ALGAE levels in the Canning River have risen again in the past two weeks but the two species currently blooming are not known to be harmful to humans, according to Swan River Trust scientists.
However, the Department of Health has warned people to avoid eating shellfish, cockles or oysters from the Swan and Canning rivers for the time being.
The Trust is continuing to monitor the species, known as Gymnodinium and Karlodinium, which can bloom to reach high numbers under favourable conditions.
“Alga such as Gymnodinium favours the current conditions in the Canning River, where salinities are high and there is some degree of stratification in the water,” principal scientist Kerry Trayler said.
“They can compete with other species by moving up and down the water column to find ideal conditions of food, light, temperature and shelter.”
Dr Trayler said Karlodinium was a free-floating microalgae found in most WA waterways and typically increased in density between mid-summer and autumn.
This same species also was found in the Swan and Canning rivers in January
Dr Trayler said that water in the affected area may look reddish-brown and have an oily surface scum.
There is no evidence Karlodinium affects humans but it can be toxic to fish, while Gymnodinium could strip oxygen from the water column and produce foul odours.
“If oxygen levels become critical, this will affect fish and other animals in the river,” she said.
“We are keeping a close watch on the algal bloom and Trust officers will inspect for affected fish.”
Dr Trayler said that as a precaution, anyone swimming in affected areas should rinse off with fresh water as soon as they leave the water.
The community is asked to report sightings of slow-moving or sluggish fish to 9278 0900 or after hours on 0419 192 845.
More details at www.swan rivertrust.wa.gov.au.