Study on the effect of aeration system on the mass transfer performance of supersaturated total dissolved gas.

ABSTRACTThe release of supersaturated total dissolved gas (STDG) from dams has been linked to the development of gas bubble disease, which can ultimately result in the death of fish. In order to minimize the impact of STDG on aquatic ecology, the effect of aeration on mass transfer at the air-liquid interface is taken into account. This paper selects four commonly used aerators to carry out indoor aeration tower experiments under different aeration conditions (aeration aperture, aeration water depth, and aeration volume), exploring aerators that can efficiently promote STDG release. The results indicated that the diaphragm aerator was found to have the greatest effect on STDG release, followed by corundum and spin mix aerator. In contrast, a pinhole aerator was found to have the least beneficial impact on STDG release. The increase in the release coefficient for the diaphragm aerator in comparison to the pinhole aerator is 32%. A prediction model for the aeration system was developed based on the mass transfer mechanism at the gas-liquid interface. The parameters in the model were determined using experimental data, which effectively improved the model's prediction accuracy. The findings of this study may serve as a reference point for the selection of the most suitable aerator in the actual engineering of STDG mitigation by aeration technology.

TDG prediction model improvement by analysis and validation of experiments on a dam model.

The combination of aerated flows and a high-pressure environment in a stilling basin can result in the supersaturation of total dissolved gas (TDG) downstream of hydraulic projects, posing an ecological risk to aquatic populations by inducing gas bubble disease (GBD) or other negative effects. There is limited literature reporting TDG mass transfer experiments on a complete physical dam model; most existing research is based on measurements in prototype tailwaters. In this study, TDG mass transfer experiments were conducted on a physical model of an under-constructed dam, with TDG-supersaturated water as the inflow, and TDG concentrations were meticulously monitored within the stilling basin. The measurements indicate that the TDG saturation at the outlet of the stilling basin decreased by 13.7% and 10.6% compared to the inlet for the two cases, respectively. Subsequently, an improved TDG prediction model was developed by incorporating a sub-grid air entrainment model and a phase-constrained scalar model. The numerical simulation results were compared with experimental data, indicating a maximum error in TDG saturation at all measured points of less than ± 3%. Moreover, the TDG saturation showed an error of only ± 0.3% at the outlet of the stilling basin. This model has broad applicability to various flow types for obtaining TDG mass transfer results and evaluating mitigation measures of TDG supersaturation to reduce the harmful effects on aquatic ecosystems.

Tolerance threshold of a pelagic species in China to total dissolved gas supersaturation: from the perspective of survival characteristics and swimming ability.

Total dissolved gas (TDG) supersaturation downstream of dams can occur in the Yangtze River basin and is known to cause stress and even death in fish. Consequently, it is important to establish tolerance thresholds of endemic fish to protect local aquatic resources. We conducted experiments to assess survival characteristics and swimming ability of bighead carp, an important commercial fish dwelling in the Yangtze River, to evaluate its tolerance threshold to TDG supersaturation. The typical external symptoms of gas bubble trauma (GBT) were observed and the time when the fish lost equilibrium and died were recorded. The results showed that the mortality occurred when TDG level exceeded 125%, with obvious symptoms such as exophthalmos and bubbles on the head. The interval between loss of equilibrium and mortality decreased with an increase in TDG level. Neither exposure time nor TDG level significantly affected the critical swimming speed (Ucrit) of fish exposed to non-lethal exposure (110%, 120% and 125% TDG) over a 7 day period. Significant reductions in Ucrit were found under 130% and 135% TDG conditions when the exposure lasted 52.0 h and 42.9 h, respectively. The Ucrit also significantly decreased after exposure of 1.6 h under 140% TDG condition. Moreover, after exposure to 140% TDG for 39.2 h, 135% TDG for 56.5 h and 130% TDG for 95.9 h, bighead carp were transferred into air saturated water to recover for 24 h or 48 h; however, swimming performance remained impaired. The results of this study indicate that 125% TDG was the highest TDG level where limited mortality was observed and the swimming ability was not impaired, showing that 125% TDG can be set as the tolerance threshold of this species to guide the operation of dams in the Yangtze River Basin.

Causes and consequences of gas bubble trauma on fish gill function.

Total dissolved gas supersaturation (TDGS) occurs when air mixes with water under pressure, which can be caused by features such as hydroelectric dams and waterfalls. Total dissolved gas supersaturation can cause harmful bubbles to grow in the tissues of aquatic animals, a condition known as gas bubble trauma (GBT). As gills are the primary gas exchange surface for most fish, it is through the gills that elevated total dissolved gases enter the blood and tissues of a fish. We describe the role of the gills in admitting TDGS into the body and discuss potential effects of bubbles in the gills on blood oxygen and carbon dioxide diffusion, blood ion and pH homeostasis, and nitrogenous waste excretion, as well as downstream effects on aerobic swimming performance.

TDG generation in a ski-jump spillway with a fully or partial-flip bucket.

Ski-jump spillways are frequently used as discharge structures for high dams during floods with high energy heads. The selection of bucket types at the end of spillways has a pronounced effect on the hydraulics of jet characteristics, such as trajectories and entrained air features. However, there is no literature reporting how changes in the bucket types influence TDG generation. This study compares the hydraulic characteristics and TDG mass transfer properties of a hydraulic project under construction using both the traditional fully-flip bucket and the partial-flip bucket configurations. The results indicate that, the use of the partial-flip bucket at the end of the spillway significantly disperses the water flow and yields better energy dissipation effects. At low flow rates (lower than 400 m3/s for the dam in this study), there is little difference in the downstream TDG saturation between the traditional fully-flip bucket and the partial-flip bucket, the average difference is 1.6 % in three cases with a low flow rate. However, at high flow rates (higher than 400 m3/s), the partial-flip bucket generates more TDG compared to the traditional fully-flip bucket, reaching up to 6.2 % at the maximum flow rate. This phenomenon stems from significant changes in the hydrodynamics of the stilling basin at high flow rates due to variations in the flip bucket type. When strict control of TDG generation is necessary downstream of dams, the use of the partial-flip bucket should be carefully considered. This is because, at high flow rates, the partial-flip bucket might result in higher TDG saturation than the fully-flip bucket.

Interaction of total dissolved gas supersaturation and suspended sediment on swimming performance of bighead carp (Hypopthalmichthys nobilis).

When dams discharge floodwaters, the river downstream often becomes supersaturated in total dissolved gases (TDG) and contains high volumes of suspended sediments (SS). Supersaturated TDG and high SS concentrations in water may affect fish physiologically in ways that manifest in swimming performance. Despite increasing awareness of the separate effects of TDG supersaturation and SS, knowledge about their synergistic effects remains unknown. To explore the interactive effects of TDG and SS on the swimming performance of bighead carp, the juveniles were exposed to 100, 110, 115, 120, 125, 130, 135, and 140% of TDG-supersaturated water with SS concentrations of 0, 50, 100, and 150 mg/L, respectively, and the critical swimming ability speed (Ucrit ) and burst swimming ability speed (Uburst ) were measured. The results indicated that the swimming ability (Ucrit and Uburst ) decreased when TDG levels and SS concentrations increased. TDG and SS did not interact significantly to decrease both Ucrit and Uburst . In contrast, exposure to TDG alone significantly decreased both Ucrit and Uburst , whereas exposure to SS alone decreased only Uburst . In addition, our results suggested that there was a negative linear relationship between TDG and fatigue time. Swimming ability can decline significantly due to high TDG levels (>130%). Therefore, high TDG levels (>130%) should be restricted during reservoir operation to prevent the stress caused by TDG.

Molecular mechanisms of physiological change under acute total dissolved gas supersaturation stress in yellow catfish (Pelteobagrus fulvidraco).

During the dam discharging period, the strong aeration of high-speed water leads to the supersaturation of total dissolved gas (TDG) in the downstream water, which causes gas bubble disease (GBD) in fish and threatens their survival. TDG supersaturation has now become an ecological and environmental issue of global concern; however, the molecular mechanism underlying the physiological effect of TDG supersaturation on fish is poorly known. Here, we comprehensively investigated the effect of TDG supersaturation on Pelteobagrus fulvidraco at the histopathological, biochemical, transcriptomic, and metabolomic levels. After exposure to 116% TDG, P. fulvidraco exhibited classic GBD symptoms and pathological changes in gills. The level of superoxide dismutase was highly significantly decreased. Transcriptomic results revealed that heat shock proteins (HSPs) and a large number of genes involved in immunity were increased by TDG stress. A key environmental sensor PI3K/Akt/mTOR pathway was significantly stimulated for defence against stress. Integrated transcriptomic and metabolomic analyses revealed that key metabolites and genes were upregulated in the triacylglycerol synthesis pathway and that amino acid levels decreased, which might be associated with TDG supersaturation stress. The present study demonstrated that TDG supersaturation could cause severe physiological damage in fish. HSP genes, immune functions, and energy metabolic pathways were enhanced to counteract the adverse effects.

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.

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.

Production of total dissolved gas supersaturation at hydropower facilities and its transport: A review.

Total dissolved gas supersaturation (TDG) is a common issue in hydropower facilities as a result of water conveyance structures that increase the amount of air entrainment from the atmosphere and dissolved into the water. Water with TDG supersaturation can negatively impact fish, aquatic invertebrates and their habitats. This study comprehensively reviewed the physical mechanisms of TDG generation and predictive TDG generation models at various facility types. To establish TDG mitigation strategies, it is essential to develop predictive tools for TDG generation that consider both facility geometry as well as the hydrology of the downstream environment. Applications of TDG prediction at different discharge modes included plunging flows, trajectory jets, plunging jets, free-falling jets, and submerged jets were discussed. TDG transport models in downstream rivers involving mixing and dissipation were introduced, which can be integrated with TDG generation models into a platform to describe TDG distribution in river systems. Subsequently, risk ranking procedures for assessing the degree of TDG risk on fish were provided. Potential measures for mitigating TDG supersaturation were reviewed and included engineering, operational, and technical solutions. Outcomes from this review considered a diverse suite of studies on TDG issues in regulated rivers and allowed for recommendations to reduce uncertainties and improve environmental performance at facilities where TDG risks occur.

River habitat assessment and restoration in high dam flood discharge systems with total dissolved gas supersaturation.

The success of river habitat restoration relies on accurate assessment proxies. However, determining how to quantitatively assess the impact of multiple stressors during flood discharge from high dams in riverine ecosystems and where and how to implement more reliable recovery interventions remain challenges. Here, we developed a bottom-up mechanistic framework for assessing the effects of total dissolved gas supersaturation (TDGS) and hydrodynamics on fish habitat quality and applied it to the downstream river reach of the Xiangjiaba Dam in Southwest China. The results showed that the available habitat area of river sturgeon was the smallest, while Chinese sucker had the largest available habitat area among the three target species under all discharge scenarios. Although the TDGS levels were evenly mixed laterally, the habitat suitability index indicated that the suitable habitats were primarily within both sides of the river reach under all scenarios, which is contrary to findings based on the traditional TDGS risk assessment model. The traditional TDGS risk assessment model overestimates the impact of dams on habitats. This divergence reflected the sensitivity of the habitat assessment to fish habitat preferences, fish tolerance to TDGS and the biological response of fish under TDGS. Additionally, the priority areas for restoration can be identified by habitat suitability index with lower values. We simulated twenty-four schemes and found that interventions such as stone groups, ecological spur dike, water-retaining weir and river dredging can enhance habitat suitability for fish species under multiple stressors, providing novel insights into where and how to mitigate the impact of TDGS. Our findings offer a transferable framework for the quantitative evaluation of fish habitat and implementation of restoration management during dam flood discharge periods, thus providing a new perspective for biodiversity conservation and habitat restoration in dam-regulated rivers with TDGS around the world.

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 6 days 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 10 hours 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 8 hours. 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.

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.

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.

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.

Total dissolved gases induced tolerance and avoidance behaviors in pelagic fish in the Yangtze River, China.

Total dissolved gas (TDG) supersaturation caused by dam operations can cause fish gas bubble disease (GBD) and even fish kill. Few studies have examined the effects on pelagic species. Here, we examined the tolerance and avoidance characteristics of silver carp (Hypophthalmichthys molitrix), a pelagic fish widely distributed in the Yangtze River basin in China, under stress caused by TDG supersaturation. Silver carp had an average mortality rate of 7.5% ±â€¯1.8%, 92.5% ±â€¯1.8%, and 97.5% ±â€¯1.8% under 130%, 140% and 150% TDG supersaturation for 72 h of exposure, respectively. The average median lethal time (LT50) of silver carp was 18.1 h and 8.0 h under 140% and 150% TDG supersaturation, respectively. Bubbles and congestion appeared in the fins, gills and skin of silver carp. Silver carp can detect and avoid high TDG supersaturation. Significant avoidance behaviors were displayed by silver carp and the final avoidance rate was over 80% under 130% or above TDG conditions. The results of this study indicate that 130% TDG supersaturation triggered silver carp avoidance behaviors, and can be considered as the tolerance threshold.

Experimental investigations on the dissipation process of supersaturated total dissolved gas: Focus on the adsorption effect of solid walls.

Hydropower, which utilizes energy from fast-moving water, can help alleviate the energy crisis and promote economic development. For safety and ecological purposes, dams must discharge periodically. This discharge process, which occurs with the supersaturation of total dissolved gas (TDG), affects the fish and other aquatic organisms living in downstream river areas. Previous studies have shown that the supersaturated TDG dissipation process is closely related to hydraulic properties such as the flow rate, water depth and turbulent kinetic energy. Additionally, the presence of solid walls such as vegetation leaves in water can adsorb dissolved gases in water, thereby promoting the supersaturated TDG dissipation process, and the adsorption effect is closely related to the solid wall material. However, systematic studies on how solid walls in water can quickly absorb dissolved gases from water and rapidly reduce the TDG saturation of water are lacking. Herein, a series of experiments was conducted to study the dissipation process of supersaturated TDG under the action of solid walls in water and to reveal the adsorption effect of solid wall surface properties on dissolved gas. The results showed that the surface roughness and hydrophobicity (contact angle) are the key factors related to the adsorption effect of solid walls on the TDG in water. Generally, the dissipation rate of supersaturated TDG first increased and then decreased with increasing surface roughness. The supersaturated TDG dissipation rate increased monotonically with increasing contact angle. Based on the experimental data, the adsorption coefficient, which represents the adsorption effect of the solid wall for dissolved gas in water, was proposed, and a prediction formula between the adsorption coefficient and the contact angle of the solid wall was established. These results can provide theoretical support for the utilization of the solid wall adsorption effect to mitigate the adverse effects of supersaturated TDG and protect fish.

Effect of Total Dissolved Gas Supersaturation on the Survival of Bighead Carp (Hypophthalmichthys Nobilis).

To assess the effect of TDG on the survival of different sizes of pelagic fish, bighead carp (Hypophthalmichthys nobilis) were subjected to TDG supersaturated water at levels of 125, 130, 135, and 140%. The results showed that apparent abnormal behaviours and symptoms of gas bubble disease (GBD) were observed in bighead carp. The survival probability of large and small juvenile bighead carp declined with increasing TDG levels. The median survival time (ST50) values of large juvenile bighead carp were 74.97 and 31.90 h at 130% and 140% TDG, respectively. While the ST50 of small fish were 22.40 and 6.72 h at the same TDG levels. In comparison to the large juvenile bighead carp, the small juvenile bighead carp showed weaker tolerance to TDG supersaturated water. Furthermore, acute lethality experiments after chronic exposure to TDG were initiated to further investigate the effect of TDG on bighead carp. The juveniles were first subjected to 115% TDG supersaturated water for 96 h. After chronic exposure, live fish were immediately transferred to TDG supersaturated water at levels of 125, 130, 135, and 140%. The results demonstrated that no fish died under chronic exposure and few fish exhibited slight GBD symptoms. The ST50 values for bighead carp subjected to acute exposure after chronic exposure were 61.23 and 23.50 h at 130 and 140%, respectively. Compared with the bighead carp subjected to acute exposure, bighead carp subjected to multiple exposures were more vulnerable to TDG.

Effects of Total Dissolved Gas Supersaturation in Fish of Different Sizes and Species.

Two endemic fish in the upper Yangtze River, the Rock Carp (Procypris rabaudi) and Prenant's Schizothoracin (Schizothorax prenanti), were used as research objects in this study to assess the effects of total dissolved gas (TDG) supersaturation on fish of varying sizes. Fish were exposed to TDG-supersaturated water at the levels of 145, 140, 135, 130, and 125%. The results showed that fish swam slowly, responded clumsily, and then exhibited spiral swimming performance after a period of exposure to TDG-supersaturated water. Fish exhibited exophthalmos, body swelling, gill bleeding, and caudal fin bleeding when they died in the TDG-supersaturated water. With the increase in TDG supersaturation, the tolerance capacity of fish to supersaturated TDG significantly reduced. At high supersaturation, the difference in survival time between species was not significant, while fish with smaller sizes showed greater tolerance capacity. At low supersaturation, the tolerance capacity of fish was mainly affected by species, and the influence of size was relatively small. With the decrease in TDG supersaturation, the catalase (CAT) activity first increased and then decreased. Rock Carp displayed significantly less activity than Prenant's Schizothoracin on exposure to TDG-supersaturated water. At high supersaturation levels, the CAT activity of Prenant's Schizothoracin of small size was greater than that of large Prenant's Schizothoracin. In contrast, small Prenant's Schizothoracin showed less CAT activity at low TDG levels than did large individuals.

The Tolerance Characteristics of Resident Fish in the Upper Yangtze River under Varying Gas Supersaturation.

In circumstances where total dissolved gas (TDG) levels are variable, the peak TDG and duration are expected to be the dominant drivers of fish survival. Focusing on the peak TDG and duration in natural rivers, a laboratory experiment and field experiments in the upper Yangtze River were conducted with Prenant's Schizothoracin (Schizothorax prenanti), a rare species inhabiting the upper Yangtze River, to examine the tolerance characteristics of fish under varying gas supersaturation levels. The results of the field experiments showed that TDG supersaturation in natural rivers changed greatly during the flood period due to reservoir regulation. The survival of fish was affected by TDG levels, water depth and TDG fluctuation range. A high TDG level, and shallow compensatory water depth caused fish mortality in the field experiment to be higher in September than in July. The results of the laboratory experiment showed that fish tolerance was lower under fluctuating TDG supersaturation than under constant TDG supersaturation. The tolerance of fish to TDG supersaturation varied depending on peak TDG and duration. Under the fluctuation range of 115-125%, fish survival in the 6 h-6 h cycle was significantly different from that in the 8 h-8 h cycle. The fluctuation cycle did not affect fish survival at the fluctuation range of 110-130%. Intermittent lower TDG supersaturation does not significantly increase the tolerance of fish. This study revealed the tolerance characteristics of resident fish in the upper Yangtze River to TDG supersaturation, which provides a reference to the ecological operation of reservoirs and may contribute to the protection of aquatic organisms.