A die-off of epic proportions has wiped out the whole fish population in a reservoir in Dong Nai province, southern Vietnam, raising alarms over the temperatures rising to unprecedented highs causing serious environmental changes in Southeast Asia.

Hit by a massive heatwave, fishermen are working against the clock to gather the hundreds of thousands of dead fish — around 200 tonnes — covering the 300-hectare Song May reservoir.

Vietnam’s southern region experienced a terrible drought in April, with temperatures soaring to 40 degrees Celsius (104 degrees Fahrenheit), distressing the farmers trying to sustain their crops.

Local residents and media are attributing the fish die-off to the drought, heatwave, and last but not the least the issues with the reservoir management.

Recent images show fish stacked on top of each other, covered in mud, desiccated, and disintegrating.

As there has been no rainfall for weeks, it is being theorised that he lake’s water level was too low for the animals to survive.

Nghia, a local from the province’s Trang Bom district, told AFP that the fish “died for lack of water,” adding: “Our life has been turned upside down over the past 10 days because of the smell.”

Even with numerous buckets and nets, removing all the fish appears to be almost an impossible task.

In images, the reservoir bed can be seen visibly dried up, cracked and muddied, with the clustered dead fish floating in the remaining water.

Nghia told AFP that the effort to renovate the reservoir included a pump being brought in to remove mud so that the fish would have more space to move.

The reservoir supplies water for irrigation of crops in the Trang Bom and Vinh Cuu districts, and its management had previously released water from it in an attempt to save crops downstream, Nghia said.

Tuoi Tre newspaper reported that extra water initially planned to be released into the reservoir for the fish was instead diverted downstream because of the “unrelenting heatwave,” according to AFP.

Authorities are investigating the mass die-off as efforts continue to remove the dead fish, AFP said.