Hundreds of rotting fish wash up on WA riverbanks

About 700 fish have washed up dead on the banks of the Collie River in Western Australia.

The fish, which were found in the Lower Collie River between the Australind Bypass bridge and Eaton Drive bridge in the state's south-west, likely died due to low dissolved oxygen levels in the river, the Department of Water and Environmental Regulation (DWER) has said.

"Dissolved oxygen levels have fallen rapidly following recent intense heavy rainfall," a statement said.

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Staff from the department have collected several samples from the site and put up signs warning the public to avoid the area as a precaution.

The affected fish are mostly black bream and mullet.

After the heavy rainfall causes water to flow into the river catchment, the organic matter and sediment that gets shaken up can rise to the surface, the DWER statement explained.

When this low oxygen content rises to the surface of the river, it can cause fish to suffocate.

DWER said the rain was the first significant rainfall since October 2023, adding that the particular part of the river system was prone to sudden changes in water quality.

"The layer of trapped saltwater near the bottom can become deoxygenated over time, posing a significant risk to the entire river system if mixed suddenly," it said.