Two newly-acquired manta rays at Resorts World Sentosa (RWS) have died, adding to the list of casualties at the Marine Life Park.
Since 2010, RWS has lost four of its 27 wild-caught bottlenose dolphins.
A Marine Life Park spokesman yesterday said the giant rays died while in quarantine, "despite the best efforts and round-the-clock care provided by the husbandry and veterinary teams".
She declined to give further details, but a source told The Straits Times that both creatures died earlier this year.
Marine animals are quarantined before being exhibited to ensure they are disease- and parasite-free, and to give them time to get acclimatised to the temperature and other water conditions.
News of the latest deaths comes after RWS' announcement last week that it is starting a conservation project for manta rays.
Only a handful of aquariums worldwide have these ocean giants because of their size - certain species can grow up to 7m in width and weigh more than two tonnes. They face a high risk of extinction in the wild.
The aquarium's three existing manta rays, which first went on show in 2012, are doing well, said the spokesman.
Animal welfare activists have for years spoken out against keeping large, wide-ranging animals such as dolphins and manta rays at the marine park, particularly those caught in the wild, because they do not fare well in captivity.
Mr Louis Ng, chief executive of advocacy group Animal Concerns Research and Education Society, said the group is "saddened by the loss".
"We are not against the park, if they keep species that are not as wide-ranging as dolphins and manta rays," he said, adding that reef fish, because of their small home range, may do better in captivity.