A Singapore casino resort's acquisition of dolphins from the Solomon Islands for its marine park has contributed to the depletion of the species there, an animal welfare group said Friday.

Singapore-based Animal Concerns Research and Education Society (ACRES) released a video on the depletion of the Indo-Pacific bottlenose dolphins as part of a campaign to pressure Resorts World Sentosa (RWS) into freeing them.

RWS acquired 27 dolphins from the Solomons between 2008 and 2009 despite the availability of information "advising against the purchase", ACRES said in a statement.

It cited a report by the International Union for Conservation of Nature that said catching dolphins from the islands would be detrimental to the survival of the species there.

The dolphins, now down to 25 after two died, are currently in the Philippines for training and will be shipped to the marine park in Singapore sometime next year. The park is set to open without them later this year.

The large mammals were at the centre of a legal tussle this week after a Philippine court temporarily blocked their transport to Singapore following a civil suit filed by animal-rights activists.

They alleged the dolphins' capture from the Solomon Islands violated an international treaty on the trade of endangered animals and plants.

However, the Philippine government said Thursday it has approved the export of the dolphins after the ban on transporting them was lifted.

Resorts World "should have performed due diligence before they acquired the dolphins", said ACRES chief executive Louis Ng, who called for the animals to be released back into the wild.

A spokesman for the RWS Marine Life Park said the resort will comply with all the requirements of an international treaty on the trade of endangered animals and plants.

"In the lead up to the opening of Marine Life Park, we are sparing no effort and time to ensure our thousands of marine animals get the best care they could possibly get," the spokesman told AFP.

"We urge ACRES to focus on areas where it can constructively contribute to marine conservation, rather than engage in online antics to encourage netizens to harass or heckle us and our Facebook fans."

The spokesman stressed that the park's team of experts and specialists are animal lovers themselves who have worked in reputable zoological facilities across the world.