Enhancing fish passage over large on-stream dams in south-western Australia: a case study

Instream barriers are known to have major negative impacts on aquatic ecosystems; particularly on migratory fishes. These impacts include exclusion from critical habitats (particularly spawning habitats), reduced colonisation, genetic fragmentation, and increased rates of density dependent mortality below barriers. Construction of fishways can overcome many of the impacts of barriers on migratory fishes by providing passage over, through, or around artificial barriers. In southwestern Australia, instream barriers are one of several major stressors on highly endemic (82%) freshwater fishes; many of which are potamodromous and migrate to spawn during the seasonal high flow period. Moreover, climate change has made the allocation of surface water more challenging due to a severe (~50%) reduction in surface flow over the past ~40 years. This study describes the design and construction of the largest fishway system built to date in this region and tests its functionality. The rock-ramp fishway system was located on Rushy Creek (an ephemeral tributary of the Blackwood River) and included a bypass and spillway fishway with an overall lift of 4.5 m. In spring 2010 and 2011, three of the eight native fishes (Western Minnow (Galaxias occidentalis), Western Pygmy Perch (Nannoperca vittata), and the Blue-spot Goby (Pseudogobius olorum)) in Rushy Creek were shown to pass upstream and downstream on the fishway system. Higher and more sustained flows in 2011 likely resulted in greater upstream fish passage in that year compared with 2010; highlighting the flow dependence of successful fish passage through fishways, which will have implications in terms of their functionality in drying climatic regions both in terms of changes in migration cues and fishway passage success. However, hydrological conditions during peak flow in both years also probably exceeded the swimming performance of the Western Pygmy Perch thereby preventing it negotiating the system during the early part of its spawning period, as opposed to the Western Minnow and the Blue-spot Goby both of which successfully negotiated the system during August and September. The findings highlight the importance of understanding species life-histories and swimming abilities, and have implications for future planning and design of fishways in this region to ensure they are appropriate for fish passage under future flow scenarios