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.
Teen boy killed after being struck by two cars while crossing road in Warnbro
Free supplies, health checks, advice for Mandurah pet owners
Rockingham Detectives ask for public's help to find wanted man
LISTEN: WA Police release triple-0 call from shooting murder of bikie boss Nick Martin at Kwinana Motorplex
Police pursuit comes to crashing end in East Rockingham
Tensions boil over in Rockingham council chambers as motion to reinstate weekly red bin collections is defeated
WA's first public National War & Service Animal Day commemoration to be held at Dawesville War Memorial
New road safety laws to be named in memory of Furnissdale hit-and-run victim
UK nuclear submarine docks at Rockingham's HMAS Stirling