The Murray-Darling is one of the most regulated river systems in the world. In the lower River Murray, downstream of the Darling River junction, flow and connectivity are impacted by upstream dams, 10 sequential weirs and over 7 km of tidal barrages, profoundly impacting the ecological integrity of riverine and estuarine ecosystems. The Basin plan aims to rehabilitate aquatic ecosystems in the Murray-Darling Basin (MDB), primarily through the reallocation of water to the environment. To achieve this, substantial volumes of environmental water (1000s GL) are now available to augment flow in the Basin’s rivers.
Measuring ecological response to the allocation of environmental water is essential to inform future management and to justify water use to a multitude of stakeholders. Fish are an integral and charismatic component of aquatic ecosystems and form a major objective of environmental flow allocations throughout the MDB. In this paper, we use two case studies to review fish responses to environmental water delivered to the lower River Murray. Over a four-year period (2013–2016), 500–1000 GL/annum of environmental water was used to supplement the regulated flow regime of the lower River Murray. Over this period, fish assemblages in the main channel of the lower River Murray trended to those characteristic of drought conditions, being dominated by generalist species and with an absence of recruitment of flow-cued spawning species (e.g. golden perch). In contrast, abundances of diadromous fishes at the Murray barrages (the freshwater–saltwater interface) increased as environmental water was used to promote connectivity through the Barrages to facilitate key life history processes. We explore the mechanisms behind these responses, and relationships with flow volume and timing. This knowledge will inform future flow management to achieve ecological outcomes in the MDB.