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Flood discharge containing excessive suspended sediments (SS) can result in total dissolved gas (TDG) supersaturation downstream of dams. It has been confirmed that high TDG levels and excessive SS cause serious threats to fish survival. To explore the effects of TDG supersaturation on the hatchability of Chinese sucker (Myxocyprinus asiaticus Bleeker) eggs, eggs were exposed to 100%, 125%, 130% and 135% TDG supersaturated water. The results showed that the hatching rates were 78.25%, 75.35%, 71.30% and 67.50% at the 100%, 125%, 130% and 135% TDG respectively. The hatching rate decreased with increasing TDG levels. Furthermore, acute lethality experiments were carried out to investigate the interaction of TDG supersaturation and SS on Chinese sucker. Half-year-old juveniles were exposed to TDG supersaturated water with SS concentrations of 0, 200, 600 and 1,000 mg/L, and the TDG levels were 125%, 130%, 135% and 140% respectively. The fish exhibited abnormal behaviours and signs of gas bubble disease. Mortality increased with increasing TDG levels and SS concentrations. The Chinese suckers only survived for 2.5 hr at 140% TDG and 1,000 mg/L SS. Moreover, an increase in SS in TDG supersaturated water could cause a decrease in the median lethal time (LT 50 ). When the TDG level reached 140%, the LT 50 declines significantly at 600 and 1,000 mg/L SS. The interaction of TDG supersaturation and SS was significant at LT 50 (F = 22.81, df = 9, p < 0.001). The results indicate that the combined effect of TDG and SS could be great on Chinese sucker. K E Y W O R D SChinese sucker (Myxocyprinus asiaticus Bleeker), hatching rate, median lethal time (LT 50 ), suspended sediment, Total dissolved gas supersaturation

High total dissolved gas (TDG) levels and excessive suspended sediment (SS) concentrations pose serious threats to fish survival during flood season. However, little information is available on the effects of TDG supersaturation with varying SS concentrations on fish. In this study, laboratory experiments were performed to investigate the effects of TDG supersaturation with varying SS concentrations on five-month-old river sturgeons (Acipenser dabryanus). The test fish were exposed to combinations of SS concentrations (0, 200, 600 and 1,000 mg/L) and TDG levels (125, 130, 135 and 140%), and their mortality and median lethal time (LT50) were quantified. The fish showed abnormal behaviors (e.g., quick breathing, fast swimming and an agitated escape response) and symptoms of gas bubble disease (GBD). SS increased the mortality of river sturgeon exposed to TDG supersaturation. Furthermore, the LT50 values at 125% TDG were 4.47, 3.11, 3.07 and 2.68 h for the different SS concentrations (0, 200, 600 and 1,000 mg/L, respectively), representing a significant decrease in LT50 with increasing SS. However, at higher TDG levels (130–140%), there was no significant increase in LT50 with increasing SS. Therefore, river sturgeon showed weak tolerance of TDG-supersaturated water with SS.

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