Thousands of dead grey mullet, a popular farmed fish, washed up at Sungei Buloh Wetland Reserve yesterday, startling visitors there.
In the afternoon, as the high tide went out, dead fish brought in by the tide were left along river banks, though native halfbeak fish in the Sungei Buloh Besar river seemed unaffected.
The Agri-Food and Veterinary Authority also said Singapore's coastal fish farms in the western Johor Strait had not been hit by die-offs, and no plankton blooms or any abnormalities had been detected.
The agency cautioned it would take enforcement action against farms found to be illegally dumping farm waste into the water.
Meanwhile, the National Parks Board said it would be removing the dead fish at the reserve.
At the Lim Chu Kang jetty, fish farmers said their farms were not hit, and suggested the dead fish could have been washed over from Malaysia.
Some were taking precautions such as lowering their nets so fish could swim deeper for more oxygen.
Earlier this year, 39 fish farms on both sides of the Johor Strait suffered mass fish deaths due to low oxygen levels in the water, and some 160 tonnes of fish died.