Total Dissolved Gas Research Articles

Comparative transcriptome analysis provides insights into the TDG supersaturation stress response of Schizothorax davidi

In the dam discharge season, the supersaturation of total dissolved gas (TDG) in the downstream channel can seriously affect the survival of aquatic organisms. However, few studies have revealed the mechanism by which TDG supersaturation affects the physiology of fish thus far. The present study was conducted to study the mechanism of the effect of TDG supersaturation on Schizothorax davidi, a species that is very sensitive to gas bubble disease. S. davidi was exposed to 116 % TDG supersaturation stress for 24 h. Serum biochemical tests showed that the aspartate aminotransferase and alanine aminotransferase levels after TDG supersaturation exposure were significantly decreased compared to those in the control group, while superoxide dismutase activity was significantly increased. RNA-Seq of gill tissues identified 1890 differentially expressed genes (DEGs), which consisted of 862 upregulated genes and 1028 downregulated genes, in the TDG supersaturation group vs. the control group. Pathway enrichment analysis revealed that the cell cycle, apoptosis and immune signaling pathways were affected by TDG stress. The results of this study may contribute to our understanding of the underlying molecular mechanism of environmental stress in fish.

Effect of total dissolved gas supersaturation and flow velocity on survival and swimming ability of juvenile Schizothorax prenanti

Abstract Although developing large-scale hydropower cascades in the upper Yangtze River effectively improves the hydropower resource utilization, it produces total dissolved gas (TDG) supersaturation. In the flood season, the high level of TDG supersaturation (TDGS) frequently occurs in the downstream of dams, causing migratory fish to suffer from gas bubble trauma (GBT) and reducing their survival and swimming ability. Currently, there is a deficiency in particular approaches to evaluate the ecological hazard posed by TDGS on migratory fish as they traverse different flow velocities within their migratory routes. This study assessed the vulnerability of juvenile Schizothorax prenanti (S. prenanti) to GBT from the static setting to 9.0 BL/s during exposure to nominal levels of 100%, 110%, 120% and 130% TDG. The mortality occurs when the flow velocity surpasses 6.0 and 7.5 BL/s in 100% and 110% TDG levels, respectively. For fish exposed to 120% and 130% TDG levels, the relationship between survival time and flow velocity is an approximately inverse bell-shaped curve with increasing velocity. The optimal velocity of maximal survival time of juvenile S. prenanti is 3.0 and 4.5 BL/s in 120% and 130% TDG-supersaturated water. Both TDG level and flow velocity significantly affect burst swimming speed (Uburst) and critical swimming speed (Ucrit). The cases involving GBT showed substantial declines in Uburst and Ucrit, exceeding 6.0 BL/s and TDG levels greater than 120%. The results may contribute to formulating a specific management strategy for hydropower operation during the migratory period and conserving vulnerable species in the Yangtze River.

Construção do defletor na UHE Colíder para minimizar o percentual total de gases dissolvidos (TDG)

ABSTRACT During voluntary or involuntary spills, the total dissolved gas (TDG) may increase with potential of causing gas bubble disease (GBD) in affected fish. Bubbles entrained during spill events in Colider are transported by plunging jets to deep, high pressure, regions in the tailrace where dissolution is enhanced increasing TDG concentrations. The most common alternative to minimize TDG supersaturation downstream of hydropower dams is to install deflectors on the spillway face. In order to reduce TDG levels downstream of the Colider's spillway, Copel retrofitted the four spillway bays with deflectors, finalizing the construction in November 2021. The design of the deflectors were assisted with two studies. The first study comprised the development of a physical model in 1:60 scale, and other study was a numerical model based on the open-source toolbox OpenFoam.

Backpack electrofishing does not contribute to external signs of gas bubble trauma in sculpins

AbstractWe exposed prickly sculpin Cottus asper and reticulate sculpin Cottus perplexus to electroshock and sham treatments in a controlled laboratory setting to determine if backpack electrofishing contributed to or exacerbated external signs of gas bubble trauma (GBT) in fish exposed to elevated total dissolved gas (TDG) levels. Fish were exposed to 115, 120 and 125% TDG (measured as percent of saturation) for various amounts of time (24–144 h) then subjected to a 5‐s electroshock or sham treatment of no electroshock. Fish were examined to determine the incidence of GBT pre‐ and post‐treatment, and all aspects (i.e., dorsal, ventral, left and right sides) of each fish were photographed. Across all TDG levels in all trials, no sculpin showed any change in GBT incidence following treatment. Analysis of GBT signs evident in photographs of 68 fish found no evidence of change in mean pre‐ and post‐treatment gas bubble sizes in fish in any TDG trial, nor were any new gas bubbles found following treatment. We are aware of no physiological mechanism by which backpack electrofishing can cause GBT in sculpin, and as such, we believe using this gear should not increase GBT incidence rates in sculpin.

Research on the release relationship between dissolved oxygen (DO) and total dissolved gas (TDG) in standing water

Abstract Supersaturation of dissolved oxygen (DO) and total dissolved gas (TDG) is generated by high dam discharge, excess oxygen production in photosynthesis and increasing temperature in water, which may directly lead to fish suffering from ‘gas bubble disease’ or death. In this paper, under a series of experimental aeration conditions in standing water, it was concluded that aeration had a positively promoting effect on releases of supersaturated DO and TDG, while aeration aperture and aeration depth had inhibitory effects on them. For single factor analysis, aeration had the greatest effect on the release of DO and TDG, the second effect on DO was that of aeration depth and the smallest effect was that of aeration aperture, but the second effect on TDG was that of aeration aperture and the smallest effect was that of aeration depth. Most importantly, the release coefficient of DO was greater than that of TDG, and a quantitative relationship between the release coefficient of DO and TDG and aeration conditions, respectively, was established. An exponential function relationship of the release coefficients of DO and TDG was also established. The results of the research have important guiding significance and theoretical value for reducing the harm caused by supersaturated DO and TDG.

Biochemical, transcriptomic and metabolomic responses to total dissolved gas supersaturation and their underlying molecular mechanisms in Yangtze sturgeon (Acipenser dabryanus)

With the rapid development of hydropower facility construction, the total dissolved gas (TDG) generated by dam discharge is seriously threatening the survival of fish and has become an ecological environmental issue of global concern. However, how TDG affects fish physiology and the underlying molecular mechanism remain poorly known. In this study, Acipenser dabryanus, an ancient living fossil that is a flagship species of the Yangtze River, was exposed to water supersaturated with TDG at a level of 116% for 48 h. A comprehensive analysis was performed to study the effect of TDG supersaturation stress on A. dabryanus, including histopathological, biochemical, transcriptomic and metabolomic analyses. The histopathological results showed that mucosal-associated lymphoid tissues were seriously damaged after TDG supersaturation stress. Plasma catalase levels increased significantly under TDG supersaturation stress, while superoxide dismutase levels decreased significantly. Transcriptomic analysis revealed 289 upregulated genes and 162 downregulated genes in gill tissue and 535 upregulated and 104 downregulated genes in liver tissue. Metabolomic analysis revealed 63 and 164 differentially abundant metabolites between the control group and TDG group in gill and liver, respectively. The majority of heat shock proteins and genes related to ubiquitin and various immune-related pathways were significantly upregulated by TDG supersaturation stress. Integrated transcriptomic and metabolomic analyses revealed the upregulation of amino acid metabolism and glycometabolism pathways under TDG supersaturation stress. Glycerophospholipid metabolism was increased which might be associated with maintaining cell membrane integrity. This is the first study revealing the underlying molecular mechanisms of effects of TDG supersaturation on fish. Our results suggested that acute TDG supersaturation stress could enhance immune and antioxidative functions and activate energy metabolic pathways as an adaptive mechanism in A. dabryanus.

Experimental investigations on the growth of wall-attached bubble in total dissolved gas supersaturated water.

Due to dam discharge, waterfalls, sudden increases in water temperature and oxygen production by photosynthesis, the total dissolved gas (TDG) in water is often supersaturated, which may have serious effects on aquatic ecology. When the atmospheric pressure is lower than the TDG pressure in water, the supersaturated dissolved gas in water will slowly release into air. Wall-attached bubbles were formed during the TDG release process. The generation and departure of wall-attached bubbles influence the release process of TDG in water. To simulate the growth period of the wall-attached bubbles under different pressures, a decompression experimental device was designed to record the supersaturated TDG release process. Based on experimental data and mathematical calculations, the quantitative relationship between the bubble growth rate and environmental pressure was obtained. The supersaturated TDG dissipation rate increases monotonically with increasing relative vacuum degree. Applied the calculation method about the wall-attached bubble growth rate, a formula of the supersaturated TDG adsorption flux was proposed, and a prediction method of the TDG release coefficient was established. The simulation results show that with the increasing relative vacuum degree, the TDG release coefficient increases correspondingly, and the adsorption from wall surface area can be obviously promoted. This study provides an important theoretical basis for the accurate calculation of the TDG release process and provides a scientific basis for the accurate prediction of the spatial and temporal distribution of supersaturated TDG under different pressure and solid wall conditions.

Mortality risk evaluation methods for total dissolved gas supersaturation to fish based on a mitigation measure of utilizing activated carbon

The proper water chemical composition of aquaculture water is very important for fish farming in reservoirs or fish multiplication stations. Gas bubble disease (GBD) is mainly caused by total dissolved gas supersaturation (TDGS) in water and is a common problem that affects the healthy growth of fish. Extensive measures have been taken to mitigate TDGS levels in water where fish live, while methods for quantitatively evaluating the mitigation effect of the proposed measures on fish exposed to TDGS are still lacking. In this paper, an activated carbon (AC) adsorption experiment for supersaturated total dissolved gas (TDG) dissipation was conducted, and the experimental results indicated that AC addition could effectively accelerate supersaturated TDG dissipation. Based on fish tolerance experiments conducted by Huang (2010), two models, including a mortality risk degree evaluation model and a mortality rate calculation model, were developed to quantitatively evaluate the mortality risk mitigation effect of AC addiction on fish exposed to unsteady TDGS levels. Application of the results of the mortality risk degree evaluation model has shown that AC addition can help alleviate the mortality risk of fish suffering from TDGS. Application of the results of the mortality rate calculation model has also demonstrated that the final mortality rate of the fish group in the case with AC addition was lower than that of the case without added AC, and the final mortality rate decreased as the specific surface area and dosage of AC increased. Furthermore, an equation that related the required AC mass and a given harvested fish mass was established. This paper provides a reference for evaluating the effects of various mitigation measures to alleviate the risk posed to fish by TDGS.

Tolerance and Growth of Adult Schizothorax prenanti Exposed and Re-Exposed to Supersaturated Gas Downstream of a Dam

Total dissolved gas (TDG) is suggested to be one possible explanation for the extensive mortality of endemic fish suffered from gas bubble disease (GBD). We investigated the effects of water depth on adult Schitzothorax prenanti susceptibility to GBD in TDG supersaturated water 11.5 km downstream of Dagangshan dam in a two-year (2016–2017) live cage study. The probability of survival significantly increased at greater depth. The mortality of fish at 1–2 m was reduced to 50% of that at 0–1 m and the mortality rate for fish at 2–3 m was reduced to 25% of that at 0–1 m. Fish that survived in-situ TDG exposure in 2016 were relocated to equilibrated water and observed for 274 d to investigate sublethal effects of GBD. The surviving fish showed a substantial reduction in growth compared to the control group. During the flood discharge period in 2017, fish were re-exposed to TDG supersaturated water after a period of recovery (274 d) in equilibrated water. The mortality rate of re-exposed fish decreased to 35% compared to newly introduced fish. Our results contribute to the protection of aquatic organisms and the improvement of eco-friendly hydroelectric dam operations in the Yangtze River.

Effects of total dissolved gas supersaturation and sediment on environmental DNA persistence of grass carp (Ctenopharyngodon idella) in water

Environmental DNA (eDNA) technology has become an alternative tool for monitoring aquatic communities due to its sensitive, economical, and non-invasive properties. However, the application of this technique is often limited by the complexity of environmental conditions, which often poses a barrier to the transmission of biological information. Here, we conducted a series of experiments with grass carp as the target species to evaluate the effects of total dissolved gas (TDG) supersaturation and sediment on the persistence of eDNA under different flow conditions. The results showed TDG supersaturation promoted eDNA decay in still water but with no significant effect in flowing water for rapid dissipation of TDG. For sediment, its presence accelerated the decay of eDNA no matter the flow conditions. The grass carp eDNA showed an exponential decay pattern in water and the decay rate constant decreased gradually with time. Our study highlights the importance of integrating experimental results with the natural environment and provides an important reference for species monitoring using eDNA technology in aquatic ecosystems with high dams built.

Experiments about the removal of supersaturated total dissolved gas from water environment by activated carbon adsorption.

Water environment conditions directly support aquatic life. It is important to maintain a suitable water environment to improve the efficient use of water resources. Supersaturation of total dissolved gas (TDG) in the water will cause fish suffer from gas bubble disease and even mortalities. Measures should be taken to mitigate the adverse effect of supersaturated TDG. Considering the adsorption effect of porous medium, activated carbon (AC) was utilized in this experiment to explore the effect of AC on supersaturated TDG removal. The effects of AC properties, AC dosage, and initial TDG saturation were investigated. The results showed that adding AC in the water could effectively accelerate the supersaturated TDG removal rate, which was positively correlated with the AC specific surface area and dosage. Meanwhile, the average dissipation rate of TDG increased and then decreased with increasing initial TDG saturation. The adsorption characteristics of AC on supersaturated TDG were also explored. The maximum equilibrium adsorption capacity and removal rate were 0.262 mg/g and 48.5% respectively. It was concluded that the adsorption process of AC on supersaturated TDG conformed to the Langmuir equation and pseudo-first-order kinetic model. Recycling test indicated that the used AC could be reused after drying. It was hoped that this research could contribute to improving water environment and ensuring the healthy development of the aquatic livings.

Exposure Risk of Fish Downstream of a Hydropower Facility to Supersaturated Total Dissolved Gas

AbstractFish exposed to supersaturated total dissolved gas (TDG) levels can develop gas bubble trauma (GBT) which can lead to sublethal effects or mortality. Access to refugia in areas of high TDG that allows for hydrostatic (depth) compensation can mitigate exposure risk and GBT occurrence. The goals for this study were to examine resident fish habitat and depth use and assess exposure risk to elevated TDG levels related to hydropower operations in the Columbia‐Kootenay system in British Columbia. Modeling was used to predict TDG levels for three operational cases (low, medium, and high spill rates). Acoustic telemetry was used to track rainbow trout Oncorhynchus mykiss (RT) and mountain whitefish Prosopium williamsoni (MW) reach and depth residency. Telemetry results did not differ among operational scenarios and aligned with known biological/life history characteristics for fluvial or fluvial‐adfluvial species. Within‐species MW reach residency appeared to be reflective of seasonal habitat selection for spawning, foraging, and refuge movements. Within‐species RT reach residency appeared to follow habitat association patterns reflective of RT ecology. A risk assessment revealed that RT had a significantly higher TDG exposure risk relative to MW, but sufficient depth refugia habitat was available to mitigate exposure risk and GBT occurrence in both species. The results suggested that TDG exposure risk and actual risk depend on the interplay between species‐specific ecology and TDG patterns generated by hydropower facilities. The ecological and TDG patterns in this study suggested that system‐ and species‐specific studies will be required to generate detailed TDG exposure predictions for management decision‐making.

Prediction model and application of machine learning for supersaturated total dissolved gas generation in high dam discharge

Supersaturation of total dissolved gas (TDG) caused by high dam discharge is an ecological risk that cannot be ignored in the operation of hydropower stations. The establishment of an efficient and concise TDG generation prediction model is of great significance to the water ecology and water environment protection of hydropower development reaches. The flow conditions and the process of water-gas mass transfer in discharge and energy dissipation are very complicated and difficult to observe in the field, bringing difficulties to the establishment of prediction model and parameter calibration. Increasingly abundant observations make it possible to establish an efficient machine learning prediction model for supersaturated TDG. In this study, extreme learning machine (ELM) and support vector regression (SVR) were used to establish the prediction model. The main influencing factors of supersaturated TDG, obtained by the analysis of the physical process of the generation of supersaturated TDG, were used as the input of the machine learning model. Then, this research took Dagangshan hydropower station and Xiluodu hydropower station as objects, and established machine learning prediction model for supersaturated TDG with several years of observation data in different discharge scenarios. Four models, including ELM, SVR, GA-ELM and GA-SVR, were obtained through genetic algorithm optimization. The relative errors of the simulation results of each model are mostly less than 5%, mean absolute error (MAE) values less than 1.6%, and root mean square error (RMSE) values less than 2.5%. The results showed that these models are highly accurate and time-saving. Based on this, TDG saturation in downstream of Dagangshan hydropower station with different discharge scenarios was simulated by machine learning model, on which the discharge optimization scheme was put forward. The proposed models, as an important supplement to the prediction of supersaturated TDG, enjoy practical significance and engineering value.

Experimental study of the growth period of wall-attached bubbles

Abstract Due to dam discharge, waterfalls, sudden increases in water temperature and oxygen production by photosynthesis, the total dissolved gas (TDG) in water is often supersaturated, which may have serious effects on aquatic ecology. Wall-attached bubbles formed during the TDG release process and the generation and departure of wall-attached bubbles influenced the release of TDG from water. Therefore, an experiment was performed to simulate the growth of wall-attached bubbles at various water flow velocities, a quantitative relationship between the wall-attached bubble growth period and flow velocity was obtained. Another quantitative relationship, between the wall-attached bubble departure diameter and turbulent kinetic energy of flowing water, was also determined. The analysis results of the TDG release rate proved that the adsorption of TDG on solid walls was considerably affected by flow velocity. The analysis models the TDG release mechanism through complex experiments and provides a method to better identify sites for supersaturated TDG adsorption. This study serves as an important theoretical basis for revealing the mechanism by which solid surfaces promote the release process of supersaturated TDG in natural water.

Improved Model for Predicting Total Dissolved Gas Generation With the Residence Time of the Water in the Stilling Phase

Quantitative predictions of total dissolved gas (TDG) super-saturation are essential for developing operation schemes for high dams. Most TDG generation prediction models have various shortcomings that affect the accuracy of TDG super-saturation estimation, such as oversimplification of influencing factors and uncertainty in parameter values. In this study, the TDG generation process was divided into three parts, gas-liquid mass transfer process in the stilling phase, dilution resulting from the water jet plunging into the stilling phase, and outflow of TDG–super-saturated water from the stilling phase, while considering the water body and bubbles in the stilling phase as a whole. The residence time of the water in the stilling phase (Tr) was introduced to estimate mass transfer time, along with dimensional analysis methods. The properties of TDG generation were evaluated experimentally under varying Tr values. Based on the theoretical analysis and experimental results, a basic water renewal model was proposed and was validated using experimental data. Furthermore, prediction results of this model were compared with those of a classical empirical model and mechanical model based on observed data from a field survey at Xiluodu Dam. The results show that the relative errors between the predicted and experimental measurements were all less than 5%, indicating that the developed prediction model has a good performance. Compared with the mechanism model, the developed model could reduce the standard error (SE), normalized mean error (NME), and error of maximum (REMAX) by 60, 96, and 15%, respectively. Meanwhile, the developed model could reduce the SE, NME, REMAX by 17.4, 36, and 23%, respectively, compared with the empirical model. Considering all the error indexes, it can be concluded that the prediction performance of the water renewal model is the best among the three models. The proposed model was also more generically versatile than the existing models. Prediction results of water regeneration model for TDG could aid the drafting of governing strategies to minimize the risk of super-saturated TDG.

Swimming performance of a pelagic species in the Yangtze River under different exposure modes of the total dissolved gas supersaturation.

During flood discharges of upstream dams in the Yangtze River, the pelagic fish have a stress risk from total dissolved gas (TDG) supersaturation in the river water. This study took the silver carp as the object and systematically evaluated the effects of TDG supersaturation levels and exposure time on their critical swimming speed (Ucrit) at different temperatures. The external symptoms of gas bubble disease were found when TDG levels exceeded 130%. Both exposure time and TDG level did not significantly impact the Ucrit of fish under 6days of non-lethal exposure (110%, 120%, 130% TDG) with lower or higher water temperature. Significant differences in Ucrit were found among different exposure times at 11.0 ± 1.0°C under 10hours of lethal exposure (135%, 140%, 150% TDG) and the Ucrit reduced by 59.88%, 83.32%, and 92.40%, respectively. TDG level had a significant impact on the Ucrit at 21.0 ± 1.0°C when exposure time exceeded 8hours. Ucrit at 21.0 ± 1.0°C water were significantly greater than those at 11.0 ± 1.0°C water where conditions had the same TDG supersaturation and exposure time. Differences in Ucrit between temperatures ranged from 3.24 to 6.12 BL/s under non-lethal exposure and from 6.38 to 13.88 BL/s under lethal exposure. The results of this study can provide a reference for fish conservation during flood discharge.

A System Model for Total Dissolved Gas Risk Assessment Due to Multidam Spill Operations

AbstractSpill operations of hydropower facilities can generate supersaturated total dissolved gases (TDGs) that can negatively impact fish residing in downstream habitats by causing gas bubble dise...

Effects of Total Dissolved Gas Supersaturation on the Survival of Juvenile Procypris rabaudi and Juvenile Myxocyprinus asiaticus at Varying Water Depth in a Natural River.

Total dissolved gas (TDG) supersaturation, which can be caused by flood discharge, results in gas bubble disease (GBD) in fish and threatens their survival downstream of dams. TDG supersaturation has become a serious environmental problem in the Yangtze River. Few studies have evaluated the effect of TDG supersaturation on fish in natural rivers during periods of flood discharge. To estimate fish tolerance to TDG supersaturation under natural conditions, juvenile Myxocyprinus asiaticus and juvenile Procypris rabaudi were exposed to TDG-supersaturated water for 96 h at various depths (0-0.3 m, 0.3-1.3 m, 1.3-2.3 m and 0-2.3 m) during periods of flood discharge of Dagangshan hydropower station. The results showed that juvenile Procypris rabaudi and juvenile Myxocyprinus asiaticus exhibited obvious GBD signs. An increase in exposure time decreased survival probability of the two species. Deeper water depths can increase the tolerance of juvenile Procypris rabaudi to TDG supersaturation in natural rivers during periods of flood discharge while it cannot improve the survival of juvenile Myxocyprinus asiaticus. Compared with juvenile Myxocyprinus asiaticus, juvenile Procypris rabaudi showed weaker tolerance of TDG supersaturation in shallow water, and juvenile Procypris rabaudi were more vulnerable to TDG supersaturation than juvenile Myxocyprinus asiaticus even if the TDG level (116%) was low.

Effect of total dissolved gas supersaturation on the survival of common carp (Cyprinus carpio) and silver carp (Hypophthalmichthys molitrix).

Flood discharge results in total dissolved gas (TDG) supersaturation downstream of a dam during the flood period. Fish suffer death from gas bubble disease (GBD) caused by TDG supersaturation. Nonetheless, current studies mainly attach importance to the survival of benthic fish affected by TDG supersaturation in the Yangtze River in China. Few studies have attempted to investigate the survival of pelagic fish influenced by TDG supersaturated water and compare the tolerance characteristics to TDG supersaturation between benthic and pelagic fish. To identify the survival of fish species that inhabit the various water layers affected by TDG supersaturation, silver carp (Hypophthalmichthys molitrix) (pelagic fish) and common carp (Cyprinus carpio) (benthic fish) were chosen to conduct an acute exposure experiment of four different TDG supersaturation levels (125%, 130%, 135% and 140%). The findings illustrated that the two fish species both exhibited evident aberrant behaviours of maladjustment in TDG supersaturated water. Obvious GBD symptoms were also found in the test fish. The survival probability of silver carp and common carp decreased with increasing levels of TDG supersaturation. The median survival time (ST50 ) values of the silver carp exposed to four levels of TDG supersaturated water (125%, 130%, 135% and 140%) were 26.84, 7.96, 5.56 and 3.62 h, respectively, whereas the ST50 values of common carp were 53.50, 26.00, 16.50 and 11.70 h, respectively. When compared with common carp, silver carp had a weaker tolerance to TDG-supersaturated water and were vulnerable to GBD. It shows that levels above 125% are not safe for common carp survival. In terms of the tolerance threshold value, silver carp merits further investigation because it showed lower tolerance to TDG than did common carp.

Impact of TDG supersaturation on native fish species under different hydropower flood discharge programs.

Total dissolved gas (TDG) supersaturation caused by the operation of hydropower stations can threaten the survival and swimming performance of fish species. Different modes of hydropower flood discharges (regular vs. intermittent) from the Dagangshan hydropower station in China were studied in July and September 2017 to study the impact of TDG supersaturation on two native fish species in the downstream Dadu River. The average TDG supersaturation value was 114.3% in July under the regular discharge mode. In September, the supersaturation of TDG fluctuated in response to the intermittent discharge mode with an average TDG of 119.3%. Apparent gas bubble trauma was found on young-of-the-year (YOY) Prenant's schizothoracin and elongate loach in cages at different water depths during the flood discharge period. The mortality rate of YOY Prenant's schizothoracin and elongate loach in cages with water depths of 0-1m were 16.25% and 2.5%, respectively, in July. The fluctuating TDG levels with higher peaks in September caused higher fish mortality rates. The final mortality rates of YOY Prenant's schizothoracin and elongate loach in cages with water depths of 0-1m were 75% and 33.75%, respectively. Fish in the cages at a 0-3m water depth survived better than those in the cages at a 0-1m water depth. The critical swimming speeds (Ucrit) of YOY Prenant's schizothoracin and elongate loach without exposure to TDG supersaturation were 11.64 and 16.76 BL s-1, respectively. Ucrit decreased significantly after experiencing the flood discharge period and recovered to the normal level after experiencing the corresponding interval period.