Light glinted off the bodies of thousands of fish in Sungei Api Api river yesterday morning.
The mesmerising sight surprised many residents who lived near the river in Pasir Ris.
The mass of swimming fish also caused the water surface to ripple.
"I thought it was raining at first," said Mr Mohammed Rahim Dasmin, 46, who passed the 1.2km river while cycling to work at 7.30am.
The storekeeper has lived in the area for more than 10 years, but it was the first time he had witnessed such a sight.
Housewife June Chua, 60, was on her way to the market when the crowd gathered at the river caught her attention.
Distracted, she walked over and was amazed to see the sight of thousands of fish.
"It was very beautiful," said Madam Chua.
However, a different sight - and smell - greeted her when she passed the site again in the evening to pick up her grandchildren from school.
Then, the stench of dead fish washed up along the river bank hit her even before she saw them.
Another resident, Madam Wati Mansor, whose home faces the river, compared the pungent smell to that of a fish market.
"I was cooking chicken for lunch and thought to myself, 'Why is the smell of fish so strong?'"
The 47-year-old housewife said the foul smell lingered in her second-storey flat throughout the afternoon.
Ms Haydee Allanigue was jogging around her estate in Pasir Ris and was about 100m from the river bank when she detected the odour of dead fish.
Like many others, the 33-year-old Pasir Ris resident was puzzled.
"I know it's fish, but why are there so many here?" she asked.
When The New Paper arrived at the scene at 5.30pm, workers were already clearing away the fish.
A 31-year-old foreman, who wanted to be known only as Mr Wanz, claimed that his team had been hard at work since about 8am.
"I have no idea when we're going to finish," he said.
Used in cooking
He believed that the dead fish is ikan tamban, a species commonly used in cooking.
In a joint statement, the Public Utilities Board (PUB) and National Environment Agency (NEA) said they were informed of dead fish at the mouth of Sungei Api Api this morning.
When they arrived at the site, dead fish were also found along the beach at Pasir Ris.
They were likely to have been washed in by the tide and some of them were deposited on the river bank of Sungei Api Api.
PUB assures the public that this incident has no impact on drinking water quality.
The water from the river is not used for drinking.
NEA is investigating the cause of the incident.
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