Optimal Discharge from a Hydropower Plant for Fish Reproduction: An Aquatic Habitat Model

Abstract: Hydropower is the dominant source of electricity in Sweden, constituting 45% of the national supply. While pivotal for decarbonization and climate goals, its environmental impact on freshwater biodiversity is concerning. In this case study of a hydropower plant in Helge River, Sweden, the aim was to find a flow suitable for trout (Salmo trutta) and salmon (Salmo salar) reproduction. A terrain model was constructed with bathymetric data gathered in a field study. Then, seven flow scenarios were simulated in 2D space in this order: 0.5, 1, 2, 3, 4, 5 and 6 m3/s. The resulting hydraulic maps were classified in a rule-based habitat model based on preference curves for salmon and trout in Finnish and Norwegian rivers, and the Weighted Usable Area (WUA) was calculated. The habitat model suggests that more flow is better, as WUA increased all the way up to the highest flow scenario of 6 m3/s. The greatest benefit to habitat area by increasing flow was observed up to 2 m3/s, and slowing after, particularly for ‘good’ quality habitat. In conclusion, there is great potential for improvements to fish conditions with the implementation of a minimum discharge requirement in future operational permits for the hydropower plant.