One of Perth's worst mass fish kills in years has prompted the agency responsible for managing the Swan and Canning rivers to appeal to people to reduce the amount of nutrients entering the rivers.
More than 10,000 mainly black bream were killed in the mid reaches of the Swan River between July 27 and August 5 in the latest outbreak of a toxic algal bloom in the city's waterway.
The microalgae, known as karlodinium veneficum, prospers in saline water.
It exploded between Garratt Road Bridge in Ascot and the Narrows in the city after Perth's dry July failed to flush the rivers.
The outbreak brought to about 13,000 the number of fish killed in the rivers this winter.
It prompted the Swan River Trust to call on the community to help reduce nutrient flows into the river system.
Trust principal scientist Kerry Trayler confirmed one of the agency's oxygenation plants, which are used to prop up anaemic parts of the river, broke down in the days leading up to the latest fish kills.
Dr Trayler said the malfunctioning plant at Guildford was upstream of the worst affected part of the river and it was restored as the algal bloom spread on July 28.
She said the Guildford plant's closure was unlikely to have contributed to the number of fish killed and noted the plant at nearby Caversham continued to operate.
"The main contributor to both low oxygen conditions and algal blooms is the influx of high levels of nutrients and organic matter into the waterways," Dr Trayler said.
"The whole community has a role in reducing these pressures."
Recfishwest boss Andrew Rowland said so many deaths of such a popular recreational fishing species as black bream was a worry.He said it should prompt action to help improve the rivers.
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