Animal Concerns Research & Education Society, or Acres, wants all dolphins in Underwater World's Dolphin Lagoon to be returned to the wild.

Five of the Indo-Pacific humpbacked dolphins came from Thailand. One male was born at the lagoon last November.

'We are targeting Dolphin Lagoon first as we want to clean up our own backyard, before approaching other dolphinariums,' said Acres president Louis Ng.

He said dolphins are often deprived of food to encourage them to perform, and research done on captive dolphins does not reflect what happens in their natural environment,'so there is no need to keep them in captivity'.

Mr Ng, a biology graduate, started Acres in 2001 to improve the welfare of animals in captivity and foster compassion. It now has more than 100 members, he said.

They have started a month-long 'Suffering, Not Smiling' campaign, to stop 'suffering and continued exploitation of dolphins' and teamed up with the World Society for the Protection of Animals to spread their message to marine institutions around the world.

But Underwater World's curatorial director, Mr Bruce Mackay, said: 'Jumbo is not aggressive. Its behaviour is natural instinct during mating, where it will compete with the other male dolphin for attention.

'And we removed its teeth because they were fractured or decaying'.

As for research work, they have been studying the reproductive cycle of the pink dolphins, which ultimately contributed to the birth of Splash last November, he said.

The dolphins are fed 8kg of food, five times a day, and never deprived of food to induce a performance.

Said Mr Mackay: 'Every day, the valves to the lagoon are opened to allow fish to swim in, and the dolphins have been hunting these fish, on top of the food we provide.'