| Summary: | Total dissolved gas (TDG) supersaturation, which can be caused by flood discharge, results in gas bubble disease (GBD) in fish and threatens their survival downstream of dams. TDG supersaturation has become a serious environmental problem in the Yangtze River. Few studies have evaluated the effect of TDG supersaturation on fish in natural rivers during periods of flood discharge. To estimate fish tolerance to TDG supersaturation under natural conditions, juvenile <i>Myxocyprinus asiaticus</i> and juvenile <i>Procypris rabaudi</i> were exposed to TDG-supersaturated water for 96 h at various depths (0–0.3 m, 0.3–1.3 m, 1.3–2.3 m and 0–2.3 m) during periods of flood discharge of Dagangshan hydropower station. The results showed that juvenile <i>Procypris rabaudi</i> and juvenile <i>Myxocyprinus asiaticus</i> exhibited obvious GBD signs. An increase in exposure time decreased survival probability of the two species. Deeper water depths can increase the tolerance of juvenile <i>Procypris rabaudi</i> to TDG supersaturation in natural rivers during periods of flood discharge while it cannot improve the survival of juvenile <i>Myxocyprinus asiaticus</i>. Compared with juvenile <i>Myxocyprinus asiaticus</i>, juvenile <i>Procypris rabaudi</i> showed weaker tolerance of TDG supersaturation in shallow water, and juvenile <i>Procypris rabaudi</i> were more vulnerable to TDG supersaturation than juvenile <i>Myxocyprinus asiaticus</i> even if the TDG level (116%) was low.
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