Over-abstraction of water places unsustainable pressures on river ecosystems, with the impacts amplified under drought conditions. Freshwater fishes are particularly vulnerable due to associated changes in water quality, and habitat availability, condition and connectivity. Accordingly, fish assemblages are ideal indicators of the impacts of drought and over-abstraction. The Murray-Darling Basin (MDB), south-eastern Australia, terminates at the Ramsar listed Coorong and Lower Lakes, which comprise Lake Alexandrina and Lake Albert. Over-abstraction and extreme drought during the last decade has placed these lakes under severe environmental stress. The purpose of this study was to investigate shifts in fish assemblages caused by substantial water level recession and salinization in the Lower Lakes. Small-bodied fish assemblages were sampled at the beginning and several years into the drought. Off-lake habitats held diverse fish assemblages in 2003, but most sites were dry by 2009. Remaining habitats were disconnected, salinities increased substantially, and aquatic vegetation shifted from freshwater to salt-tolerant species. There was a substantial decline in the proportion of specialist species, especially diadromous and threatened species, and an emerging dominance of generalist freshwater and estuarine species. The findings warn of the inevitable ecological impact of over-allocating water for human use in drought-prone regions, and highlight the need for adequate environmental water allocations. This study also emphasises that understanding the ecological attributes of a fish species, and the subsequent assignment to a functional group, will help predict vulnerability to decline and extirpation.
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The 2003 component of this study was supported by the South Australian Catchment Water Management Board, the Department for Environment and Heritage, The Goolwa to Wellington Local Action Planning Board and Native Fish Australia (SA). Data for several sites were supplied by Jason Higham, formerly of SARDI Aquatic Sciences. The 2008–2009 component of 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 was managed by Adrienne Frears. Field assistance was by Brian Degger and Thomas Barnes. Sampling was conducted in accordance with the University of Adelaide’s Animal Ethics Policy and the Fisheries Management Act 2007. The authors are grateful to Professor Angela Arthington and two anonymous reviewers for their very helpful comments.
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Wedderburn, S.D., Hammer, M.P. & Bice, C.M. Shifts in small-bodied fish assemblages resulting from drought-induced water level recession in terminating lakes of the Murray-Darling Basin, Australia. Hydrobiologia 691, 35–46 (2012). https://doi.org/10.1007/s10750-011-0993-9
- Threatened species