Diadromous and Estuarine Fishes of the Murray-Darling Basin: The Importance of Freshwater Flow and Connectivity

Globally, estuarine associated fishes are threatened by river regulation and subsequent reduction in freshwater flow, and habitat fragmentation. In the Coorong Estuary, at the end of Australia’s Murray-Darling Basin, upstream extraction of water for consumptive use has reduced mean annual end-of-system discharge to a third of natural, and freshwater and estuarine habitats are disconnected by a series of low-level tidal barrages. This has had profound impacts on the estuarine ecosystem including changes in salinity regime, reduced productivity and obstruction of fish migrations. Hydrological extremes of drought and flood from 2006–2014 exacerbated the impacts of river regulation and highlighted the vulnerability of estuarine and diadromous fishes. We present comparisons of fish assemblage structure (species composition and abundance) and distribution, as well as the movement and population dynamics of a diadromous species, congolli (Pseudaphritis urvilli), between disparate hydrological periods as empirical evidence of the need to provide freshwater flow and facilitate connectivity in estuarine environments. These data are discussed in the context of adoption and utilization by environmental managers in the development of environmental flow and fish passage programs in the Coorong, which may in turn inform the rehabilitation of estuarine associated fish populations in other arid and semi-arid river systems.