Biodiversity decline and diverted rivers – the Murray, the Darling and the fish kill

May 14, 2019 - 10:43am

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Last week, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), released its 2019 Global Assessment Report on Biodiversity and Ecosystem Services. This landmark report does not bode well for our planet, claiming that “nature is declining globally at rates unprecedented in human history — and the rate of species extinctions is accelerating”. While, unfortunately, this finding is not surprising, it is still always sobering to have the facts confirmed at global scale. My dear Swedish colleague, Marie Stenseke (@MarieStenseke), has been Co-chair of the Multidisciplinary Expert Panel of IPBES for several years now, and I know how hard they have worked. (One of their big challenges, not yet solved, has been to find appropriate terminology for ‘nature’, both with and without human influences.)

Also released last week, a Nature article ‘Mapping the world’s free-flowing rivers’ by Grill et al., was a further sobering reminder of how few of the world’s rivers are still free-flowing along their entire length. The authors of that article remind us how important free-flowing rivers are for supporting diverse, complex and dynamic ecosystems, all a vital part of facilitating the survival of diverse species, including humans. The filling of Lake Eyre is currently reminding Australians of the gift we have in those unregulated systems. In their beautiful exhibition, Water Lore: Learning from the Drylands, Gini Lee and Antonia Besa provoke us to think about what urban communities can learn from areas where huge levels of water variability are the norm.

Biodiversity and the flow of rivers have been front of mind for the first months of 2019. My year kicked off with being asked to join a multidisciplinary expert panel convened by the Australian Academy of Science to provide advice to the federal Leader of the Opposition, Hon Bill Shorten MP, about the reasons behind two distressingly large-scale fish deaths in the Murray-Darling river system, including circumstances that may have exacerbated the immediate causes of fish death.

On 15 December 2018 tens of thousands of dead fish were reported along a 30 km stretch of the Darling River near the town of Menindee in New South Wales where dead silver perch (a critically endangered species), bony herring, golden perch and Murray cod (a vulnerable species) were seen in the vicinity of the Old Menindee Weir and Menindee Pump Station. On 6 January 2019, a second, larger fish kill event involving hundreds of thousands of fish was reported along the same stretch of river. Unusual amongst the deaths was the age of the Murray cod, with many individuals aged 20 years or older amongst those found dead.

It was a challenge and privilege to work with this group of scholars. We rapidly brought together our combined data, literature and disciplinary expertise to analyse the reasons behind the swathe of fish deaths, including two further fish deaths on 28 January 2019 (millions of fish killed) and commencing on 4 February 2019 that occurred as we were undertaking our work.

The final peer-reviewed report is available as: ‘Investigation of the causes of mass fish kills in the Menindee Region NSW over the summer of 2018-2019'. We concluded that the immediate cause of the fish deaths was thermal ‘stratification [of the water profile] and then mixing of a large volume of oxygen-depleted bottom water with the smaller oxygenated surface layer’. Importantly, these immediate causes were found to be in-turn attributed to the ‘interaction between a severe (but not unprecedented) drought and, more significantly, excess upstream diversion of water for irrigation’, thus ‘the root cause of the fish kills is that there is not enough water in the Darling system to avoid catastrophic decline of condition through dry periods’. This highlights how this is an issue of water management and river diversion, rather than one beyond human control. As with climate change denial, the downplaying of human impacts under the guise of ‘drought’ or ‘natural variability’ also downplays the extent to which we can and must take action on these issues.

I will be discussing some of these issues and their emotional dimensions on 16 May 2019 in Melbourne as part of a panel at the Wettenhall Extinction Lecture 2019 which will focus on diverse ideas and perspectives relating to the impact of extinction on people and living systems in Australia.




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