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DWER was notified on 30 June and sent officers to the river where they observed dead fish in a 100-metre area along the north shore and adjacent to the boat ramp.

Officers identified the species as Bony Herring (75 per cent) and Black Bream (20 per cent). The remaining five per cent were Whiting and Hardyhead.

Fish specimens were collected by DWER and will be assessed by the Department of Primary Industries and Regional Development. DWER is analysing phytoplankton samples.

Fish deaths are often water quality related such as reduced dissolved oxygen and changes in salinity, which is consistent with the recent rains and high tides in the area.

DWER’s Principal Scientific Officer Tim Storer said that like most river systems where fish kills occur, organic material from the Greenough catchment builds up in the deeper areas near the coast.

“This is particularly true in systems where sand bars reduce flushing to the ocean,” Dr Storer said.

“Seasonal rain events or strong tides bring in additional material and mix what has accumulated over the dry season.

“This provides food for bacteria and phytoplankton which consume oxygen as their populations grow – and in extreme events can result in a rapid drop to concentrations below tolerance of species.”

Contact with dead fish should be avoided and the Department of Health advises against eating or handling fish found in these circumstances.