Lethal ‘blackwater’ in Barwon-Darling river prompts fears of another mass fish kill

Floodwaters carrying "blackwater" lethal to aquatic life are moving south in the Barwon-Darling river towards the Menindee Lakes, raising fears the situation may cause a repeat of events in 2019 where over a million native fish were killed.

Blackwater events are caused when heavy rains wash leaf litter that has collected on a floodplain into a major river such as the Barwon-Darling, turning it the colour of dark tea.

Australian National University (ANU) ecologist Dr Matt Colloff said they were dangerous to aquatic life as the organic compounds plants used for protection against insect attack leached out of leaf litter and dissolved into the river.

"Bacteria then break down the dissolved organic compounds and release carbon dioxide," Colloff said. "In the process of metabolising carbon, the bacteria use up the oxygen in the water."

Colloff said once concentrations of dissolved oxygen fell below about 2-3mg/L, fish suffocated because they could not draw enough oxygen from the water through their gills to survive.

As blackwater events move slowly and can stretch for kilometres, there is no way for fish and other aquatic life to swim through it. Depending on the geography of the area, they can be left with nowhere to hide as the waters approach, resulting in a mass die off.

The Menindee Lakes were hit by a mass die off in 2019 that is thought to have killed more than a million native fish, devastating stocks of bony herring, Murray cod, silver perch and golden perch.