Two otters believed to be from the Pasir Ris family were found dead near the Changi Sailing Club.
A member of the public who discovered the otters on Wednesday alerted Facebook page OtterWatch yesterday morning.
Photos shared by OtterWatch show one of the otters covered in mud in a large metal cage under a boardwalk, while the other had been washed up on the beach.
Avid otter watcher Jeffrey Teo, who is part of OtterWatch, told The Straits Times that the otter in the cage was "in a bad shape".
It had cuts on its face and a monitor lizard was seen chewing on its body, he said, adding that the carcass will be handed over to the Wildlife Reserves Singapore for a post-mortem.
However, the carcass of the other otter was no longer there by the time volunteers arrived.
National University of Singapore biology lecturer N. Sivasothi, who heads the Otter Working Group, told ST that the otters were likely from the Pasir Ris family, comprising 10 adults and four pups.
They usually roam the waters off Pasir Ris and Changi. But in November last year, they made a surprise appearance on the tarmac of Changi Airport.
Mr Sivasothi said that the otters, just like other wild animals, could have been looking for food when they became trapped. He added that if fish had been used as bait, then this could have led the otters to enter the cage to get to it.
Members of the public can alert water agency PUB to suspicious activities in Singapore's reservoirs.
In June last year, a dead otter was found in a cage at Marina Promenade in the Kallang Basin. A man was caught later the same day as he was setting up traps in the area.
It is illegal to use trapping cages in reservoirs or waterways, even in areas where fishing is allowed.
Those caught trapping any animal or doing something that hurts fauna in a reservoir may be fined up to $3,000.