Acute Effects Of Total Dissolved Gas Supersaturated Water On Different Juvenile Fish

Total dissolved gas(TDG)supersaturated water greatly threatens the survival of fish downstream of dams when food discharge of the high dam occurs.TDG supersaturated water has become an increasingly serious ecological risk in hydropower projects.The effects of TDG supersaturated water on fish have been studied abroad for a long time,and the results are relatively rich.However,the main research results abroad are about 125% TDG level caused by low-head flood discharge,and it cannot be directly applied to high dam projects in China.Most studies in China have focused on the impact of TDG on the survival of endemic benthic fish in the Yangtze River,and few studies have evaluated the survival of pelagic fish.Fish species of different water layers and sizes show different tolerances to TDG supersaturation.The Yangtze River is the most developed area of freshwater fisheries in China.The annual food discharge of the high dam is an important period for the growth and development of juvenile fish.Juvenile fish are very vulnerable to the TDG supersaturated water produced by food discharge of the high dam.The purpose of this study was to investigate the effects of TDG supersaturated water on juveniles of different water layers and sizes,and to further summarize the impact of TDG supersaturated water on fish.The data obtained in this study can provide basic data and reference basis for establishing protection measures of the Yangtze River,evaluating the impact of high dam project on aquatic ecosystem,and protecting aquatic ecological environment in the downstream of high dam.Carp(benthic fish)and silver carp(pelagic fish),the main economic fishes in the Yangtze River,were selected as the research objects to explore the tolerance of TDG levels water on different water layers fish.In addition,grass carp and bighead carp were selected as the research objects to explore the tolerance of different sizes juvenile fish to TDG supersaturated water.The main results were as follows:(1)Abnormal behaviors such as breathing rapidly,swimming up and down faster,floating head,were observed on carp and silver carp exposed to 125%~140% TDG water.The similar signs of gas bubble disease(GBD)that were frequently found in fish include haemorrhages of gills,abdominal swelling,bubbles on caudal fins,but the probability of GBD were different.(2)At 125%,130%,135% and 140% TDG levels,the median survival time of juvenile carp were 53.51 h,26.03 h,16.49 h and 11.74 h,respectively,and that of juvenile silver carp were 26.84 h,7.79 h,5.57 h and 3.63 h,respectively.The median survival time,survival condition and tolerance decreased with increasing TDG levels.The time of GBD,abnormal behavior and death on juvenile silver carp was earlier than that of juvenile carp,and the median survival time of juvenile silver carp was relatively short.It indicated that the survival condition of pelagic silver carp was weaker than that of benthic carp,and silver carp was vulnerable to TDG supersaturated water.(3)The prevalence rates of GBD in large juvenile grass carp and juvenile bighead carp were slightly higher than that in small juvenile grass carp and juvenile bighead carp.However,the signs of abdominal swelling,tail scale shedding and mouldy tail were only found in small juvenile grass carp and juvenile bighead carp.(4)Compared with the median survival time of small juvenile grass carp,the median survival time of large juvenile grass carp after exposure to 130,135 and 140% TDG were decreased by 45.53 h,35.38 h,and 33.75 h,respectively.The tolerance of TDG supersaturated water decreased with the increases of weight and length in grass carp.The tolerance of small juvenile grass carp was better than that of large juvenile grass carp.In addition,the tolerance of grass carp at the low TDG level(125%)was less affected by the size of the fish,but the tolerance of grass carp was significantly affected by the size of the fish in high TDG levels(130% ~ 140%).(5)At 130%,135%,and 140% TDG supersaturated water,the median survival time of small juvenile bighead carp were 24.14 h,18.24 h,7.73 h,and that of large juvenile bighead carp were 77.08 h,69.41 h,33.88 h.It indicated that with the increase of TDG level,the tolerance of bighead carp significantly decreased,and large juvenile bighead carp had stronger adaptability and tolerance in TDG supersaturated water.In addition,at 130% and 135% TDG water,large juvenile bighead carp survived partially,but all small juvenile bighead carp did not survive.The survival curves of small juvenile bighead carp and large juvenile bighead carp were significantly different under the same conditions,which further indicated that the tolerance of bighead carp to TDG supersaturated water was affected by the size of the fish.