Close to a thousand fish went belly up at Bishan-Ang Mo Kio Park on Monday night, in a repeat of a similar incident last year.
National water agency PUB said that the latest incident at Kallang River, which runs through the park, affected about 800 to 900 fish of varying sizes. They were mainly cichlids, a diverse family which includes the popular aquarium fish luohan and the food fish tilapia.
Fish specimens have been sent to the Agri-Food and Veterinary Authority of Singapore for examination, and investigations are ongoing, said a spokesman for PUB. It is monitoring the situation.
While the cause of the mass fish deaths is a mystery for now, experts The Straits Times spoke to suspect that, like the incident in February last year which killed about 400 fish, it could be related to the hot weather and reduced rainfall.
Dr Tan Heok Hui, a fish expert from the National University of Singapore's (NUS) department of biological sciences, explained that this could have led to low oxygen levels in the water. When there is less rain, the water becomes clearer, so sunlight can better penetrate and algae grows faster, explained Dr Tan, who is also a museum officer at the Lee Kong Chian Natural History Museum.
Water samples that the PUB collected for testing showed that the raw water quality, including dissolved oxygen levels, was within the normal range, it said.
The agency received a report of dead fish at around 7.40pm on Monday, and cleanup operations since then have been largely completed, said the PUB yesterday.
"As of now, the water quality of the river in Bishan Park is normal, live fishes can be seen in the water and the public can continue with their activities at Kallang River at Bishan-Ang Mo Kio Park," said the PUB spokesman.
When The Straits Times visited the park yesterday morning, cleaners were still removing the dead fish. Residents in the area said that the dead fish gave off a pungent smell which wafted over to the main road nearby.
Bishan resident Chen Shanshan, 24, who lives at the block next to the park, said that she could see many dead fish floating in the water even from her eighth-floor flat.
"I saw passers-by gawking and taking pictures and then later, when I was crossing the road, I could smell the dead fish stench," said the communications associate.
Dr Chou Loke Ming, adjunct research professor at the Tropical Marine Science Institute at NUS, said the mass fish deaths are worrying, and that it could happen again.
"Most would point to the changing climate and that may indeed be true, but we need to investigate to be sure," he said.