Study On Rational Flow Of High Dam Discharge Based On The Effect Of Water Temperature On Fish Spawning Behavior

With the development of economy and the constant growth of population,the demand of water resource and the need of flood control are increasing.For this reason,a large number of water conservancy projects were built around the world.However,despite the great economic and social effects exerted on flood control,power generation,navigation and irrigation,these hydraulic projects have brought a series of negative impacts to river ecosystem.The construction of high dams and great reservoirs has changed the original hydrological situation of the river.Besides,the cold water discharged by the dams has changed the downstream water temperature,influencing the activities of downstream aquatic organisms.As the representative organism of river ecosystem,fish's physiological and ecological behaviors are also affected by the cold water release.In order to relieve the negative impact that the hypothermia water releases exerted on downstream fish,a lot of studies have been carried out and various measures have been taken out to solve the cold water problem both in China and abroad,such as stratified water intake,ecological regulation and so on.Previous studies mainly focus on whether the water temperature meets the minimum requirements for fish spawning,while the relationship between gonadal development and water temperature during the fish breeding process is seldom studied.In addition,when taking engineering measures to improve the downstream water temperature,the current existing scheduling modes hardly take the demand for water temperature during the fish spawning into consideration.Thus,based on the response of the downstream target fish spawning behavior to water temperature,the present paper studied the dam discharge which affects the fish's demand for the microhabitat water temperature by analyzing the variation of water temperature along the downstream of the high dam.The major results and conclusions are as follows:(1)Based on historical data and the survey on the fish in the Xiluodu-Xiangjiaba section of the downstream of the Jinsha River,seventeen potential fish species were selected.These selected fish were studied from the following aspects: the characteristics of the Spawning characteristics,the economic value and feeding grounds and so on.According to the screening principle,a matrix for comparison and judgement was constructed and ranking weights of these fish species were calculated by using the Analytical Hierarchy Process(AHP).Coreius heterodon was selected as the most representative target fish based on the requirement for temperature during the spawning period in the target region.Additionally,according to the fish survey and historical data,it was determined that the suitable temperature for Coreius heterodon spawning is 20-22 ?.Furthermore,through analyzing historical data,we found the biological zero temperature of gonadal development for Coreius heterodon was 8 ?,and the accumulated temperature of laying eggs was 1510.7±226.6 ?.(2)A two-dimensional water temperature model of Xiluodu-Xiangjiaba was established.The simulation results showed that the downstream temperature had been obviously delayed and flattened after the construction of the dam,which was characterized by lower water temperature in spring and summer but high water temperature in autumn and winter.The water temperature in low temperature period is not low and vice visa.Combining the suitable water temperature and the accumulated temperature of fish spawning,the water temperature of spawning sites during fish reproduction period was analyzed.It was found that,for Coreius heterodon,the time for reaching the accumulated temperature was delayed due to the construction of the dam;the time for laying eggs was put off and the egg-laying duration was shortened.At the same time,low temperature water discharged from the dam mitigated the influence of meteorological factors on the water temperature of the spawning sites during the spawning period,constantly increasing the suitable water temperature for spawning in spawning sites.(3)Increasing the discharged flow can reduce the water level of Xiluodu reservoir and reduce the distance from the intake to the water surface so as to increase the water temperature around the intake during the heating period,mitigating the delay effect of the water temperature of the Coreius heterodon spawning sites.As a result,the time for reaching the accumulated temperature of the spawning was shifted to an earlier time,which was conducive to the early occurrence of spawning activities and increase the time of spawning.Raising the elevation of the intake can increase the water temperature around the intake and effectively improve the water temperature distribution of the Coreius heterodon spawning sites,which was conducive to the development of gonads in the breeding period of the Coreius heterodon,and can advance the time of reaching the accumulated temperature of the spawning,making the Coreius heterodon spawn ahead of time and increasing the duration of the spawning period.On the other hand,raising the elevation of the intake shortened the duration of the optimal spawning water temperature during the spawning period.Based on Coreius heterodon's demand for water temperature during the breeding period,the discharged flow should be increased during the wintering period,and the water level in front of the dam in the reservoir should be reduced in advance to shorten the distance between the intake and the water surface during the heating period and increase the water temperature around the intake.These measures were beneficial to alleviate the current situation of low temperature discharge from the dam and effectively improve the water temperature of the spawning sites of Coreius heterodon during spawning period,reducing the adverse effects of low temperature discharge on the spawning of Coreius heterodon.