A touch too much – interactions of various species of freshwater fish and crustacea at a simulated fish protection screen

Fish protection screens are being increasingly applied to prevent fish entrainment at water diversions and intakes. Screen success is thought to depend upon selecting the most suitable mesh size and ensuring appropriate water velocities around the screen. It is generally hypothesised that if fish are smaller than the mesh size, they may become entrained through the screen or, if they are larger, become impinged upon the screen. Both situations can lead to injuries or mortality and need to be resolved for the fish protection screen to be effective. International screen specifications suggest that approach velocities should generally range between ≤ 0.1 and 0.3 m/s to minimise adverse fish interactions. These specifications are based on swimming performance of a small range of target fish species, mostly from North America. It is uncertain to what degree these specifications provide protection for a wider range of species. To determine suitable approach velocities for 17 freshwater fish species and two crustacean species, we simulated screen encounters at a 3 mm mesh wedge-wire screen panel in a specialised flume, under various approach velocities (ranging from 0 to 0.3 m/s). Fish and crustacean behaviour, including species-specific impingement susceptibility and entrainment across the range of approach velocities tested, were analysed. Impingement was effectively reduced at velocities ≤ 0.1 m/s for a wide range of fish across different life stages. Approach velocities that exceed 0.1 m/s were unsuitable for most tested fish species and resulted in impingement. At 0.3 m/s, impingement susceptibility and detachment ability varied across the range of fish and crustacean species tested. Species-specific behaviour such as volitional screen contacts or flow responses played a crucial role in screen encounters and should be considered when designing screens globally to protect aquatic biota.