Regular monitoring showed levels of the microalgae Karlodinium veneficum had significantly increased in the two weeks before the Easter weekend.
Swan River Trust systems manager Mark Cugley said there was no evidence Karlodinium veneficum could affect humans, but it could be toxic to fish.
‘In this instance, it is estimated about 70 fish including black bream and yellow tail grunter were affected,’ Mr Cugley said.
‘Fish kills tend to occur where oxygen conditions in the water column are also depleted, as is currently the case below the Kent Street Weir.’
Karlodinium is a free-floating microalgae that is usually present when the river is low.
The algae increases in density between mid-summer and autumn each year and blooms are triggered by excess nutrients entering the river system when it rains. Mr Cugley said the Trust was working to reduce nutrients such as nitrogen and phosphorus that algae fed on.
‘This helps stop algae from growing excessively and causing an algal bloom,’ he said.
‘The health of the Canning River is also being boosted by the construction of a new oxygenation plant to help reduce the impact of algal blooms and prevent fish kills.
The State Government is investing $2.4 million to build a third plant for the Canning River in Langford and upgrade existing plants upstream from Kent Street Weir.’
The third plant will almost double the oxygen relief provided to the Canning River and is expected to be completed later this year.
Report sightings of slow-moving or sluggish fish to 9278 0900 or after hours to 0419 192 845.