Effects Of Dissolved Gas Supersaturation On Fish Resource In Middle And Lower Reaches Of Jinsha River

One of the negative environmental impacts associated with hydropower is the excessive presence of dissolved gas in water,known as supersaturation.This condition can cause fish to develop bubble disease,leading to severe consequences,including death.Given the challenges posed by climate change,hydropower has gained significance as a low-carbon and environmentally friendly energy source.However,the issue of dissolved gas saturation resulting from the construction of numerous large dams has become increasingly prevalent in hydropower-affected river systems.Therefore,it is crucial to investigate the effects of dissolved gas supersaturation on fish populations.This research focuses on studying the middle and lower sections of the Jinsha River and specifically examines gas supersaturation patterns downstream of the Xiluodu hydropower station.Furthermore,a numerical model is employed to predict the extent and distribution of gas supersaturation in the river system.The study also utilizes a specially designed supersaturated water generation device to simulate and assess the tolerance of various fish species to gas supersaturation.In addition,in situ toxicity tests were conducted in the Xiangjiaba reservoir to evaluate fish tolerance to gas supersaturation under natural conditions.To better understand the mechanisms by which fish avoid gas supersaturation,an analysis of the natural gas distribution in the relevant stream was conducted.This analysis,combined with an examination of fish resource characteristics,enables an assessment of the impact of gas supersaturation on fish.The study identifies the fish species affected by gas supersaturation and proposes measures to mitigate its detrimental effects based on the evolving trends in fish resources.The main findings of this research are as follows:1.The occurrence probability,degree,and distance affected by gas supersaturation are related to several factors.The likelihood of supersaturation induced by high dams is influenced by the progress of power station operations.As the generation units gradually come online,the flow of electricity generation increases while the waste flow decreases.Consequently,the occurrence probability of gas supersaturation tends to decrease,and the degree of supersaturation gradually weakens.The pressure of the released water and the waste flow are the primary factors that impact the extent of supersaturation,exhibiting a positive correlation with the degree of gas supersaturation.The rate at which dissolved gas saturation diminishes depends on the morphology of the river and the presence of tributaries.In the reservoir,the attenuation rate is 4.04% per 100 km,while in the reach without tributaries,it is 7.55% per 100 km,and in the reach with significant tributaries,it reaches 20.71% per 100 km.Throughout the study period,the nearest affected stream was observed to be located 98 km downstream from the Xiangjiaba dam,while the furthest affected stream was found 198 km downstream from the Xiangjia Dam.2.The occurrence probability and degree of gas supersaturation in the lower reaches of Jinsha River are high,and the distance of their influence is long.Based on the principles governing gas supersaturation and the mathematical model,it is observed that downstream gas supersaturation occurs when the waste water discharge exceeds 20%of the generation discharge.When the abandoned flow is equal to the generation flow,gas saturation can exceed 110%.Furthermore,if the discharge rate exceeds twice the generating rate,gas saturation can surpass 120%.Assessment results indicate that all 14 hydropower stations within the study area have the potential to induce gas supersaturation.Among these stations,five(Jin’anqiao,Ludila,Wudongde,Baihetan,and Xiluodu)could impact river sections exceeding a length of 495 km.For seven stations(Liangjiaren,Liyuan,Ahai,Longpan,Guanyinyan,Jinsha,and Yinjiang),the affected river length ranges between 247 km and 495 km.Lastly,the remaining two power stations(Longpanshui and Xiangjiaba)have an influence on river sections of less than 247 km in length.3.The tolerance to gas supersaturation is considerably different for different kinds of fish and different stage.Dissolved gas supersaturated water did not exhibit significant toxic effects on the hatching rate of fish eggs.However,as the larvae developed,the toxic effects of gas supersaturation on fish gradually intensified.Different fish species display varying tolerances to gas supersaturation.In natural conditions,fish can actively overcome the effect of gas supersaturation.When gas supersaturation occurs,fish with strong swimming capabilities exhibit active behavior by swimming deeper into the water to mitigate the impact of gas supersaturation.This suggests that fish can actively evade gas supersaturation in water by employing a compensatory mechanism related to water depth.Genetic analysis showed that the mechanism of fish’s death caused by gas supersaturation was the same as that was caused by anoxia.The results of the study demonstrate that gas supersaturation produces similar effects on fish as oxygen deficiency does,leading to fish mortality due to insufficient oxygen levels in the circulatory system.4.Gas supersaturation seriously effet fish stocks.The findings reveal that gas supersaturation has minimal impact on larvae and juveniles of 111 fish species that breed prior to the flood season.It exhibits a positive influence on the hatchability of 53 fish species that breed during the flood season.However,it poses a significant lethal threat to young and juvenile fish,leading to a potential decline in early-stage resources.Five fish species that breed after the flood season show no apparent effects on their early resources.Among the 63 fish species that have become rare and endemic due to the construction of cascade power stations,gas supersaturation exacerbates the endangerment of 29 fish species that have disappeared from reservoirs.This could result in the loss of 18 early fish species.To sum up,the frequency and degree of gas supersaturation in the downstream affected by factors such as the mode of water abandonment,the flow rate of water abandonment.According to the structure and characteristics of 14 power stations in the middle and lower reaches of the Jinsha River,the probability of gas supersaturation in the region is high,and the influence range is far.Gas supersaturation in the region has a significant adverse effect on the breeding fish in flood season,aggravates the endangered degree of endemic and protected fish that gradually disappear due to the habitat of the cascade power station reservoir,and leads to the endangered degree of endemic and protected fish that can maintain a certain population resource affected by the cascade power stations.The systematic implementation of reservoir ecological control and water ecological protection measures will mitigate the adverse effects.