Thousands of dead fish from coastal fish farms were found in Lim Chu Kang in the past few days, some in the sea and on the shore.
The Agri-Food and Veterinary Authority (AVA) said yesterday that about 90,000kg of fish from farms in the area had died due to the recent hot and dry weather spell.
It said the weather, combined with a lack of rainfall, resulted in low levels of dissolved oxygen in the waters near the farms. Rainfall churns the water, which helps to dissolve atmospheric oxygen in it.
The AVA added that other factors, such as a slight rise in the water's temperature, had also contributed. It has been monitoring the water's condition and had alerted the farms to take precautions such as installing standby aeration systems to add oxygen to the water, said a spokesman.
"As a result, most farms were not affected. The situation has since stabilised and returned to normal. We will continue to work closely with the farms to address any issues they may face," she said, adding that the dead fish came from four farms and included species such as milkfish and mullet.
Singapore has had other instances of mass fish deaths. In December 2009, for example, a plankton bloom killed 400,000 fish in farms off Pasir Ris and Pulau Ubin.
Fish farmers in Lim Chu Kang told The Straits Times that they use water pumps and other machinery to artificially churn the water when there are prolonged periods without rain.
However, this may not help if the natural oxygen levels are too low, said Mr Ong Kim Pit, who has run a fish farm in Lim Chu Kang for 20 years.
Some of the fish can still be sold, but fish farmers are supposed to hire private contractors to dispose of the remainder responsibly, added Mr Ong, who is in his 60s.
Nature enthusiast Ria Tan, who runs the WildSingapore website, told The Straits Times she saw dead fish strewn over the Lim Chu Kang mangroves yesterday morning.
"It looks like some of the fish farmers just dumped the dead fish into the sea and they washed up here," she said.
"It's disgusting. If you had a chicken farm and your chickens died, you wouldn't dump them all over the road."
