After a sudden downpour on April 4, Nguyen Tien Dung, 40, and his colleagues at the state-owned Ho Chi Minh City Urban Environment Co., Ltd (Citenco) had to spend two hectic days removing dead fish from the Nhieu Loc-Thi Nghe Canal, which flows through Districts 1, 3, Binh Thanh, Phu Nhuan, and Tan Binh.

They collected 14 tons of fish from a three-kilometer section of the 8.5-km (5.28 miles) canal, making it one of the most serious such incidents in almost a decade since Nhieu Loc-Thi Nghe was cleaned.

Dung and 20 others had rushed to the scene at 3:30 in the morning after being informed that dead fish were floating all over the canal.

He said: "Even the previous evening I had noticed a lot of dead fish in the water and its change of color. Though I’d come prepared, I was still shocked to see so many fish belly up all over the canal."

Since 2012, after taking over the job of cleaning the canal, he has been taking out dead fish almost every year soon after the rains arrive, but what he saw that morning left him dismayed: the fish covered almost the entire water surface visible between garbage and water hyacinth plants.

Dung and his team had to work as fast as possible since the huge volume of dead fish posed a serious threat to the water environment.

The city Department of Agriculture and Rural Development’s fisheries division provided a clear explanation for the mass death.

It said the sudden heavy rains flushed out a large volume of wastewater lying in the drains, which then covered the canal surface and significantly reduced the oxygen in the water.

The sudden rush of wastewater also stirred the layer of sludge at the bottom, which usually contains toxic gases. The gases were released and they rose up and dissolved in the water.

"The dual impact from below and above led to a sudden change in the fish’s living environment and killed them," the division said.

Nhieu Loc – Thi Nghe was once arguably the most polluted water body in the city.

A project to dredge and clean it began in 1993 at a cost of VND1.6 trillion ($69.3 million). Embankments and roads along its two banks were built and households living along it were relocated.

Ten years later an even bigger clean-up was undertaken, this time with sewers built to ensure wastewater was no longer released into the canal, with over $300 million provided by the World Bank.

But the story of fish dying en masse in the year’s first rains has become a regular one, the worst being in 2016, when 70 tons were collected.

Jolted by this, in 2017 the Steering Center for Urban Flood Control Program proposed installing an oxygen system in the canal at a cost of VND134 billion to save the fish.

But since it costs VND99 million ($4,290) a day to operate the system, the proposal remains on paper.

The Department of Natural Resources and Environment said the cost is unacceptable since the main function of the canal is to drain the city and not act as a fish farm.

According to the fisheries division, to avoid the annual fish deaths, the city needs to install a system to collect and treat wastewater all along the canal at a cost of VND40 trillion ($1.73 billion).

And with authorities still unable to arrive at a decision, Citenco has come up with its own temporary solution for the problem: reducing the canal’s fish population by catching them and releasing them in other rivers.

Truong Van Ho, head of the garbage collection team, said it is necessary for the company and his team to receive timely warnings so that they could shut the drains and let rainwater dilute the residual wastewater before flushing it into the canal.