Posted: Feb 7, 2019

Photo: Nicolas Rakotopare

Many Australian freshwater fish species are in decline, with some populations in the Murray-Darling Basin estimated to be 10% of what they were prior to European settlement. Culverts, dams, and weirs play a role in dwindling fish numbers by fragmenting fish habitat and diverting or blockading flow, impeding a fish’s ability to migrate and evade predators. Culverts, especially, force water into a confined space, increasing flow velocity and presenting a barrier for fish that cannot swim fast or for long in currents. Small-bodied and/or juvenile fish are most at risk. Dr Jabin Watson, from the University of Queensland, and his team with the Threatened Species Recovery Hub studied the swimming ability of native fish in a biohydrodynamics laboratory and found that traditional ‘remediation’ methods designed to alleviate flow in culverts, such as baffles, generate turbulence that can disorientate fish. By taking into account a property of fluid mechanics called the ‘boundary layer’ – a thin layer of slower water generated by a fluid moving across a solid surface – Watson and colleagues discovered a novel way to help juvenile fish navigate culverts and pipes. When a longitudinal beam is added along a culvert wall, the additional surface close to the culvert corner merges the boundary layers from the three surfaces, resulting in a reduced-velocity channel big enough for small fish to swim through. This breakthrough has successfully enabled the passage of golden perch (Macquaria ambigua), juvenile Murray cod (Maccullochella peelii, above), Empire gudgen (Hypseleotris compressa), freshwater catfish (Tandanus tandanus), Agassiz’s glassfish (Ambassis agassizii) and Pacific blue eye (Pseudomugil signifier) in laboratory trials.

Watson JR. 2018. Ecological Engineering 122. DOI: 10.1016/j.ecoleng.2018.08.008