NSW report says a temperature plunge from 46C to 28C contributed to the ‘unprecedented’ death of ‘hundreds of thousands of fish’

This article is more than 1 year old

This article is more than 1 year old

Little or no flow in the Darling River and a plunge from high temperatures are likely to have led to conditions that caused a catastrophic drop in oxygen levels prior to extensive fish kills at Menindee.

An interim investigation by the New South Wales Department of Primary Industries found the fish deaths were likely to have been caused by “several related and compounding factors resulting in low oxygen in the river”.

The report also confirmed the second incident on 6 January involved the “unprecedented” death of “hundreds of thousands of fish”.

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The report blamed “thermal stratification in which a a warmer surface layer of about one metre of water sat above a cooler deeper layer (2-3m) of water with very low dissolved oxygen”.

“These conditions were conducive to blue-green algal blooms in the surface layers,” it said.

Facebook Twitter Pinterest Algal bloom on the Darling River at Tolarno Station, south of Menindee. Photograph: Mike Bowers/The Guardian

In the first incident that killed “tens of thousands of fish” before Christmas, rain fell on 15 December 2018 and temperatures dropped, “which appears to have mixed the water layers, which resulted in very low dissolved oxygen throughout the water column”.

“Another substantial drop in air temperatures from 46C to 28C on January 4 and 5 associated with cold fronts passing through the region again caused layers of water with different dissolved oxygen levels to mix, reducing the overall dissolved oxygen available,” the report said.

“High algal content in stock and domestic flow releases drawn from Lake Pamamaroo, which increased oxygen demand and consumption, further reduced dissolved oxygen available to fish.”

But the report also noted that stratification and subsequent changes in temperature were not enough in themselves, as otherwise there would be regular fish kills in NSW dams, where the same phenomenon occurs.

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The species affected were predominantly bony herring, but also included hundreds of golden perch and silver perch, as well as dozens of Murray cod, mostly large adults. The largest was 127cm long.

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It said the preliminary study of the ear bones, known as otoliths, taken from larger Murray cod, golden Perch and silver perch, suggest the fish were between 17 and 25 years old.

“Further research will examine where they were born,” the report said.

The deputy premier and minister for regional NSW, John Barilaro, said the fish kill was unprecedented and a major ecological disaster.

“We know there is anger and frustration following this incident but we are working hard to support far-west communities with practical measures,” Barilaro said.

He pointed to the six aerators the government had installed to provide small fish refuges in the Darling, which are untested as a strategy to preserve fish in a river.

Facebook Twitter Pinterest NSW Department of Primary Industries workers install aerators to put oxygen back in the river above the weir at Menindee. Photograph: Mike Bowers/The Guardian

He also announced that the NSW Cross Border Commissioner, James McTavish, would be immediately appointed as Regional Town Water Supply Co-ordinator, to oversee the work to ensure water supply and quality in towns and communities all over NSW.

“This could include the provision of funding for water carting to towns at risk of low water supply, or the provision of bottled or chilled water to schools or health care facilities in these communities,” Barilaro said.

University of NSW professor of environmental science Richard Kingsford, an expert on the Menindee Lakes, said the report was “a good report as far as it goes”.

“It’s a good scientific assessment of the proximate causes,” he said.

But he said the references to other fish kills during the millennium drought in the report underplayed the seriousness of these incidents.

The millennium incidents had been nowhere near the scale of the recent incidents – they had involved thousands of deaths not hundreds of thousands, he said.

“It’s all about what was the condition of the river, not just in the short term but longer term.

“There is an absence of good data on water quality,” he said. “It doesn’t identify the status of blue-green algae before or after the fish kill.”

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The report also revealed that small numbers of carp also died, but the huge deaths of native fish meant that the non-native species, which are regarded as a pest, could well become more dominant after the incidents.

The DPI staff said they observed the reaches of the river where the kills occurred appeared green and discoloured, with clumps of algae scums visible. A strong musty or organic odour was also reported.

During the second fish kill, staff said they observed “fish noticeably gasping (known as aquatic surface respiration) near the Menindee town”.