The Coorong is the unique estuary of the Murray-Darling Basin, located at the terminus of the Murray–Darling River. Freshwater flow plays a pivotal role in maintaining estuarine habitat and functionality and influencing fish population dynamics. This study incorporated 15 years field data (2007–2022) from the Coorong, to investigate spatio-temporal variations in fish species richness/diversity, assemblage structure, and the abundance and distribution of key species. The long-term period encompassed varying hydrological conditions (including drought and flood/high-flows), allowing to explore the effect of flow regime on fish species of different life-histories and assemblage changes, and to determine key environmental drivers.
Along the increasing salinity gradient in the Coorong (fresh/brackish to hypersaline), fish species richness decreased; similarly, the abundances of key species decreased, except for smallmouth hardyhead (Atherinosoma microstoma), a highly salt-tolerant species. The overall fish abundance was primarily driven by two small-bodied species, with sandy sprat (Hyperlophus vittatus) and smallmouth hardyhead dominating the northern and southern Coorong, respectively. Large-bodied fish (e.g. yelloweye mullet (Aldrichetta forsteri), congolli (Pseudaphritis urvillii), and bony herring (Nematalosa erebi)) were most abundant in the Murray estuary.
Flow and salinity were the primary drivers shaping fish assemblage structure in the Coorong. During drought/lack-of-flows (2007–2010) with higher salinities, fish assemblages were dominated by marine species in the Murray estuary, and the South Lagoon became unsuitable fish habitat due to its extreme hypersalinity (>4x seawater). Under high-flows (e.g. 2011–2013, 2016-17, 2021-22) with lower salinities (more freshwater–brackish conditions), fish assemblage changed with increased species richness and greater contributions of freshwater, estuarine and diadromous species. Salinity strongly influenced fish distribution in the Coorong, with a pronounced abundance decrease for most species when salinities exceeded 50 ppt. This study improved the understanding of flow and salinity effects on estuarine fish and will inform management to improve population resilience in estuaries.