Millions of fish have been dying in Australia’s major rivers

Ruby Prosser Scully New Scientist 26 Feb 19;

Fish have been struggling to breathe and dying by the millions on the banks of Australia’s largest river system. Experts say that without serious change, it will continue to happen.

Poor management, excess upstream irrigation and drought led to three mass deaths of endangered fish species during December and January in the Murray-Darling Basin. These deaths included Murray cod fish that were decades old, according to an investigation by the Australian Academy of Science that was published last week.

Craig Moritz at the Australian National University in Canberra, who chaired the investigation, says the sight of millions of dead fish should be a wake-up call. He described the mass fish deaths as a mainland equivalent of the coral bleaching events that have been hitting the Great Barrier Reef.

Food security

Much of Australia’s economy and food security depends on the Murray-Darling Basin, a complex river system with 2.6 million nearby residents, including more than 40 Aboriginal nations.

The Australian Academy of Science investigation was initiated by the opposition Labor party. The government has also launched its own separate investigation, but the panels from both have agreed on the immediate cause of the fish deaths.

Warm, still water created the perfect environment for blue-green algae to bloom. Algae are important for providing oxygen in the upper layers of water, but when they die, they fall to the bottom of the riverbed where microorganisms that deplete oxygen in the lower layers feed on them.

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A cold snap caused the oxygenated upper layer and the larger, unoxygenated, lower layer of water to mix. Fish that had been surviving in the shallow top layers quickly ran out of oxygen and suffocated.

The government-commissioned interim report highlighted role of “exceptional climactic conditions” in exacerbating the situation.

But Moritz’s report said the drought hadn’t been unprecedented, and that excess upstream irrigation led to a lack of water and a “catastrophic decline” of the system’s condition through dry periods.

Culum Brown at Macquarie University says focusing on extreme weather conditions is “nonsense” and that management of the Murray-Darling river system had ignored the predictable results of climate change.

“Sticking your head in the sand and pretending climate change does not exist is not an appropriate management action,” says Brown.