The effects of gas supersaturation on the density of brown trout in two Norwegian rivers

Abstract

Rivers are under pressure from different stressors, one of which is gas supersaturation. Gas supersaturation occurs when the total dissolved gases (TDG) in the water exceed 100%. Unregulated rivers normally hover around 100% TDG. Hydropower plants can create supersaturated levels higher than 200%, and waves of high TDG can reach as far as 30 km downstream from the hydropower outlets. Gas supersaturation can be toxic to animals with aquatic respiration, such as fish, benthic invertebrates and zooplankton, leading to physiological stress and mortality. In this study we have investigated how gas supersaturation affects the density of brown trout, Atlantic salmon and minnow in Modalen River and Otra River. Electrofishing was done to estimate the density, and TDG was measured using automated sensors. Downstream from the Brokke outlet in Otra River, the highest TDG value in 2023 was 199%; downstream from Hellendsfoss in Modalen River, the highest was 121% in 2023. We found that the density followed a pattern in sampling sites downstream of the Brokke hydropower outlet: the closer to the outlet, the lower the density. This pattern was not present in the Modalen River. Our results indicate that high spikes of TDG do negatively affect the density of brown trout and minnows. We recommend setting a maximum allowable limit of 110% TDG at the median depth downstream from hydropower plants and requiring the hydropower plants to monitor their TDG values downstream of their outlets.