Effects of gas supersaturation on migrating Atlantic salmon smolt (Salmo salar) in Evangervatnet

Total dissolved gas supersaturation (TDGS) downstream of power plants may cause harmful effects on the fauna in many rivers. The harmful effects of TDGS are known as gas bubble disease (GBD). TDGS has been proposed as one possible cause of mortality of migrating Atlantic salmon (Salmo salar L.) smolt in Evangervatnet, Norway. The present study investigated whether TDGS had negative effects on the survival of Atlantic salmon smolt downstream of the power plant in Evangervatnet. Four complementary sub-studies were conducted from April to June 2016: (1) Atlantic salmon held in surface cages (0-1.15 m depth) and submerged cages (1.15-2.5 m depth) upstream and downstream the power plant for 4 and 14 days. (2) Electrofishing and snorkeling observations of wild fish. (3) Capture of Atlantic salmon smolt in traps. (4) Towing of Atlantic salmon smolt across Evangervatnet. The fish were examined for GBD by visual observations by use of a stereo microscope. The surface caged smolt were also examined for histology pathology and blood cortisol. The smolt showed no mortality or signs of GBD during the field studies, suggesting that the smolt tolerated the TDGS levels in Evangervatnet. The TDGS levels at the cage locations ranged between 102.7 to 112.8% and medians between 105.6 to 107.5% (TDGS: lowest upstream the power plant and higher downstream the power plant). The density of salmon and trout (Salmo trutta) was similar at higher levels of TDGS close to the power plant as in the control area upstream, indicating that the fish distribution was not negatively affected by the power plant. The median cortisol levels in the smolt held in the cages were above the normal background levels at all sampling sites after both 4 days and 14 days of TDGS exposure, which indicates that the smolt were stressed. The high cortisol levels were found both at low and at higher levels of TDGS, suggesting that handling prior to blood sampling was the main contributor to the high cortisol levels. As a conclusion, TDGS values in Evangervatnet appeared to have no negative effects on the survival of Atlantic salmon smolt downstream of the power plant