Water temperature(WT) variation at downstream of the dam after impoundment of the Three Gorges Reservoir (TGR) influenced the critical life cycle stages of the Four Major Chinese Carps to a certain extent.The degrees of WT variation in the Middle Yangtze River after impoundment of the TGP were quantitatively assessed,then effects of WT variation on the Four Major Chinese Carps were analyzed.25 WT parameters with ecological interpretations by Indicators of WT Alteration method were generated based on daily data of the Yichang hydrologic station from 1983 to 2013.The variation degree of 25 parameters after impoundment of the TGR was calculated by the range of variability approach,the linear regression method by F-test was used to evaluate trend of WT parameters with high alteration.Then relationship between spawning time of the Four Major Chinese Carps to variation of WT parameters was analyzed,to study the response of four carps spawning to WT change.The results showed:the total alteration of WT had reached to 44.2%,7 parameters representing high alteration in WT were found out,which were mean monthly WT respectively in January,April,October,November and December,arising date in the lowest WT,and arising date that WT arriving in 18℃,the corresponding WT alteration degree had reached to 87.9%,75.8%,74%,100%,100%,74%,89.9%,these all gave explanations of obvious change in WT alteration downstream of the dam after impoundment of the TGR.Compared to the same period before impoundment of the TGR,the average released WT were 2.3℃ and 1.7℃ lower in April and May during drawdown period,thereby the significant change of arising date that WT arriving in 18℃ was occurred,from 19 June in pre-impoundment period to 4 May in post-impoundment period,averagely postponed by 15 days.The spawning season of the Four Major Chinese Carps was closely related to the arising date that WT arriving in 18℃.Due to the lower released WT by the TGR,the first spawning time of the Four Major Chinese Carps had postponed from April to mid-May,and the whole spawning season had been shortened.The results might provide scientific reference for the experimental ecological operation of the TGR on natural reproduction of the Four Major Chinese Carps.
