One of 25 dolphins being transferred to a Singapore oceanarium despite protests from activists died during its flight to the city-state on Thursday, the resort said.

Wen Wen, a male dolphin aged about 10, died suddenly less than an hour before the flight from the Philippines landed, a Marine Life Park spokesperson told AFP in a statement.

The spokesperson of the park -- which opened to the public for the first time earlier Thursday and is part of the Resorts World Sentosa (RWS) casino -- said the dolphin appeared fine when medically examined before the flight.

"We are deeply saddened... he will be sorely missed," the spokesperson said.

The other 24 bottlenose dolphins had arrived and were acclimatising to their new home.

"No effort or resources will be spared in ensuring the health and well-being of our dolphins and all marine animals at Marine Life Park," the statement said.

Wen Wen is the third dolphin to die out of 27 which RWS acquired from the Solomon Islands in the South Pacific between 2008 amd 2009.

Wildlife activists in the Philippines -- where the dolphins were kept and trained before being exported to Singapore -- filed a lawsuit last month to stop them from being flown out.

They said the dolphins' capture violated an international treaty on the trade in endangered animals and plants.

A court in the Philippines initially agreed to a temporary ban on transferring the dolphins but another court overturned it.

A Singapore-based animals rights group also opposea the inclusion of the dolphins in the marine park, saying catching them from the Solomon Islands is detrimental to the survival of the species there.

The remaining 24 dolphins are due to make their public debut at the park's twin attractions the S.E.A Aquarium and Adventure Cove Waterpark only next year.

The aquarium is touted as the world's largest with 100,000 marine animals from over 800 species in 45 million litres (12 million gallons) of water.