The warmer weather in recent days may have caused thousands of fish to die in waters near Lim Chu Kang jetty and in nearby fish farms.
The dead fish started surfacing in the sea about two or three days ago, said Mr Simon Ho, communications officer at the Fish Farmers Association of Singapore.
"Because of the higher temperatures these few days, there's a lack of oxygen in the water. I believe the fish died as a result," he noted.
He added that fish farmers in the area sent motorboats out to remove and dispose of the dead fish.
About four or five farms are said to have been affected, with each reporting a loss of about one to two tonnes of fish on average - worth $3,000 to $4,000.
A spokesman for the Singapore Food Agency (SFA) said preliminary investigations suggested that low levels of dissolved oxygen were the cause.
He added that the SFA has worked with farmers to develop contingency measures for such events, such as providing aeration and reducing feeding or not feeding the fish.
Mr Ho said deaths like this happen "one or two times in a year" and the phenomenon is "not unusual".
He added that some fish farms have machines to replenish the oxygen in the water.
Mr Ho, who has been a fish farmer for around 10 years, said it is possible to tell with experience that the oxygen conditions in the water are below normal levels. Fish behaviour can alert farmers as well.
"When the oxygen in the water is insufficient, some of the more active fishes will move less than usual," he added.
A mass death occurred in 2015 when around 600 tonnes of fish died in a span of two weeks due to a plankton bloom, with farms near the East Johor Strait the worst hit.
A bloom happens when plankton multiplies rapidly due to warmer temperatures in the water. Plankton sucks oxygen from the water, threatening marine life.
It is unclear if the fish deaths this time occurred because of the same reason.
"These are costs that small fish farms like us have to bear from time to time," Mr Ho added.
"With nature, you never can tell. Anything can happen."