Effect of Total Dissolved Gas Supersaturation on the Survival of Bighead Carp (Hypophthalmichthys Nobilis)

SIMPLE SUMMARY: Total dissolved gas (TDG) supersaturation severely threatens the survival of fish downstream of hydraulic structures in the Yangtze River due to flood discharge. However, existing findings mostly demonstrate the effect of TDG on the tolerance of benthic fish in China. Few studies have attached great importance to investigating the impact of TDG supersaturation on pelagic fish. Furthermore, varied flow of flood can lead to varied TDG levels in the spill season. Fish resided in the downstream of high dams may experience a period of chronic exposure at a low TDG level before the arrival of flood peak. However, rare achievement has reported that the survival situation of fish exposed to TDG supersaturated water with a high level after receiving chronic exposure. This study investigates the tolerance of bighead carp (Hypophthalmichthys nobilis) (pelagic fish) to TDG supersaturation and determine the effect of a high level of TDG supersaturation on its survival after chronic TDG exposure. The results showed that TDG led to apparent abnormal behaviours and gas bubble disease signs in bighead carp. The tolerance of bighead carp declined with increasing TDG levels. Compared with the large juvenile bighead carp, the small juvenile bighead carp are more sensitive to TDG. Furthermore, the results indicated that bighead carp subjected to acute exposure after long term chronic exposure are more vulnerable to TDG. The data from this study may provide valuable information related to protecting aquatic organism diversity and establishing water quality standards related to TDG supersaturation. ABSTRACT: To assess the effect of TDG on the survival of different sizes of pelagic fish, bighead carp (Hypophthalmichthys nobilis) were subjected to TDG supersaturated water at levels of 125, 130, 135, and 140%. The results showed that apparent abnormal behaviours and symptoms of gas bubble disease (GBD) were observed in bighead carp. The survival probability of large and small juvenile bighead carp declined with increasing TDG levels. The median survival time (ST(50)) values of large juvenile bighead carp were 74.97 and 31.90 h at 130% and 140% TDG, respectively. While the ST(50) of small fish were 22.40 and 6.72 h at the same TDG levels. In comparison to the large juvenile bighead carp, the small juvenile bighead carp showed weaker tolerance to TDG supersaturated water. Furthermore, acute lethality experiments after chronic exposure to TDG were initiated to further investigate the effect of TDG on bighead carp. The juveniles were first subjected to 115% TDG supersaturated water for 96 h. After chronic exposure, live fish were immediately transferred to TDG supersaturated water at levels of 125, 130, 135, and 140%. The results demonstrated that no fish died under chronic exposure and few fish exhibited slight GBD symptoms. The ST(50) values for bighead carp subjected to acute exposure after chronic exposure were 61.23 and 23.50 h at 130 and 140%, respectively. Compared with the bighead carp subjected to acute exposure, bighead carp subjected to multiple exposures were more vulnerable to TDG.