More than a thousand scaly mackerel were found washed up on a Rockingham beach yesterday.
Palm Beach remains open, but a serious health warning has been issued as investigations commence.
Inspectors from the Department of Primary Industries and Regional Development received reports of dead fish on Tuesday, but inspectors found none.
By Wednesday morning there were hundreds washed up on shore.
Some locals suspect brine discharged from the Perth Seawater Desalination Plant could be the cause but the Water Corporation dispute the claim.
They told Seven News "Comprehensive monitoring is conducted to ensure there are no adverse environmental impacts on the marine environment from our operations."
Other fish remain alive and unharmed in the area - the only fish affected at the moment is the scaly mackerel.
The beach currently remains open but visitors are being warned not to touch or eat the fish, and to stay well clear of them while swimming.