© Department of FisheriesThe fish washed up in Wickham.
The department was made aware of the incident, which occurred at Wickham, around 50km east of Karratha, by a member of the public on Tuesday, the ABC reports.
A fisheries official visited the scene and estimated that at least 4000 fish of the same species were strewn across the beach.
The Department of Fisheries northern region manager Peter Godfrey said he could not recall such an incident ever occurring in the Karratha area.
"We do occasionally get reports of small fish kills, but certainly there has been nothing major in the area," he said.
Samples taken at the site have been sent to Perth for analysis by the department's fish health pathologists.
Mr Godfrey said there could be a number of possible explanations for the mass fish deaths and it was too early to speculate.
"It could be some sort of fish disease or it could be environmental factors, such as an algal bloom, or hot water conditions, or stranding by the tides," he said.
Mr Godfrey has advised locals to avoid the area, not to take samples and not to eat the fish.
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