SINGAPORE - One of the first otters that appeared in central Singapore has died, a community of otter watchers said on Thursday (June 8).

The father of a family of otters which live at the Singapore River began showing signs of illness last Thursday, said Mr Jeffery Teo, who is part of the Otter Working Group, made up of representatives from the public and various government and other agencies involved in the welfare of otters.

The formerly robust otter, estimated to be about seven years old, "got weaker and smaller, he was seen vomiting and struggled to swim and eat", he said.

The otter would also sleep on a grass patch while the rest of the family swam in the river, he added.

Otter watchers found blood in the otter's spraint, or dung, in the past week.

The last sighting of the otter by the group was on Wednesday at Jiak Kim Bridge. He was trying to eat but was too weak, and watching over the family but resting.

"He had no energy to join them," said Mr Teo.

Weak and lethargic otter dad at Singapore River at Tuesday (June 6) evening

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The Otter Working Group was informed and an operation involving Wildlife Reserves Singapore and the Animal Concerns Research & Education Society (Acres) was planned for this week.

"As long as dad was with the family, we could not isolate him because trapping would cause unforeseen stress to the rest of the otters," Mr Teo said.

They were closely monitoring the otter and planning how to isolate him, but it was too late.

The otter dad was not seen away from his family before this, but has been missing for more than 36 hours as of Friday morning.

OtterWatch, a Facebook page dedicated to Singapore's otters, posted a photo of the otter on Thursday afternoon, with the phrase, "In Memory of 1st Otter in our City. Marina Dad, 2010 - July 2017".

Some suspect that the otter might have accidentally ingested rat poison.

"Recently, Marina dad is looking very tired. Fresh blood has been observed from his poo these 2 days. We are concerned if he has been poisoned," Mr Nick Soo posted on Facebook on June 6.

"Anyone know of a way to ascertain whether he has been poisoned?"

A resident in the area had also posted on Facebook that her dog was poisoned.

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"My dog was recently poisoned along the Singapore River," she wrote on the Nature Society's Facebook page, saying that rodent poison had been left in the ground cover and bushes along the river. "(It) is a busy walking area with a lot of dogs, kids and otters. Any of them can easily eat the poison without leaving the concrete path," she wrote. "There were no signs to warn of this. Is this legal? And if so, how do we change this?"

The otter, affectionately known as Ah Huat to otter watchers at Gardens by the Bay, first appeared at the Gardens in 2013 with his mate, which gave birth to five pups in 2014.

They had two more litters since, and moved to the Singapore River last year after losing their territory at Gardens by the Bay to the Bishan otter family.

Smooth-coated otters can live from eight to 10 years in the wild.