Bigger than expected: Species- and size-specific passage of fish through hydropower screens

Abstract

Mitigating the adverse ecological impacts on stream ecosystems caused by hydropower expansion is a major challenge. To prevent fish from turbine entrainment and to reduce injury and mortality risk, physical barriers such as fine screens with horizontally or vertically oriented bars are frequently installed at turbine inlets. In this study, the species- and size-dependent protection from turbine entrainment of different types of fish protection screens (FPSs) were investigated at five hydropower sites by a net-based monitoring of downstream moving wild fish and hatchery-reared test fish during different seasons (15,223 individuals from 40 species). Across different screen types and fish species, considerably larger individuals were able to pass the FPSs than what would have been expected from common models estimating the physical passability of these mechanical barriers. The examined FPSs with 15 mm and 20 mm bar spacing could be passed by adult barbel (Barbus barbus L.), brown trout (Salmo trutta L.) and European perch (Perca fluviatilis L.) up to a total length (TL) of 32 cm (15 mm FPSs) and 34 cm (20 mm FPSs), respectively. In addition, the 20 mm FPSs could be passed by Danube salmon (Hucho hucho L.) up to 39 cm TL. Consequently, thresholds from modelling and rules of thumb for estimating maximum TLs capable of passing 15 mm and 20 mm FPSs were exceeded by up to 135% for these species. The results of this study suggest that fish species other than eel can also squeeze through physical barriers narrower than body dimensions. No physical fish protection was realised by the investigated 15 mm and 20 mm FPSs for many small-bodied species such as bullhead (Cottus gobio L.), gudgeon (Gobio gobio L.) and spirlin (Alburnoides bipunctatus Bloch) with maximum TLs smaller than 20 cm. This also holds true for juveniles and sub-adults of larger species, which can pass these physical barriers. Since a large part of the downstream moving fish generally consists of small species or small individuals, these fish sizes must be given greater consideration in physical fish protection concepts at hydropower plants.