Dead fish were spotted in Sungei Buloh Wetland Reserve earlier this week, with parts of Sungei Buloh Besar turning black.
There was also a strong stench - not unlike the smell of rotten eggs - along the main bridge in the reserve when The Straits Times visited yesterday.
Despite several decomposing fish carcasses on the banks of Sungei Buloh Besar, halfbeaks, a native breed of fish, were still seen swimming in the river.
Housewife Sharon Choy, 50, a frequent visitor to the reserve, said: "On Tuesday, when I looked out from the main bridge, I could see the dead fish lining the banks of the river. I told my husband, even if 100 people came to help, they would not be able to clean everything up."
The water, however, was still clean and had its "usual colour" when she visited.
Nature enthusiast Ria Tan, who runs wildsingapore.com, a wildlife website, said she saw about 40 dead fish along the river yesterday morning.
Most of them looked like the sort of large market-size farmed grey mullets, she added.
National University of Singapore biology lecturer N. Sivasothi said mass fish deaths are usually due to two factors - low oxygen levels and poisoning.
Low oxygen levels could be a result of increased algae growth, with algae competing with fish for oxygen. If there are large amounts of fertiliser or sewage upstream, the fish could have been poisoned and flowed into Sungei Buloh.
It was important to monitor the water quality of the Johor Strait, and ensure that offshore fish farms have proper waste disposal facilities, he added.
A spokesman from the Agri-Food & Veterinary Authority of Singapore (AVA) said there has been no report of abnormal fish deaths from our coastal fish farms so far. "We are investigating the fish deaths in Sungei Buloh with the National Parks Board," he added.
The National Parks Board yesterday said it is removing the dead fish from the reserve.
In a similar incident in April, thousands of dead grey mullet were washed up in the wetland reserve.
The AVA said then that Singapore's coastal fish farms in the western Johor Strait had not been hit by die-offs and that there had been no plankton blooms or any abnormalities detected.
The agency had also cautioned that it would take enforcement action against farms found to be illegally dumping waste in the water.
Additional reporting by Priscilla Goy
This article was first published on July 25, 2014.
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