Nghe An’s Department of Agriculture and Environment said on Tuesday that the provincial Fisheries and Fisheries Surveillance Sub-department had issued a preliminary report on the incident, which affected fish farms in Anh Son Commune.
Heavy rain and thunderstorms hit the area on the evening of May 3. By the following morning, the Lam River had turned yellowish and murky, although no unusual odours or oil slicks were detected.
Farmed fish species including grass carp, black carp and catfish suddenly stopped feeding, surfaced and died rapidly, with mortality rates reaching nearly 100 per cent, authorities said.
The dead fish showed no external abnormalities or signs of haemorrhaging, although mucus was found on the gills.
Some wild fish species in the river, including carp, loach and bagrid catfish, were also seen gasping at the surface before washing ashore dead. The incident stopped later the same day.
Local authorities said 13 households operating 19 fish cages were affected, with total losses exceeding 8.6 tonnes.
Residents collected the dead fish and either sold them for VND 50,000-VND 60,000 (USD 1.9- USD 2.3) per kilogramme or distributed them to local villagers.
Fish farmers said cage farming had been carried out on the Lam River in the area since 2009 under standard technical procedures, with no previous cases of seasonal mass fish deaths typically associated with disease outbreaks.
Tests conducted on May 5 showed key water-quality indicators remained within acceptable levels for freshwater fish, including transparency of 40 cm, pH of 7.5, alkalinity of 110 and dissolved oxygen levels of 5 mg per litre.
However, based on the rapid and widespread deaths affecting both farmed and wild fish, officials said the incident was likely caused by sudden environmental fluctuations that reduced water quality, particularly dissolved oxygen depletion or the presence of toxic gases.
“This was an environmental incident affecting a wide area, not a typical disease outbreak,” the fisheries agency said in its report.
The agency added that strong river currents and the rapid development of the incident had complicated efforts to determine the exact cause, while no live fish remained for further sampling and analysis.
Authorities have proposed inspections into potential wastewater discharges near Kim Nhan 1 village in Anh Son Commune and said responsibility would be considered if violations were found.