Study On The Mechanism And Effect Of TDGS On Fish Swimming Behavior

China has nearly 100,000 dams,forming a storage capacity of approximately 9trillion cubic meters.Although the development of hydropower has brought renewable energy,it causes a series of ecological and environmental problems,such as the impact of total dissolved gas on aquatic organisms.When the high dam project releases the flood,the entrainment effect causes a large amount of air to enter the water.The watervapor mixture enters the energy dissipation pool under the dam where the high pressure causes excessive dissolution of gas into the water,resulting in Total Dissolved Gas(TDG).Therefore,the water in the downstream reach of river is in a state of Total Dissolved Gas Supersaturation(TDGS)for a long time,which causes the fish to suffer from Gas Bubble Disease(GBD).GBD adversely affects their growth and development and even leads to death.The research on the influence of TDGS on fish behavior usually has two parts:field investigation and laboratory evaluation.More systematic and in-depth studies have been carried out on important endemic fish under TDGS stress,including lethal effects,avoidance and perception effects,and the ability to use compensation water depth.The research on fish swimming behavior is limited to the effect of TDGS on fish swimming ability and the descriptive evaluation of abnormal behavior.The mechanism of its influence is not explained from the perspective of toxicology and molecular biology.In addition,the existing experimental results are all obtained at extremely low flow rates.The behavioral response of TDGS and flow rate synergistic stress,and the law of typical swimming behaviors affected by TDGS stress have not been carried out.This caused engineering mitigation measures to fail to fully consider the changes in fish swimming behavior in the TDGS environment.This dissertation focuses on the model organisms: Danio rerio,endemic fish Schizothorax prenanti,and Schizothorax davidi.It is firstly studied the effects of TDGS on the hatching rate,growth and gene expression of fish eggs,and then the effects of TDGS on the acute lethality,swimming ability,energy metabolism and gene expression of experimental juveniles.From the molecular toxicological level,the mechanism of TDGS’s influence on fish swimming behavior is analyzed in different life stages.After that,It is studied the co-stress of TDGS and flow rate Schizophyllus survival and swimming ability.Finally,it is studied the influence of TDGS stress on typical swimming behaviors such as long-distance bursting ability,maximum ascent distance,hydraulic factor response and abnormal behavior patterns.The results of the thesis can provide a theoretical and experimental basis for the analysis of fish escape possibility and the construction of avoidance areas in the process of flood discharge.The main findings are as follows:(1)The mechanism of the influence of TDGS on early swimming behaviorThe phenotypes of Danio rerio,Davi’s schizothoracin and Prenant’s schizothoracin under TDGS stress were observed.It showed that 120% and 130% of TDG saturation have acute toxicological effects on fish eggs,adversely affect the growth and development of larvae,and reduce the activity of larvae.Larval RNA-seq analysis showed that: TDGS reduced Danio rerio’s neurotransmitters and acetylcholine,resulting in neuronal and muscle tissue development defects,nerve signal transmission blocked,and then decreased balance ability;TDGS reduced the larval neurotransmitters of Davi’s schizothoracin and Prenant’s schizothoracin,which leads to a decrease in swimming activity.(2)The mechanism of the effect of TDGS on fish swimming abilityThe swimming ability decline of the three experimental fishes is closely related to the exposure time,survival rate,and TDG saturation.The basal metabolic rate,exercise metabolic rate and factorial aerobic range are significantly reduced under TDGS stress.The reason for the decline in swimming ability is insufficient oxygen supply and decreased energy metabolism rate.At the same time,the decrease in oxygen consumption directly indicates the loss of respiratory function of some fish gills.The results of RNA-seq of experimental fish gills showed that TDGS exposure reduced the immune function of zebrafish gills and impaired the ammonia metabolism related to respiration;the immune function of gills decreased,and the muscle tissues and organs failed;the immunity and penetration the pressure regulation function of gills is reduced.(3)Responses of endemic fishes under the co-stress of TDGS and flow rateAt 100% and 110% TDG levels,high flow velocity(0.48-0.72m/s)will cause death of experimental fish due to fatigue.At 120% and 130% TDG levels,low velocity(0.02-0.36m/s)can delay the death of experimental fish,while high velocity accelerates death.TDG saturation is the main factor influencing survival at low flow rates.As the flow rate increases,the flow rate gradually becomes the main factor affecting survival.The results of the swimming ability experiment showed that the increased average flow velocity and TDG level will reduce the critical and burst swimming ability of the experimental fish.Average flow velocity is the main influencing factor of critical swimming ability,and TDG level is the main influencing factor of burst swimming ability.(4)The effect of TDGS on the typical swimming behavior of the endemic fishThe experimental fish were stressed by TDGS,their long-distance burst swimming passing rate did not significantly decrease,and they could pass the flow barrier faster with a shorter path;the durability and explosive ability of the experimental fish would both decrease,and the uptrack distance would be shortened.The experimental fish switched from durable swimming to burst swimming at a lower flow rate.The results of the hydraulic factor response experiment showed that the response of the experimental fish to the hydraulic factor did not change significantly before the abnormal behavior.TDGS forced the experimental fish to produce two abnormal behavior patterns,the swimming step length increased significantly or they kept spinning in a small area.This dissertation explains the influence mechanism of TDGS on fish swimming behavior from the perspective of molecular toxicology.The research results of the behavioral response of endemic fish under the co-stress of TDGS and flow velocity have accumulated basic data for the establishment of fish escape models during the flood discharge process;the research results of the effect of TDGS on the typical swimming behavior of endemic fish can provide a preliminary theoretical basis for engineering measures such as constructing shelters.