The Department of Fisheries has studied 7,000 oysters from pearl farms and carried out 800 tests looking for disease.
The investigation was launched after a large number of deaths at nine farms in the Exmouth Gulf, in the state's north-west, in October last year.
Principal fish pathologist Brian Jones says the tests have not identified a cause for the deaths.
Source: ABC
Oyster Deaths Remain A Mystery
AUSTRALIA - The cause of a large number of oyster deaths that plagued Western Australia's pearling industry remains a mystery, despite months of tests by researchers.