The deaths have affected farms operated by 50 households in Ngoc Son Ward since August 26, according to the provincial Department of Agriculture and Environment.
The affected clams, numbering around 150-160 per kilogramme, were stocked at an average density of about five kilograms per square metre.
Prolonged heavy rain associated with Storm Narra was the likely cause. Large volumes of freshwater flowing into the estuary caused a sudden drop in salinity, putting the clams under environmental stress and leading to mass deaths.
Authorities have collected three water samples and three clam samples for environmental and disease testing to determine the exact cause.
They have been instructed to clear dead clams from farming areas to prevent pollution and minimise further losses.
Farmers have also been advised to harvest market-sized clams, reduce stocking density in areas with smaller clams and suspend new stocking until water conditions improve.
Provincial agencies will continue monitoring weather and water quality in the farming areas and report developments daily.