Further mass fish kills could occur downstream of Menindee in New South Wales, as water starved of oxygen makes its way down the Darling River.
That’s according to Dr Darren Baldwin, an expert in freshwater ecology, principal consultant with Rivers and Wetlands consultancy and an adjunct research professor at Charles Sturt University.
Baldwin tells Cosmos the fish kills are a natural event resulting from ‘blackwater’ conditions. Flooding has washed large amounts of organic material into the river system. Bacteria consuming the carbon in leaf litter and other organic material, are also consuming oxygen, with the process happening faster in warm weather.
Oxygen levels in river water can be as high as 10 milligrams per litres. “Big fish tend to stress at about 4 [mg/l] and tend to die when the dissolved oxygen falls below 2 [mg/l],” he says.
In the past week, oxygen levels in the Darling River near Menindee fell to zero, and have remained low.
Baldwin says bony bream, in particular, can be quite fragile. “They don’t like changes in water temperature and they don’t like big massive falls in dissolved oxygen.”
Hotter water temperatures create additional stress for the fish.
Images from the river show the surface covered with millions of dead fish – bony bream, Murray Cod, perch and carp – near Menindee.
The blackwater events, leading to fish kills, can set off a chain reaction potentially leading to further fish kills.
“All those dead fish will start to sink. And that’s another source of carbon. […] That’s what we think happened in the 2018-19 event. The first fish kill, helped fuel the second fish kill, which helped fuel the third fish kill, and they were actually larger each time.”
Baldwin says Menindee has seen five significant fish kills since 1819, based on historical newspaper reports.
He says while the process is natural, efforts to manage rivers using weirs and dams can exacerbate the problem, allowing for the build-up of organic material over longer periods whereas in the past flood events would have been more frequent.
“You get more and more [leaf] litter building up on the floodplain. So on the Murray it mostly comes from Red Gum leaves. On the Darling there will be some fringing Red Gum and that will be the bigger input when you’ve got a flow that comes down a dry riverbed, then you’ve got the grasses and all that the other material that’s sitting out on the floodplain.”
It may take weeks for the situation to resolve. A blackwater event in the Murray in 2010 lasted for six months.