Fish Responses to Total Dissolved Gas Supersaturation: From the Perspective of Historical Experimental Data in the Upper Yangtze River

During discharge from high dams, the high-velocity water flows mix with air, causing downstream total dissolved gas (TDG) supersaturation, threatening fish survival. However, a systematic understanding on fish response patterns under the coupled influence of multiple environmental and biological factors remains limited. This study synthesized historical experimental data to explore fish tolerance and avoidance in response to TDG supersaturation in the upper Yangtze River. Economic fish species demonstrated overall stronger tolerance than the rare fish species under all TDG levels. Significant interspecific differences in economic fish tolerance were observed with increasing TDG levels. Aristichthys nobilis demonstrated stronger tolerance than Hypophthalmichthys molitrix. For the rare fish, Procypris rabaudi showed the weakest tolerance. Exposure time, TDG level, species and water temperature were identified as the primary factors influencing fish mortality. Based on these, mortality prediction formulas for rare and economic fish were established, achieving an R2 over 0.59. The avoidance ability of fish increased significantly with increasing TDG levels and reached 50% at 130% TDG. Significant interspecific differences in fish avoidance were observed at a 140% TDG level or above, and Schizothorax prenanti showed the weakest avoidance ability. TDG level was the dominant factor influencing fish avoidance ability, followed by body length. Accordingly, a prediction formula for fish avoidance rates was proposed with an R2 of 0.88. The findings offer critical insights for ecological risk management and the development of fish conservation strategies downstream of high dams.