The Canning River could get a bit smelly if an algae bloom continues to worsen between Riverton and Cannington.
The Swan River Trust is monitoring elevated levels of single-cell algae, gymnodinium, which can bloom to reach high numbers.
Trust principal scientist Kerry Trayler said while this particular species is not known to be harmful to humans or produce toxins; it can strip the water of oxygen and produce foul odours.
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Dr Trayler said fish and other animals in the river could be affected if oxygen levels continue to drop in the areas between Riverton and Bacon Street, Cannington, however tidal movements could push the bloom significantly downstream.
Fish that are affected by low oxygen levels will be slow-moving and sluggish.
The Department of Health has maintained its long-standing advice to the community not to eat mussels, cockles or oysters from the Swan and Canning Rivers.