Tolerance and avoidance mechanisms of the rare and endemic fish of the upper Yangtze River to total dissolved gas supersaturation by hydropower stations

Several superhigh dams (greater than 200 m in height) and many high dams have been built in the upper Yangtze River basin in recent years, and these dams have made total dissolved gas (TDG) supersaturation a serious environmental problem. A few studies have examined the tolerance and avoidance characteristics of rare and endemic fish in TDG‐supersaturated water in the upper Yangtze River over the past 10 years. These studies focused on specific species and specific sizes and did not identify a regular pattern that can be applied to all resident fish in the upper Yangtze River. However, elucidating this type of pattern is crucial for fishery management and dam operations in the upper Yangtze River. Data on the median lethal time (LT₅₀), horizontal avoidance percentage, and vertical water depth of three rare and endemic species in the upper Yangtze River from previous studies were used in the current work. An exponential relationship was found between LT₅₀ of fish and TDG supersaturation. The avoidance percentage of fish has a linear relationship with TDG supersaturation. Fish in the upper Yangtze River can use depth compensation to avoid the threat of TDG when the saturation is 130% or above but rarely avoid the threat of TDG when the saturation is less than 125%. We also described the tolerance and avoidance characteristics of fish in a TDG‐supersaturated river downstream from a super‐high dam. When the dam discharged a two‐year flood, the LT₅₀ values of fish downstream ranged from 9.1 to 27.5 hr regardless of depth compensation. The avoidance percentage of resident fish ranged from 8.4 to 44.8%. The resident fish swam to a water depth of 2.43–3.33 m to avoid the threat of TDG.