Why Millions of Dead Fish Appeared in Australian River

A small Australian town has been inundated with millions of putrid-smelling dead fish following a mass-dying event.

The phenomenon is ongoing in the Darling River in the town of Menindee, which is situated around 580 miles west of Sydney. It has a population of around 500 people.

Experts believe the deaths may be due to recent flooding in the area, and the subsequent receding flood waters resulting in lower oxygen levels.

"The reports from late yesterday, early this morning ... [suggested] we were looking at thousands, potentially tens of thousands, of predominantly bony bream—which is a native species—that have died," Cameron Lay, director of freshwater environments at the New South Wales Department of Primary Industries (DPI) Fisheries, told ABC News Australia.

Other fish involved in the mass-dying were Murray cod, golden perch, silver perch and carp, according to local media.

"Those estimates are well and truly into the millions now. We are seeing tens of [miles] where there is fish really as far as the eye can see, so it's quite a confronting scene," Lay said.

dead fish river
Thousands of dead fish floating on October 16, 2017 in the Confuso river in Villa Hayes, Paraguay. A mass death death event has occurred in Australia, with millions of fish floating dead due to low... NORBERTO DUARTE/AFP via Getty Images

The DPI said the fish deaths were likely caused by low oxygen levels from recent floodwaters receding, which subsequently washed tons of organic matter into the river. As these decay, the bacteria and other microbes used up a lot of the oxygen dissolved in the water, with little left for fish. These effects are exacerbated by the warmer weather, meaning that fish naturally require higher levels of oxygen. The water also holds even less oxygen.

"Cause is attributed to large floodwater draining back into the river with high nutrient loads and hot weather," the DPI explain in a statement on its website. "This contributes to Dissolved Oxygen (DO) depleting overnight resulting in fish deaths. Some dead fish are reported to be on the floodplain having been stranded in off channel waterbodies as the river levels recede."

Mass fish deaths are a fairly common occurrence and can be triggered by a number of different environmental issues, ranging from water quality decreases to algal blooms and chemical pollution.

"Ultimately, fish kill events happen because the quality of the environment cannot sustain fish life," Joy Becker, an associate professor with the University of Sydney, told ABC News Australia.

"Causes of fish kills can be environmental, chemical, or possibly related to infectious disease agents including opportunistic pathogens or a combination of all these factors."

This massive die-off of fish in the Darling River is the third mass kill to hit the Menindee area since 2018, and is thought to be on par with the previous largest event, which occurred in 2019.

Around one million fish were killed after a massive 25-mile algal bloom decimated the local population. Algal blooms can both sap the oxygen out of waterways, causing fish to asphyxiate, but also produce toxins that actively kill the fish.

Similar issues arise as a result of algal blooms in the U.S., with a red tide of Karenia brevis algae currently resulting in many tons of dead fish to wash up on beaches across Florida.

The millions of fish in the small Australian town will be removed in the coming days by local authorities. Residents of the town are also being provided with clean water, with higher-oxygen waters also being released into the rivers.

Do you have a tip on a science story that Newsweek should be covering? Do you have a question about mass fish deaths? Let us know via science@newsweek.com.

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Newsweek is committed to challenging conventional wisdom and finding connections in the search for common ground.

About the writer


Jess Thomson is a Newsweek Science Reporter based in London UK. Her focus is reporting on science, technology and healthcare. ... Read more

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