Influence of Channel Regulating Structures on the Transportation and Dissipation of Supersaturated Total Dissolved Gas

Bubble dissolution during the flood discharge creates high total dissolved gas (TDG) concentration zones downstream of the dams. The dissipation of supersaturated TDG is a very slow process. Thus, the elevated TDG may remain through the water body for hundreds of kilometers downstream and lead to gas bubble disease (GBD) and even mortality in fish. To improve the navigation conditions of waterways, dikes (i.e., a solid structure) of varied sizes and shapes are commonly constructed. However, this would affect the dissipation and transportation of the supersaturated TDG. It would significantly change the turbulence intensity and hydropressure of the flow, which dominates the dissipation of TDG. Therefore, TDG distribution in the waterway differs from that in the natural river. In this study, a numerical simulation of the TDG at the Yangtze River’s upper reaches (one of the inland waterways in China) was conducted with the establishment of a two-dimensional TDG dissipation model. The effect of the dikes’ size and shape was analyzed to assess the influence of the regulation structures on the dissipation and transportation of the supersaturated TDG. Meanwhile, simulation in the study area with the natural topography was also set as blank control. Based on that, impact evaluation of TDG supersaturation on fish under different simulation scenarios was made. This study can provide a scientific basis for reducing the adverse effect of supersaturated TDG in fish and the construction of ecological waterway therefore.

Fish die-off in river and reservoir: A review on anoxia and gas supersaturation

Albeit massive fish mortality has an extraordinary visual impact and is certainly a fatality, we still have rudimentary understanding on how addressing this problem in the Neotropical region. The processes that lead to fish die-off events are complex and sometimes ephemeral, which can lead to incorrect diagnosis. In this review, we discuss these events in Neotropical freshwaters, both in areas impacted by dams and natural environment, with a focus on deaths mediated by the lack of oxygen (anoxia) or the excess of dissolved gases (gas supersaturation). We examine the available knowledge about the related mechanisms, lethal thresholds for dissolved oxygen (DO) and total dissolved gases (TDG) for fish, and the sequelae of gas bubble disease (GBD). An assessment of the main mortality events in Brazil in the last 10 years is also presented, as well as the best practices for monitoring, prevention, and mitigation. Finally, it is concluded that the proliferation of hydroelectric plants in the Neotropical region might contribute to the expansion of these events and, consequently, increasing of impacts on fish conservation. We consider urgent the inclusion of this topic in the licensing processes for new hydropower projects by the environmental agencies.

Acute effects of gas supersaturation on Atlantic salmon smolt in two Norwegian rivers

Total dissolved gas (TDG) supersaturation downstream of hydropower plants may cause gas bubble disease (GBD) and harmful effects in fish. Little is known about tolerance levels of TDG supersaturation on Atlantic salmon (Salmo salar Linnaeus, 1758) in natural rivers. The present study investigated the effects of TDG supersaturation on the survival of Atlantic salmon smolts at two field sites in Norway. Here, we kept smolts in cages at increasing distances from hydropower plants known to cause TDG supersaturation and at control sites. We recorded fish mortality and examined for GBD using a stereo microscope. Mortality and symptoms of GBD commenced in fish exposed to an average of 108.3% TDG (maximum 111.0%, water depth 0.55 m) for 2 days. Significant differences in time before mortality at the control sites and test sites commenced at 110.2% TDG (maximum 111.8%) for 3 days. The study indicates that Atlantic salmon may be more vulnerable to TDG supersaturation than Pacific salmonids, which are considered at risk when the TDG is above 110%. In addition, the study provides important data to link effects caused by TDG in the laboratory and in the field.

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

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 (ST50) values of large juvenile bighead carp were 74.97 and 31.90 h at 130% and 140% TDG, respectively. While the ST50 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 ST50 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.

PROBLEMS REPORTED IN THE MANAGEMENT OF FARMED Arapaima gigas IN THE PERUVIAN AND BRAZILIAN AMAZO

The present study reports three cases of problems reported on farmed Arapaima gigas in the Peruvian and Brazilian Amazon (1st case: Jaundice disease in farmed Arapaima gigas (Schinz, 1822) in Pará, Brazil; 2nd case: Gas bubble disease in farmed Arapaima gigas from the Peruvian amazon and 3rd case: Mortalities caused by fights between sexually mature females of Arapaima gigas. All cases were reported during field expeditions to fish farms. The first case was reported in Para, Brazil and describes a jaundice disease caused by the consumption of contaminated food by micotoxins. The second case was reported in Iquitos, Peru and describes a “gas bubble” disease on young A. gigas caused by inadequate management and lack of acclimation when translated from a fish pond to glass aquaria and wooden boxes with shallow water. The third case is reported in Manaus, Brazil and describes a storage experiment where it was demonstrated that breeding tanks with more than one female results in fights against females and consequently the death of some specimens.

Lethal Effect of Total Dissolved Gas-Supersaturated Water with Suspended Sediment on River Sturgeon (Acipenser dabryanus)

High total dissolved gas (TDG) levels and excessive suspended sediment (SS) concentrations pose serious threats to fish survival during flood season. However, little information is available on the effects of TDG supersaturation with varying SS concentrations on fish. In this study, laboratory experiments were performed to investigate the effects of TDG supersaturation with varying SS concentrations on five-month-old river sturgeons (Acipenser dabryanus). The test fish were exposed to combinations of SS concentrations (0, 200, 600 and 1,000 mg/L) and TDG levels (125, 130, 135 and 140%), and their mortality and median lethal time (LT50) were quantified. The fish showed abnormal behaviors (e.g., quick breathing, fast swimming and an agitated escape response) and symptoms of gas bubble disease (GBD). SS increased the mortality of river sturgeon exposed to TDG supersaturation. Furthermore, the LT50 values at 125% TDG were 4.47, 3.11, 3.07 and 2.68 h for the different SS concentrations (0, 200, 600 and 1,000 mg/L, respectively), representing a significant decrease in LT50 with increasing SS. However, at higher TDG levels (130–140%), there was no significant increase in LT50 with increasing SS. Therefore, river sturgeon showed weak tolerance of TDG-supersaturated water with SS.

Gas bubble disease in Nile Tilapia and Hybrid Red Tilapia (Cichlidae, Oreochromis spp.) under culture conditions

The supersaturated gases in water can cause gas bubble disease in fish and this can result in death. At the present study a case of gas bubble disease resulted from the gas supersaturation were investigated in Nile tilapia (Oreochromis niloticus) and hybrid red tilapia (Oreochromis sp.). Macroscopic and histological findings were obtained from the culture conditions to find more knowledge on this disease which is frequently seen in fish farming. Study was conducted as two steps in summer (August) and winter (December) months with experimentally cultured fish. According to clinical and histological examinations gas bubbles were found in the dorsal, pectoral and caudal fin, and petechial bleeding were found on pectoral, dorsal, caudal fins and the gill cover. The necropsy were revealed that the stomach and intestines were filled with gas bubbles and the gills were hemorrhagic. According to the histological findings hyperplasia and edema in the gills and hemorrhage in the liver and spleen were found, and gastrointestinal mucosa destruction was observed. The clinical findings were similar for both fishes except for a few special cases and it was observed that the effects of supersaturation were severe for both fish species.