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From boom to bust and back

Darling River fish deaths and floods

Australians will remember the great fish cull that dominated news feed the summer of 2018-2019. We saw the images of hundreds of thousands to millions of dead freshwater fish floating on the Darling River around Menindee in western New South Wales. 

The northern Murray-Darling Basin, including the Darling River, was in the grip of the most severe drought on record.  The Menindee Lakes had nearly completely dried and the Darling River upstream was reduced to a string of disconnected pools. 

The scale of the Darling River fish deaths stimulated two independent investigations into their causes, one conducted by the Australian Academy of Sciences and the other by an independent panel for the Australian Government. The authors of this latter report have now published a paper in a special issue of Marine and Freshwater Research focussed on fish kills.

The investigations concluded that the fish deaths were ultimately the result of a complex interplay between short-term weather conditions and long-term influences on flows in the Darling River.  The management context of large-scale water abstraction for irrigation, even under baseflows, combined with an increasing frequency of drought provided a toxic backdrop for the short-term localised conditions of extreme water temperatures, algal blooms and deteriorating water quality in the summer of 2018-19 – all things that imperil fish survival.

After the three catastrophic fish death events between December 2018 and January 2019, there were repeat events along the Darling River, at much smaller scales throughout 2019.  It was not only the fish that suffered, there was also mass mortality of freshwater mussels and likely many other plants and animals relying on the river systems in this arid and remote part of the landscape.  The iconic River Red Gums that line the banks of the Darling River were stressed.  However, as is the nature of these ‘boom’ and ‘bust’ river systems the rains did come in early 2020, with flows providing connection throughout the river system and making their way to Menindee, offering some relief to the ecosystems and communities along the way.

The moderate flows of 2020 have been followed by larger flows in 2021 and Menindee Lakes is again nearly full – as it was in 2016 before the severe drought and the conditions that were so catastrophic for the fish. 

Severe drought and flood cycle: January 2014 to September 2021
"While dryland rivers like the Darling River naturally cycle between periods of high flows and periods of extreme low flows the trajectory of an increasingly dry climate intersecting with unsustainable volumes of water diverted for irrigation, particularly at low and moderate flows, places this boom-and-bust balance at risk. "

 

The periods of flooding and high flows will always inevitably arrive, but it is the length of the periods of extreme low flows and drying that will determine the survival of the plants and animals that call our dryland rivers home.

Let’s hope the images of those fish have not been forgotten.   

Author

 

Dr Fran SheldonDr Fran Sheldon is Professor and Dean (Learning and Teaching) in the Griffith Sciences group at Griffith University and a Research Member in the Australian Rivers Institute. Her research explores the relationships between hydrology, physical geomorphology and ecology in river systems, particularly large dryland rivers such as those of the Murray-Darling Basin and the Lake Eyre Basin.

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