Effect of total dissolved gas supersaturation on the tolerance of grass carp (Ctenopharyngodon idellus)

Abstract

Abstract Background Total dissolved gas (TDG) caused by the rapid outflow of water from dams may threaten the survival of fish. However, few studies have assessed the impact of fish size on their tolerance to TDG supersaturation in the Yangtze River. To investigate the influences of fish size on the TDG supersaturation tolerance of fish, juvenile grass carp (Ctenopharyngodon idellus) of two sizes were subjected to TDG-supersaturated water at high levels (125%, 130%, 135% and 140%). Furthermore, varying flood flows may cause different TDG levels during the flood season. Fish may be subjected to low, chronic exposure to TDG before peak flooding occurs. However, TDG tolerance in fish subjected to high levels of TDG-supersaturated water after chronic exposure is rare. To further investigate the tolerance of juvenile grass carp subjected to acute exposure after chronic TDG exposure, juvenile grass carp were exposed to high levels of TDG-supersaturated water (125–140%) after receiving 96 h of chronic exposure (115% TDG). Results In the single acute exposure and multiple exposures (acute exposure after chronic exposure), similar abnormal behaviours and symptoms of gas bubble disease (GBD) were observed in the juvenile grass carp subjected to the TDG-supersaturated water. No abnormal behaviour or mortality was observed in fish in the first chronic exposure of the multiple-exposure treatment. As the TDG level increased, the mortality of the large and small juvenile grass carp increased. The median lethal time (LT50) for the large juvenile grass carp was 36.55, 21.75 and 6.37 h at 130%, 135% and 140% TDG levels, respectively, while the LT50 value of the small juvenile grass carp was 88.13, 61.49 and 35.88 h at the same TDG levels, respectively. In addition, the LT50 value of juvenile grass carp during acute TDG exposure after chronic exposure was 26.22, 7.54 and 5.34 h at 130%, 135% and 140% TDG levels, respectively. Conclusion The tolerance of juvenile grass carp decreased with increasing TDG levels. The large juvenile grass carp had weaker tolerance to TDG-supersaturated water than the small juvenile grass carp. In addition, compared with juvenile grass carp subjected to single acute exposure, juvenile grass carp subjected to multiple exposures exhibited lower tolerance and were more vulnerable to the adverse effects of TDG.