Authorities are investigating the incident while working to quickly remove the dead fish.
Hundreds of thousands of fish have died in a reservoir in southern Vietnam's Dong Nai province, with locals and media reports suggesting a brutal heatwave and the lake's management are to blame.
Like much of Southeast Asia -- where schools have recently been forced to close early and electricity usage has surged -- southern and central Vietnam have been scorched by devastating heat.
"All the fish in the Song May reservoir died for lack of water," a local resident in Trang Bom district, who identified himself only as Nghia, told AFP.
"Our life has been turned upside down over the past 10 days because of the smell."
Pictures show residents wading and boating through the 300-hectare Song May reservoir, with the water barely visible under a blanket of dead marine life.
According to media reports, the area has seen no rain for weeks, and the water in the reservoir is too low for the creatures to survive.
Reservoir management had previously discharged water to try to save crops downstream, Nghia said.
"They then tried to renovate the reservoir, bringing in a pump to take the mud out so that the fish would have more space and water," he said.
However, the efforts did not work, and shortly afterwards many of the fish died, with local media reports suggesting as many as two hundred tonnes' worth may have perished.
Tuoi Tre newspaper reported that the firm in charge of managing the lake had begun dredging in early 2024, initially planning to release extra water into the reservoir for the fish.
"But owing to an unrelenting heatwave, the investor released the water into the downstream area, leading to the water level going down. As a result, fish died en masse," the newspaper reported.
The reservoir is the water source for crops in the Trang Bom and Vinh Cuu districts of Dong Nai province.
Authorities are investigating the incident while working to quickly remove the dead fish.
"We hope authorities will do their best to improve the situation," Nghia said.
Southeast Asia bakes
According to weather forecasters, temperatures in Dong Nai province, 100 kilometres west of Ho Chi Minh City, reached around 40 degrees Celsius (104 degrees Fahrenheit) in April, breaking the record high temperature recorded in 1998.
The soaring temperatures are also impacting neighbouring Cambodia, where the high could reach 43 on the mercury.
On Wednesday Cambodian Prime Minister Hun Manet ordered schools to consider closing to protect teachers and students from the heat and put officials on stand-by in case of water shortages.
It follows the education minister on Tuesday ordering establishments to shorten morning classes and delay afternoon ones in an attempt to avoid the worst of the midday heat.
Hang Chuon Naron said the measures were "to prevent risks and to avoid illnesses that would harm the health" of students and teachers.
Meanwhile, in Thailand, electricity usage surged to new records on Tuesday as temperatures in the northeastern province of Udon Thani broke 44C.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)