Investigation of the causes of mass fish kills

They were the grim scenes that shocked Australia
and made headlines around the world. Millions of fish found dead in western
New South Wales. “To me, it was like the coral bleaching moment for the inland. It was an incredible wake-up call.” There were three separate fish kills that took place in rapid succession. The first happened on December 15 last year along a 30-kilometre stretch of the Darling River, near the town of Menindee. Then on January 6 this year, a larger fish kill was reported along the same stretch of river. That was soon followed by a third event on January 28, again impacting hundreds of thousands of fish including substantial numbers of threatened species: the silver perch and the Murray cod. “You can’t hide. You can’t deny it. This is a disaster. It’s a natural disaster. It’s a national disaster.” The Australian Academy of Science was asked to investigate the cause of that disaster. Panel chair, Professor Craig Moritz, and panel member, Professor Sue Jackson, travelled to Menindee to hear from locals. While there on February 1, they saw for themselves, large Murray cod in dire condition. “Very moving and affecting to actually be there, watching these huge cod, trying to do anything they could to get out of this pool. It was terribly upsetting.” The expert panel determined the immediate cause of the fish kills was insufficient oxygen. Here’s how it unfolded. A layer of warm water had formed over deep, colder water. The upper layer was more oxygenated, largely because of oxygen-producing blue-green algae. Algae flourish in warm and still conditions, particularly when the river isn’t flowing. But as algae die, they sink to the bottom, feeding microorganisms that use up dissolved oxygen. When a cool change hit the region, it mixed the smaller top layer with the larger, lower body of water. As that happened, the crucial oxygen from the top warm layer became diluted and was quickly used up. Not enough was left for the fish. Large temperature fluctuations coincided with all three fish kills. But while that was the immediate cause, it doesn’t get to the root of the problem. “The recent fish kills are certainly related to water management issues.” The panel found there isn’t enough water in the Darling system to avoid catastrophic outcomes. This is partly due to the ongoing drought. However, decades of rainfall and river flow data point to excess water extraction upstream. That’s a finding welcomed by local Indigenous elders, who have long argued that the system is being mismanaged. “We’re stuck in Menindee with the fish dying. The kids are getting sores all over them. And we’ve got no water, just about. So, where’s the water sharing plan? It’s not worth the paper it’s written on because they’re not listening to local people.” The expert panel recommends that urgent steps be taken within six months to improve the quality of water throughout the Darling River. That should include the formation of a Menindee Lakes restoration project to determine sustainable management of the lakes system and lower Darling and Darling Anabranch. It also involves improving meaningful engagement with river-based communities, including Indigenous peoples. Beyond that, the panel recommends a return to the intent of the 2012 Murray Darling Basin Plan to improve environmental outcomes. “So the best possible scenario is water in the Darling all the way to the bottom.”

local_offerevent_note October 2, 2019

account_box Gilbert Heid

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