Millions of dead fish have been discovered in the Darling River in Menindee, New South Wales. The die-off event, according to local officials and scientists, is probably caused by high temperatures and low oxygen levels as water from recent floods recedes.
Most of the dead fish are bone herring, also known as bony bream, although there are also carp, Murray cod, golden perch, and silver perch.
As flood waters recede, low oxygen levels in the water (hypoxia) are to blame for these fish deaths, according to a Facebook post from the New South Wales Department of Primary Industries. Concentrated amounts of nutrients, organic matter, and fish, especially carp and bony herring, are returning to the river channel from the floodplain.
The Department of Primary Industries reports that the region has been extensively flooded. Water loses oxygen when the flood floods recede. A local heat wave makes things worse because it makes fish need more oxygen, but warmer water stores less oxygen.
In particular, because this species is more sensitive to low oxygen levels and high temperatures, the department found that the population of bony herring frequently experiences a considerable increase during floods and subsequently a sharp decline when flood waters recede.
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Thousands of fish perished in similar occurrences in 2018 and 2019, according to Reuters, caused by dramatic temperature swings and poor water quality.
Residents are struggling with the sight and smell of dead fish in the nearby river after the area experienced high temperatures and flooding.
We had barely begun to clean up when this incident occurred, which is roughly equivalent to walking through a dried-up mess before smelling something foul. According to NBC News, local resident Jan Dening described the stench as being horrendous and described how it was appalling to see so many dead fish.
The potential effects of the dead fish in the river on human health are a concern for the locals as well.
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The stench was terrible. Geoff Looney, a local nature photographer, told NBC News that he nearly had to wear a mask. I was concerned for my own well-being. Our pumping station receives the water from the top of the reservoir to supply the town. Those who live north of Menindee claim that perch and cod are always drifting down the river.
The Department of Primary Industries said that multiple agencies were working to respond to the fish deaths, and locals were urged to call 1800-043-536 to report any relevant observations.