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.

Determining the position of a fish passage facility entrance based on endemic fish swimming abilities and flow field.

Hydropower development can significantly mitigate climate change and reduce carbon emissions, but it can also have substantial negative impacts on river environments and fish biodiversity. Fish passage facilities are built to ensure sustainable hydropower development and the biodiversity of fish populations. The locations of the entrances to these facilities play a key role in their efficiency. This study presents a reliable approach that combines the swimming ability of fish and a numerical flow field simulation to determine the optimal location for a fish passage facility entrance. In this study, we used the Gujun Reservoir upstream of the Yangtze River as a case study. A field experiment was conducted, and the swimming abilities of eight endemic fish species in the upstream region of the Yangtze River were measured. Among the tested species, the fastest induced swimming speed (0.14 m/s) was achieved by Glyptothorax sinense, while the slowest critical swimming speed (0.30 m/s) was observed for Paracobitis potanini. We propose that the velocity near the fish passage facility entrance should be higher than the maximum induced swimming speed and lower than the minimum critical swimming speed, making the suitable range between 0.14 and 0.30 m/s. On this basis, velocity fields 500 m downstream of the dam of the Gujun Reservoir under 4 operating conditions with discharge flows of 5.7 m3/s, 23.3 m3/s, 32.5 m3/s, and 41.1 m3/s were calculated. The results showed that the flow field variation downstream of the dam was between 0.1 and 0.9 m/s. After comparing the suitable areas for the target species, the left bank at location 2 was recommended as the optimal location for the fish passage facility entrance in the Gujun Reservoir.

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.

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.

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.