In the Murray–Darling Basin, river regulation and extractive water use have substantially altered flow regimes, including a significant reduction in end-of-system discharge. In the 1930s, in response to declining flow, the Murray Barrages were constructed near the rivers mouth to prevent upstream seawater intrusion. They also fragmented freshwater, estuarine and marine ecosystems and represented barriers to the movement of diadromous, estuarine and freshwater fishes. To remediate obstruction of fish movement, from 2003-2016, a series of 11 fishways were constructed on the Murray Barrages and now annually, environmental water is delivered to restore connectivity and support fish populations.
Since 2006, annual sampling of fish from the fishways has generated a long-term dataset of the migratory fish community and specifically the upstream movements of diadromous species. This included the Millennium Drought, when the barrages and fishways were shut and no freshwater was discharged for a period of >1200 days from 2007-2010, and several years high discharge, most notably in 2022/23. This presentation examines how fish assemblages and key diadromous species have responded over these two decades of highly variable hydrology, fishway operation, and environmental management.
Generally, fish communities using the fishways have become more abundant and dynamic since the drought, reflecting annual variation in river flows and salinity of the Coorong. Responses of individual diadromous species - congolli (Pseudaphritis urvillii), common galaxias (Galaxias maculatus), pouched lamprey (Geotria australis) and short-headed lamprey (Mordacia mordax), have been variable, highlighting that population trajectories differ among taxa and may be driven by a combination of environmental conditions and life-history requirements. Environmental water has been critical in supporting fishway function and populations of diadromous fishes.
These data demonstrate the value of sustained ecological monitoring for evaluating restoration outcomes and provide an evidence base to support adaptive barrage operations and future management of connectivity in the Lower Murray.