The provincial Department of Agriculture and Environment said on October 3 that officials had inspected coastal areas in Dong Trach and Bac Trach communes, where the dead fish were reported.
Seawater samples were collected for analysis to determine the cause. Officials did not take samples of the dead fish because they had already decomposed and were unsuitable for testing.
Local authorities were asked to continue monitoring the area and immediately report any new or unusual developments.
The dead fish were found along about 1.5 kilometres of coastline from the evening of September 30 to noon on October 1, according to the provincial Fisheries and Fisheries Surveillance Sub-department.
Most were small fish, mainly sardines, concentrated near the water's edge.
Local residents collected the dead fish in sacks and foam boxes before burying them to ensure environmental hygiene.
Authorities are investigating the cause.