Hot, dry spell likely behind mass fish deaths in Bishan

SINGAPORE - The deaths of hundreds of fish, found washed up along a river in Bishan-Ang Mo Kio Park, were likely due to the recent spell of hot and dry weather, experts said on Wednesday.

Residents spotted around 400 dead tilapia and mayan cichlids in clumps along the banks or against rocks along the 3km-long river on Tuesday.

Many were decomposing under the sun, causing a stench that lasted until Wednesday afternoon.

However, only two fish species were affected. When The Straits Times visited the river Wednesday morning, it was otherwise teeming with life. Numerous terrapins and other fish were seen.

The two affected species are commonly found in canals and reservoirs here, anglers said. Many of them are let loose by aquarium owners.

National water agency PUB said in a statement that it was notified about the dead fish on Tuesday morning and immediately sent contractors to clear them.

The two-day clean-up effort ended on Wednesday. It also collected water samples for testing and discovered reduced levels of dissolved oxygen in the water.

This was due to the recent hot, dry weather and may have caused the deaths of the fish, said a PUB spokesman.

Its tests also showed that the river's water quality is unaffected and there is "no contamination".

Dr Tan Heok Hui, a fish expert at the National University of Singapore (NUS), explained that dry weather could have lowered the river's water levels and increased algal growth.

This could have led to low oxygen levels as algae competed with fish for it.

The dead fish "could have a low tolerance to oxygen deprivation" compared to other species, he said. "Terrapins breathe air like we do and thus are not affected," he added.

Mass fish deaths like this are rare, said NUS marine biologist Chou Loke Ming.

Last July, thousands of fish in Lim Chu Kang farms were killed by low oxygen levels, while a plankton bloom killed 400,000 fish off Pasir Ris in 2009.

Long-time Bishan resident A.Tan, in his 50s, said he believed children - some of whom regularly play at the water's edge - should avoid the park's river "until it is cleaned up".

But the PUB assured the public that water activities at the river can continue and that it is monitoring the situation closely.

The picturesque river in Bishan-Ang Mo Kio Park is part of the Kallang River, which runs from Lower Peirce Reservoir down south to Kallang Basin.

It opened in March 2012 after having been converted from a concrete canal under a PUB programme to turn drains, canals and reservoirs into streams and lakes.

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