More than 200 tonnes of fish, mainly grass carp, silver carp, common carp and catfish, died in floating cages on Tri An Lake between May 30 and 31.
The mass die-off has left many fish-farming households facing devastating losses, with damage estimated at hundreds of millions of Vietnamese dong for each family.
Local authorities have assisted residents in collecting the dead fish and selling them to producers of organic fertiliser in an effort to recover part of the losses while mitigating environmental impacts.
Le Van Quyet, a fish farmer in Tri An Ward, said more than five tonnes of fish that were close to harvest died suddenly.
"Only a few days remained before traders were due to purchase the fish, but the first heavy rains of the season wiped out the entire stock. This is the first time in more than 10 years of fish farming that my family has suffered such serious losses," he said, estimating damages at more than VND 250 million (approximately USD 9,600).
Another farmer, Nguyen Van Dien, said his losses were lower thanks to preventive measures.
"Earlier this year, I installed oxygen aeration systems for the fish cages. As a result, losses during the recent heavy rains were limited to nearly two tonnes of fish," he said, estimating damages at more than VND 100 million (approximately USD 3,800).
Authorities in Dong Nai Province have instructed relevant agencies to urgently determine the cause of the incident and dispose of the dead fish to protect the water quality of Tri An Lake, a key source of domestic water supplies for millions of residents in Dong Nai and Ho Chi Minh cities.
According to experts, the primary cause of the fish deaths was a sudden change in water conditions following the first major rains of the season. Heavy rainfall washed soil, suspended solids, agricultural chemicals and fertilisers into the lake, sharply reducing dissolved oxygen levels.
The sudden environmental shift left aquatic species unable to adapt, resulting in widespread suffocation. Experts also noted that fish stocking densities in some floating cages exceeded recommended levels, further increasing vulnerability.
By June 1, the volume of dead fish had fallen significantly, with only about 10 tonnes recovered. However, strong odours and large numbers of flies around collection sites remained a pressing concern.
Authorities have disinfected affected areas to protect public health and maintain environmental hygiene.
Tri An Ward authorities are continuing to assess the extent of the losses and will submit a report to provincial authorities to consider support measures for affected households.
Covering approximately 323 square kilometres, Tri An hydropower reservoir is the largest artificial lake in southern Vietnam. It plays a vital role in electricity generation, water supply, agricultural irrigation, salinity control and flood regulation along the downstream Dong Nai River basin.
Officials said the mass fish mortality incident has not only caused substantial economic losses but could also affect the region's ecosystem and water resources.
Authorities are continuing efforts to collect dead fish and investigate the cause of the incident.