Why did a million freshwater fish perish?

Dead and dying fish in the Darling River near Menindee.

Dead and dying fish in the Darling River near Menindee.

When as many as a million fish died in the lower Darling River this past summer – and images of dead and dying Murray Cod, Silver Perch, Golden Perch and Bony Herring saturated the media – the ecological shock to the region was severe.

Why did these three mass fish death events happen? Could they have been prevented? Are the lives of more fish at risk?

An independent panel of scientists has examined the causes of the fish kills and devising measures aimed at preventing further mass fish deaths. The scientists urge all governments to increase their commitment to improving river management in the Murray-Darling Basin.

The panel found the fish death events were preceded by exceptional climatic conditions, including severe drought and heatwaves. Water quality was poor and fish mobility restricted.

The panel’s final report, released this week, includes 27 recommendations directed at governments and water managers and concludes that the Murray-Darling Basin Plan “is the right vehicle to deliver the changes needed to support the environment and communities”.

Summary of the multiple causes of the 2018-19 fish death events in the lower Darling river.

Summary of the multiple causes of the 2018-19 fish death events in the lower Darling river.

The panel was convened by Federal Minister for Agriculture and Water Resources David Littleproud and chaired by Professor Robert Vertessey, University of Melbourne. Other members were University of Technology Sydney freshwater ecologist Associate Professor Simon Mitrovic together with Daren Barma, Associate Professor Lee Baumgartner, Professor Fran Sheldon and Professor Nick Bond.

Associate Professor Mitrovic said the individual expertise of the scientists allowed the panel to “cover all the issues and come to a good understanding of the actions required to reduce the chances of mass fish kills happening again”.

Dr Mitrovic, who has spent more than 20 years researching water quality and algal blooms, with some work in the lower Darling River system, said several factors coincided to cause the fish kills.

“Previously good breeding seasons for fish meant that fish numbers were high in the river channels. As river flows dropped due to drought the fish got trapped and couldn’t go up or downstream,” he said.

Attempts were made to put oxygen back into the river system via aeration technology. Photo: Simon Mitrovic

Attempts were made to put oxygen back into the river system via aeration technology. Photo: Simon Mitrovic

“Thermal stratification in the river, that is, cold, low-oxygen water at the bottom and hotter water on top, caused an algal bloom. Cold weather fronts came through which caused the water to mix and subsequent high air and water temperatures meant there were low oxygen levels in the water resulting in the stranded fish dying.”

Dr Mitrovic said that there just wasn’t enough water to keep the river flowing.

“There was no available water to release,” he said.

Recommendations from the panel include initiatives to

  • reduce water take during low flow periods
  • protect the passage of environmental flows
  • acquire additional water for environment purposes
  • remove barriers to fish mobility.
A pump on the lower Darling River extracts water for the township of Menindee. Photo: Simon Mitrovic

A pump on the lower Darling River extracts water for the township of Menindee. Photo: Simon Mitrovic

Reviewing the operation of the Menindee Lakes and re-evaluating the water savings proposals for the scheme were also recommended. The panel also advised increased investment in research, monitoring and model development.

The panel stressed that for these initiatives to succeed, they must involve “authentic community engagement”.

“We want to see all Murray-Darling Basin governments commit to better [inclusion of] the views and experiences of Aboriginal people and other communities along the rivers in their decision making.”

The panel co-authored an article about their report for The Conversation: We wrote the report for the minister on fish deaths in the lower Darling – here’s why it could happen again.

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