Effects Of TDG Supersaturated Water On The Tolerance Of Juvenile Yellow Catfish(Pelteobagrus Fulvidraco)

Total dissolved gas?TDG?supersaturation generated by flood discharge from high dams leads to gas bubble disease?GBD?and mortality in fish downstream of dams,which seriously affected the function and stability of aquatic ecosystem.In recent years,many high dams have been constructed in China,and the effect of TDG supersaturation on fish downstream of high dams is becoming more and more serious.Therefore,it is an urgent scientific problem to mitigate the impact of TDG supersaturation caused by the flood discharge on fish.Many studies have focused on the acute impact of TDG on the GBD signs and tolerance of fish.However,the TDG level always fluctuates with varied discharge time and flood discharge flow during the discharge period.Fish may alternatingly experience intermittent TDG exposure to equilibrated water and TDG supersaturated water.Therefore,it is more suitable for engineering practice to carry out intermittent TDG exposure research on fish.The results may provide valuable data for formulating the scheme of reservoir operation and contribute to the preservation of biodiversity in the Yangtze River basin.The yellow catfish?Pelteobagrus fulvidraco?,an important commercial fish,usually resides in shallows in the middle and lower Yangtze River basin.According to the previous studies,juvenile fish are vulnerable to TDG supersaturation and show a weak tolerance.In this study,the juvenile yellow catfish were choosed for the continuous acute lethality experiment and intermittent exposure experiment.The aim of this study was to investigate the tolerance of yellow catfish to TDG supersaturation and to further determine the effect of intermittent TDG exposure on the survival of the species under different exposure-recovery conditions.The main contents and results are as follows:?1?Five TDG saturation levels?100%,125%,130%,135%and 140%?were set in the continuous acute lethality experiment.100%TDG was the control group which was provided with clean water.Through the experiment,the following results were got:a)obvious abnormal behaviours?e.g.,running up and down motion,side swimming?and GBD signs?e.g.,abdominal distension,haemorrhages in the gills and fins?were observed in the TDG treatment groups.b)Different levels of death were found in each TDG supersaturation group.The damage of yellow catfish increased with the increase of TDG supersaturation.When the TDG level was 140%,the mortality of yellow catfish reached 100%.At the end of the experiment,the survival probabilities of the juvenile yellow catfish were 5%,15%and 25%at135%,130%and 125%TDG,respectively.Compared with the high TDG levels??130%?,Juvenile yellow catfish showed a greater tolerance to the low TDG supersaturated water?125%?.c)The median lethal time(LT₅₀)and the tolerance of yellow catfish in TDG supersaturated water decreased with the increasing TDG supersaturation.d)During the whole experiment,abnormal behaviour,GBD signs and death were not found in the control group.?2?In the intermittent exposure experiment,the same TDG levels?100%,125%,130%,135%and 140%?used in the above continuous acute exposure experiment were set.100%TDG was the control group.Exposure time?ET?of 3h and 6h and recovery time?RT?of 3h,6h and 9h with four TDG levels were combined to get 24 different exposure-recovery conditions.Through the experiment,the following results were got:a)abnormal behaviours and GBD signs of fish during the intermittent exposure experiment were similar to those observed in the continuous acute lethality experiment.b)In condition 3-3,the mortalities of juvenile yellow catfish were 45%,75%,95%and 100%at 125%,130%,135%and 140%TDG,respectively,while the mortalities were 10%,70%,80%and 85%at the above TDG levels in condition 3-9.This indicated that the mortality increased with the increasing TDG supersaturation,and an increasing recovery time?from 3h to 9h?caused decreased mortalities in the juvenile yellow catfish.A long recovery time?9h?could greatly increase the survival time of the species.However,in condition 6-3,the mortalities of juvenile yellow catfish were65%,85%,95%and 100%at 125%,130%,135%and 140%TDG,respectively.This indicated that an increase in TDG exposure time?from 3h to 6h?accelerated the death of fish at the same recovery time.d)Thoughout the experiment,the experimental fish in the clear water control group survived well without any abnormal behaviour.?3?Compared with the continuous acute lethality experiment,the lower probability of GBD signs in the intermittent exposure experiment was found.This indicated that the juveniles were less affected by intermittent TDG exposure.When TDG level is highest?140%?,the juvenile yellow catfish in the intermittent TDG exposure?ET=3h,RT=3h?had a longer survival time?69h?than that in the continuous acute exposure experiment?38.27h?.When TDG level is lowest?125%?,55%of juvenile yellow catfish still survive at 125%TDG for 96 h under intermittent exposure?ET=3h,RT=3h?,and the survival probability was higher compared with continuous acute exposure?25%?.Moreover,the LT₅₀ values of juvenile yellow catfish in the acute exposure were 32.64h,16.36h and 8.02h at 130%,135%and 140%TDG,respectively.The LT₅₀ values of juveniles in the intermittent exposure?ET=3h,RT=3h?were 58.49h,33.96h and 32.12h at the above TDG levels.Therefore,the survival time,LT₅₀values and survival probability of juvenile yellow catfish were increased in the intermittent exposure.The tolerance to TDG supersaturation was also improved.This indicated that the juveniles that experienced intermittent exposure may have recovered in the recovery period,and their survival ability was improved in the TDG supersaturated water.