Fish behavior at the horizontal screen of a novel shaft hydropower plant

Preventing fish entrainment during their downstream passage at hydropower plants remains a major challenge in reducing the ecological impacts of hydropower production. We investigated fish behavior at the world's first innovative shaft hydropower plant with its novel screen concept, aiming at reducing fish entrainment due to the fully horizontal arrangement of the screen and low vertical suction effects toward the turbine. Based on ARIS sonar recordings, we assessed whether fish could move unhindered across the turbine intake area toward the bypass corridors at the sluice gate for safe downstream passage. For a range of species (Anguilla anguilla, Barbus barbus, Thymallus thymallus, Salmo trutta, and Hucho hucho) and operation modes (high/low turbine load), we assessed behavioral patterns such as screen avoidance, dwelling behavior, and search behavior at the screen. Contrary to the engineers' expectations, the innovative screen arrangement neither guided the fish away from the turbine intake to the bypass corridors nor prevented them from swimming vertically into the turbine shaft. Rather, fish freely moved near the screen and avoidance behavior was only rarely observed. Both the dwelling and active search behavior, which was particularly evident in eel, are directly linked to an increased risk of screen passage and subsequent turbine-related death or injuries. Our findings illustrate that consideration of fish behavior at turbine inlet structures is a crucial component which needs to be integrated with other variables such as fish mortality and injury patterns for a comprehensive evaluation and improvement of fish passage at hydropower plants.