An unknown natural event is likely responsible for a large-scale fish kill on Perth's south coast, according to the Department of Fisheries.
More than 1500 dead pink snapper and blowfish have washed up on Cockburn Sound since November 19, with authorities yet to pinpoint the exact cause of the deaths.
Microscopic examination of gill samples have shown the fish may have suffered respiratory stress, a Department of Fisheries spokesman said, but what caused the stress is yet to be determined.
"Test results received to date indicate no evidence of either natural algal or industrial toxin involvement and point towards an as yet unknown natural event as the cause of this incident," he said.
"Respiratory stress can be caused by physical irritants or chemical contaminants."
The Department of Environment Regulation confirmed it was investigating the spill of canola grain from the Kwinana grain jetty, within the Sound, between November 18 and 22 - matching the reports of dead fish.
"DERs investigation aims to establish the facts concerning the spill, including the quantity of the product and any person or persons responsible," a spokeswoman said.
But a Fisheries spokesman said the examination of the dead fish stomach contents found no evidence of canola or any other unnatural food source.
An open water swim in the Sound has been postponed as a result of the fish deaths, despite the Department of Health saying the water was still suitable for swimming and fishing.
Swimming WA said the event, planned for Saturday, was deferred by three weeks due to recent water quality testing, unfavourable weather and the continued presence of stingers.
"In the past 48 hours, SWA has collaborated extensively with the Department of Health, Surf Life Saving WA and the City of Cockburn and thanks them for their advice, support and diligence in their approach," Swimming WA said in a statement on Friday.