SINGAPORE - 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 fish started surfacing in the sea about two or three days ago, said Mr Simon Ho, communications officer at the Singapore's Fish Farmers Association.

"Because of the higher temperatures these few days, there's a lack of oxygen in the water. I believe the fishes 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 fishon average - estimated to be worth from $3,000 to $4,000.

Mr Ho said deaths like this one 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.

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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 Straits the worst-hit.

A bloom happens when the plankton multiply rapidly due to warmer temperatures in the water. Plankton suck 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 could happen."