Swimming performance of fish in TDG supersaturated water

High dams have brought significant benefits but have also led to reduced river connectivity and aquatic habitat fragmentation, negatively affecting fish activities such as migration, spawning, and foraging. Additionally, total dissolved gas (TDG) supersaturation caused by dam discharge presents a further ecological challenge, making fish highly susceptible to gas bubble trauma (GBT), which can disrupt normal behavior and even result in mortality. Consequently, fish are subjected to the dual threats of dam barriers and TDG supersaturation. While fish passage facilities can partially mitigate the barrier effect, the swimming performance of fish in TDG supersaturation exposure is critical for successful passage. This study employed a swimming tunnel respirometer (Loligo Systems SW10150, Denmark) to investigate the critical swimming speed (U_crit) and endurance of juvenile Myxocyprinus asiaticus and Procypris rabaudi in TDG supersaturated water. The results from one-way ANOVA revealed that the U_crit of M. asiaticus significantly decreased to 76% and 60% of the control group (12.31 BL/s) at 140% and 150% TDG levels, respectively. P. rabaudi showed even weaker tolerance to TDG supersaturation exposure, with significant reductions in U_crit at 130%, 140%, and 150% TDG levels, corresponding to 81%, 71%, and 51% of the control group (13.63 BL/s), respectively. Both species were able to swim for at least 200 minutes at velocities of 0.6 - 0.8 U_crit at TDG levels below 130% and showed significantly reduced endurance at TDG levels of 140% or higher. A significant decline in sustained swimming distance was observed at 130% or higher TDG levels. The swimming distance of fish decreased by at least 12% compared to the control group, with the reduction reaching 86% at 150% TDG level. It is indicated that a TDG level of 130% represents a critical threshold for fish survival. This study provides valuable insights into the behavioral responses of fish to TDG supersaturation exposure. The finding is crucial for understanding the impacts of TDG supersaturation on fish and for informing strategies aimed at mitigating the ecological risk associated with dam operations. Furthermore, this study offers vital support for the development of effective fish passage solutions.