Fish that died included invasive European carp as well as native fish such as golden perch (yellowbelly) and Murray cod.
Goulburn Broken Catchment Management Authority environmental water and wetlands manager Simon Casanelia said low dissolved oxygen levels was the likely cause.
“Floodwaters have carried large quantities of organic material such as leaves, twigs and grass into our rivers and creeks, which turns the water a dark brown or black colour. As the material is broken down it can reduce oxygen levels in the water, which can kill fish and other aquatic fauna,” Mr Casanelia said.
“The scale of the flooding means it’s difficult to improve water quality.
“In some cases, water for the environment can be used to provide refuge for native fish or help dilute the poor quality water; however, this is only possible if good quality water is available and when flow in the rivers and creeks drops to normal operational levels.”
Mr Casanelia said river managers, scientists and environmental water holders were monitoring water quality in the catchment’s river and creeks.
The Environment Protection Authority has been notified of the fish deaths.
Mr Casanelia said further fish deaths may occur throughout the catchment as floodwaters move through waterways.
“The floodwater may have an impact on water quality and aquatic fauna in the coming weeks and months, into summer,” he said.
The community is encouraged to report fish deaths to the EPA on 1300 372 842.
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