With information scarce, the incident has heightened concerns about damage to the environment, food safety and how businesses are regulated. State media raised questions about a waste pipe running into the sea from Formosa's steel plant being built in Ha Tinh province.
Formosa Ha Tinh Steel Corp. "is deeply surprised and sorry about" the fish deaths, it said in a statement on April 26, adding there is no proven link to the incident. It said its total investment in the Vietnam plant was US$10.5 billion, including US$45 million for a waste water treatment system. That system has regulatory permits and meets national standards, it said.
The Government has said there is no evidence for now to show Formosa was responsible. The fish may have been killed by toxins discharged by human activities or algal blooms known as a red tide, it said.
Even so, thousands protested including in the capital Hanoi. In Ho Chi Minh City, the crowd of 1000 carried signs reading "please return a clean sea to us" and "stop discharging waste water into the sea".
"We wanted to raise our voice to protect our environment," said Tran Thi Thu Nguyet, who joined the rally. Facebook posts also showed photos of protests in southern and central provinces.
Police detained two people they said collected and spread information on the internet to spur protests over the fish deaths, Vietnam Television reported, citing provincial authorities. One person allegedly entered the Formosa Industrial Zone last month to film and interview local people, it said.
Fishing and tourism have been hurt. Fishermen in central Quang Binh province have lost 115 billion dong (US$5.2 million), according to the local government. About 30 per cent of tourists cancelled visits to the province which houses Son Doong, the world's largest cave, during a four-day holiday starting on April 30, it said.
In an unusually candid response, the Government admitted it was slow to react, and new Environment Minister Tran Hong Ha publicly apologised. "The initial handling process was passive," the Administration said in its April 28 statement. Environmental monitoring was "untimely, inaccurate, and infrequent," in part as some systems weren't linked.
Anger spread on social media after a local Formosa official said Vietnamese should choose between their seafood industry or a steel plant. The company later apologised for the comment, according to local media.
Still, hundreds of people have used Facebook to urge the government to close the plant. "For people's health and our motherland protection, let's ask Formosa to compensate for the fishermen and kick it out of our country right away," said a user named Truong Thu Ha. More than 100,000 people signed a petition calling for US President Barack Obama to raise the issue with Vietnam in his visit this month.
"Vietnamese who are rarely consulted about and aware of investment projects face a big crisis of confidence," said Thi Nguyen, a Ho Chi Minh City-based independent environmental consultant. "Foreign investment is important but the Vietnamese Government shouldn't attract foreign investment at any cost, and this disaster proved that."
Vu Tu Thanh, the chief Vietnam representative of the U.S.-Asean Business Council, said the government must walk a line between assuaging public anger and not unnerving foreign businesses. Companies based in Vietnam from nations including Japan and the US are watching to see if the country moves to retroactively amend regulations, Thanh said.
Vietnam's Government has sought foreign investment to help it achieve this year's growth target of 6.7 per cent. The economy grew 5.46 per cent in the first quarter, versus 7.01 per cent in the last quarter of 2015 as income from crude oil and agriculture production dropped.
Taiwan was the third-largest investor in Vietnam in the first quarter, after South Korea and Singapore. It invested US$465.6 million in the period, according to Vietnam government figures.
I need to avoid sauce made out of dead fish before it spreads
Formosa has been the focus of public anger in Vietnam before. In May 2014 it and other Taiwanese businesses were attacked by protesters after China placed an oil rig in disputed waters off Vietnam's coast. Formosa said the riots, where one Chinese worker died of heat stroke, caused US$3 million in losses, and it received 30 billion dong in compensation.
"What the Government is worried about is if they are too vocal in criticising them, it will still stir up anti-Chinese sentiment and create trouble," said Burke from Baker & McKenzie, referring to Formosa. "But if they are not vocal enough, they are viewed as coddling these guys who are seen as being disrespectful of Vietnam and its environment."
The latest incident adds to concerns about food safety. Local media have reported recently that toxins are sometimes found in pork and vegetables. While the Government banned the use of the dead fish in central provinces for food or animal feed, many locals expressed concern it may be processed into fish sauce, a family staple.
"I'm stocking enough sauce for a year," said housewife Nguyen Thi Lan as she piled sauce bottles into her basket at a supermarket in Hanoi. "I need to avoid sauce made out of dead fish before it spreads."