Does tunnel wash water have an impact on vital rates and displacement of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in river Årungselva

Earlier studies on runoff water from roads and tunnels have demonstrated that such water hold toxic substances that can be harmful to aquatic biota. River Årungselva, which flows into the inner part of Bunnefjorden close to Oslo, Norway, receives discharges from the Vassum sedimentation pond, which in turn receives tunnel wash water irregularly from three tunnels: Nordby, Smiehagen, and Vassum. The main function of the sedimentation pond is to remove contaminants from the tunnel wash water through sedimentation processes before it flows into Årungselva. Earlier studies have revealed that the growth and survival of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) from Årungselva are lower below the outlet point of the Vassum sedimentation pond than above it. These studies have indicated that salmonids below the outlet point may be adversely affected by runoff from the sedimentation pond due to insufficient treatment of various contaminants. This thesis is a follow-up study to those previous studies. The aim of this study was to investigate whether there were differences in the concentration of contaminants, growth, survival, and movement between salmonids located above and below the outlet point of the Vassum sedimentation pond to investigate the potential effects of tunnel washing. These surveys were investigated through electric fishing, PIT-tagging, and recapture (detection) of salmonids, and water samples. Electric fishing and PIT-tagging was performed once during September 2020 with three capture rounds on five different stations, two above and three below the outlet point. PIT-tagged individuals were detected in two rounds with a mobile antenna, one round in February and one round in March 2021. The water samples were taken above and below the outlet point, and before and during full washing of the tunnels Nordby, Smiehagen and Vassum in spring 2021. A total of 405 individuals of Atlantic salmon and 31 individuals of brown trout were caught during one round of electrofishing in September 2020. Of these, 242 Atlantic salmon and 13 brown trout were PIT-tagged; 183 individuals were detected once and 122 were detected twice on the mobile antenna in February and March, respectively. Lower size-specific monthly survival was found for individuals below the outlet point compared with those above it. In addition, a higher degree of movement was found for individuals just below than individuals above and far below the outlet point. These results were found despite lower densities just below the outlet point. Water samples above and below the outlet point did not exhibit significant differences during full washings of the tunnels, although some heavy metals below the outlet point had marginally higher values in some samples. These results support findings from earlier studies that juvenile salmonids below the outlet point experience poorer conditions compared with individuals above it. Furthermore, this study did not adequately support that those poorer conditions found for salmonids below the outlet point were due to discharges from the Vassum sedimentation pond during tunnel washing. This was due to great variation found for various habitat variables, with sections just below the outlet point having poorer habitat conditions for Atlantic salmon than above it and further downstream