Shifts in fish assemblages indicate failed recovery of threatened species following prolonged drought in terminating lakes of the Murray–Darling Basin, Australia
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Freshwater fishes are vulnerable to changes in water quality, physical habitat and connectivity resulting from drought, particularly in regulated rivers. When adequate river flows return, the recovery of populations might depend on the duration and consequences of drought. Rivers of the highly regulated Murray–Darling Basin in south-eastern Australia terminate at two large, shallow lakes that are separated from the estuary by tidal barrages. Over-abstraction of water and widespread prolonged drought (1997–2010) placed the lakes under severe environmental stress, culminating in critical water level recession from 2007 to 2010. Concurrently, most freshwater fish populations collapsed. We investigate shifts in fish assemblages resulting from habitat inundation in the lakes following the drought. The inundation and re-connection of the lakes and fringing habitats led to a substantial reduction of salinity throughout the region, and aquatic vegetation shifted from salt-tolerant to freshwater species. Fish assemblages became increasingly characterized by common freshwater taxa (ecological generalists), including high proportions of alien species. There were no indications of population recovery for three threatened species. The findings emphasize that short-lived fishes with specialized habitat requirements are vulnerable to severe population declines during prolonged drought in regulated rivers, which might restrict their recovery when adequate flows return.
KeywordsThreatened fish Population recovery Drought River regulation Flood
This study was funded by the Murray–Darling Basin Authority’s The Living Murray program through the South Australian Murray–Darling Basin Natural Resources Management Board and the Department for Water, and it was managed by Adrienne Frears. Thanks to the landholders who granted access to study sites, including Kevin and Benita Wells, Colin and Sally Grundy, Chris and Beth Cowan, Jamie Withers, Ack and Jenny Vercoe, Lesley and Mick Fischer, Phil and Yvonne Giles, and Amanda Burger. Thank you to field assistants Anthony Camilleri and Yanima Hartman from the Ngarrindjeri Regional Authority and Colin Bailey. Thank you to Associate Professor Keith Walker for his guidance on an earlier version of the manuscript. We also thank five anonymous reviewers for their helpful advice. Sampling was conducted in accordance with the University of Adelaide’s Animal Ethics Policy and the South Australian Fisheries Management Act 2007.
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