以成团或颗粒形式分布于卵石间隙中。2004～2008年中华鲟自然繁殖季节葛洲坝至庙咀江段河床E1′和E1值的空间分布呈现出逐年渐变的趋势,从总体上看,2004～2007年E1′和E1值高值分布区值逐渐增加,分布范围也逐渐扩大,而2008年较之2007年则有相对缩减的趋势。据E1′和E1值分布特征将葛洲坝至庙咀江段卵石夹沙河床质细分为19小类的分析表明,河床质以1、2、3、5和6小类居多,并且2006～2008年中华鲟自然繁殖活动发生期间,除导流堤尾部区域外,河床质类型的空间分布没有发生明显的变化。3.采用River 2D模型计算(有限元法)和声学多普勒流速剖面仪(ADCP)原型观测的方法,对1996～2008年中华鲟自然繁殖活动发生期间的流速场特征进行了分析,并提出8个流速梯度指标对2005年12月2～10日葛洲坝至红花套江段285个测流断面的流速梯度特性进行了量化分析。对1996～2003年14次自然繁殖活动发生期间典型流量(最小流量、中值流量和最大流量)状态下流速场的计算结果表明,各批次产卵活动产卵场流速值变化幅度较大,“下产卵区”范围内流速变化尤其明显。采用ADCP对2004～2008年产卵活动发生起始日或产卵发生后2～3日内流速场的现场观测结果表明,各年度的流速场从大尺度范围来看具有一定的相似性,但从小的空间尺度来看,也具有一定程度的差异。2005～2008年的流速场较之河势调整工程进行以前(2004年以前),流速场结构有较大的变化,主要表现为导流堤周围形成了一个流速较缓的区域,V区中部的流速值减小而沿岸两侧的流速值增加。ADCP原型观测结果表明断面流速分布与断面形态密切相关,即与纵方向上的河床形态有紧密关系,但总体上看,一般表层流速高于底层流速,河槽中部的流速高于河槽两侧的流速。2004～2005年中华鲟自然繁殖期间3次流速场测量结果表明,20个断面平均流速变化范围为72.99～175.23 cm/s,平均值为128.89±26.28 cm/s;产卵区内3、4和11号断面的垂向平均流速值较其他断面约高2.10 cm/s;从1至30号断面,各断面的平均流向大约从225.91°下降至164.39°,并且产卵区内3、4和11号断面的流向变幅较大,变异系数比其他断面约高18.4%。2004～2006三个年份中华鲟产卵起始日下产卵区内底层流速值范围为108.74 cm/s～129.30 cm/s,这可能就是中华鲟产卵所需要的最适流速。垂线流速测量结果表明,从表层到底层各水层流速梯度随垂线水深值增大而减小,但各测流垂线时均垂线流速分布均可用指数函数和对数函数来描述。从整体上来看,流速脉动强度和相对脉动强度从表层到底层都是先降低后升高,垂向流速分量脉动强度的变化与流速的具有相似的规律,但垂向流速分量相对脉动强度的变化没有呈现出明显的规律,其值在不同水层深度之间并没有明显的差异,并且其值比流速相对脉动强度要大很多。采用所提出的8个流速梯度指标对庙咀处一断面重复测量9次的统计分析结果表明,所有指标变幅均小于15%,说明用这些流速梯度指标对具有紊动特性的水流进行分析是合适的。基于断面平均流速和8个流速梯度指标共9个参数采用k均值分类法将285个测流断面聚成8类的分析结果表明,整个江段以第7类、第8类和第2类区域占主要优势,这3类区域面积共占葛洲坝至红花套江段面积的88.66%,葛洲坝至庙咀江段为整个研究江段各类区域相间分布最多样化的江段。采用R型聚类方法分析表明,断面平均流速与断面8个流速梯度指标之间没有显著的相关性,即具有相同流速的区域不一定具有相似的流速梯度特征,有必要用这些流速梯度指标描绘水流的特性。4.对三峡水库运行前(1983～2002年)后(2003～2005年)葛洲坝下中华鲟产卵场产卵季节(10～11月)的水文状况进行了比较分析,结果表明,2003年三峡水库运行以来中华鲟产卵场产卵季节的水文状况发生了显著变化,产卵季节平均水温上升了1.48℃,含沙量降低到0.032 kg/m~3(仅为运行前的8.2%)。采用适合度模型的方法对三峡水库运行前(1983～1986年)后(2003～2005年)8个年份产卵季节水文状况对于中华鲟产卵的适合度逐日进行了分析。平均适合度模型的分析结果表明,虽然整个产卵季节水文状况对于产卵的综合适合度呈现出先上升后下降的趋势,但整个产卵季节水文状况的适合度都比较高,并且中华鲟的产卵活动并不总是发生在水文状况适合度最佳(即平均适合度波峰值)的时候,这说明水文状况并不是决定产卵时间的唯一因素。含沙量在产卵前的变动趋势由三峡水库运行前的稳中下降转变为运行后的稳中上升,说明含沙量的剧烈下降对中华鲟自然繁殖活动的影响可能较为复杂。5.对中华鲟历史(1963～1965年,1970～1975年)及现存(1983～2006年)产卵场共55次自然繁殖活动发生起始日的天气类型和气象要素(包括风、降水量、气温、气压、日照时数、湿度和云)状况及其变化进行了统计分析。结果表明:中华鲟自然繁殖行为的发生对阴雨天具有一定的选择性,而对多云天气则具有一定的回避性;产卵起始日前后天气类型的剧烈变化是诱导中华鲟自然繁殖行为发生的一个有利因素,现存产卵场内连续晴天天气也较容易诱发中华鲟自然繁殖行为的发生;除现存产卵场产卵起始日平均气温在第1批和第2批产卵活动之间表现出极显著差异外,产卵起始日其余气象要素在第1批产卵活动、第2批产卵活动和产卵起始日时间窗(最早和最晚产卵起始日的时间跨度范围)范围内三者之间并没有表现出显著的差异:第1批和第2批产卵活动产卵起始日的气压变动呈现出一定程度的相反趋势;与历史产卵场不同,现存产卵场产卵起始日的日照时数与水位和含沙量之间并不具有相互制约和补充的关系。表明气象状况也是影响中华鲟自然繁殖行为发生的重要因素。以上研究结果表明,河床地形、河床质、流速场、自然繁殖季节的水文状况和气象状况与中华鲟的自然繁殖活动都有一定的关系。河床形态在很大程度上决定了河床质和流速场的空间分布特性,而河床质和流速场则与中华鲟的自然繁殖活动直接相关。河床地形、河床质和流速场三者之间相互影响,三者共同形成了中华鲟发生自然繁殖活动所需要的特定空间。水文状况的变化虽然与中华鲟自然繁殖活动发生的具体时间有一定的关系,但并不显著,适宜的水文条件只是决定了一个大致的产卵时间范围。另外,气象状况对中华鲟自然繁殖活动发生的具体时间也有一定的影响。在中华鲟自然繁殖群体数量逐渐减少的情况下,如果能够提供充足的自然繁殖空问和创造有利于自然繁殖发生的条件,确保和提高中华鲟自然繁殖规模和效果,对于大规模提高幼鱼补充群体的数量具有重要意义。而幼鱼补充数量的增加对于中华鲟野生种群的保护和恢复则具有十分重要的意义。在现有的条件下,参照本研究所得出的结果对葛洲坝下现存产卵场的产卵环境进行改良,对三峡大坝和葛洲坝进行生态调度以创造适宜的产卵条件,是现实可行的保护方案。此外,本研究的结果还为以后人工建造中华鲟产卵场提供了理论依据。更多还原

【Abstract】 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.更多还原