Abstract
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.
Highlights
Both water pressure and temperature are often affected by river regulation and both these factors affect total dissolved gas (TDG)
In Atlantic salmon hatcheries, production problems may commence from 104 to 105% TDG (Marking, 1987). These results suggest that Atlantic salmon is sensitive to TDG supersaturation among salmonids
At Lake Evangervatnet, the average TDG supersaturation was relatively similar among the test sites (Fig. 4; Table 1)
Summary
Both water pressure and temperature are often affected by river regulation and both these factors affect total dissolved gas (TDG). Gas supersaturation can occur naturally in rivers, for example during rapid solar heating, algal blooms and downstream from waterfalls plunging into deep pools (Marking, 1987). It can arise from human activity in at least two situations: (1) When air is entrained into the penstock system of hydropower plants (Heggberget, 1984; Pulg et al, 2016, Pulg et al, 2020) and (2) when air is entrained in plunge pools below dams (Ebel, 1969; Backman & Evans, 2002; Weitkamp, 2008; Cao et al, 2019)
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