Fish passage hydrodynamics: insights into overcoming migration challenges for small-bodied fish

Abstract The modification and utilization of rivers in regions where small-bodied diadromous fish are prevalent has largely occurred without fully understanding the migration behaviour of these species. As a result, existing in-stream structures often prevent or restrict migration. Current fish passage design guidance generally focuses on providing average hydrodynamic conditions within the range of known critical swimming velocities for target fish species. Considerable portions of discharge capacity must be sacrificed to achieve average cross-sectional water velocities that will allow passage of weak swimmers. Furthermore, because the hydrodynamic requirements for small-bodied species are poorly understood, successful passage is still not guaranteed even when average hydrodynamic design criteria are met. Ethohydraulic research is focused on how water flow influences fish behaviour and vice versa, by studying the interaction of fish with small-scale in-flow characteristics. We discuss how an ethohydraulic approach may improve fish passage design for small-bodied fish, such as țnanga/common galaxias (Galaxias maculatus), a widespread diadromous Southern Hemisphere species. The ethohydraulic approach is discussed in detail for culverts, a commonly found structure known to impede fish passage for many small-bodied species.