The Grand Aquarium, one of the highlights of the new Aqua City zone that was officially launched yesterday, features milk fish manta rays and endangered species such as scalloped hammerhead sharks and bluefin tuna. They are among the 5,000 fish of more than 400 species in the aquarium containing 5.2 million litres of water.

The park admits 10 of the 80 endangered bluefin tuna died en route from Japan. The Hong Kong Dolphin Conservation Society accuses the park of lacking in transparency in revealing information about the deaths of some animals.

Society chairman Dr Samuel Hung Ka-yiu, who said he was quoting a park employee, said more than 10 of 40 hammerhead sharks had died too due to overcrowding in a quarantine pool.

But Una Lau, the park's public affairs director, said only just over 10 hammerhead sharks were imported.

Zeman, who denied that hammerhead sharks had died, said the deaths of the bluefin tuna were natural. 'It's quite normal ... people die and babies are born every day,' he said. 'Fish die and are born.

'The aquarium in Japan told us the tuna we got, which were caught by a reputable fisherman, were meant to be sushi. We actually saved them as they would have wound up being sushi in a restaurant.'

Hung said importing such fish created demand and led to unnecessary catching of the gravely endangered species. 'Why couldn't they import other species of tuna which are not endangered. The public can't differentiate between the species. The park is no longer the Ocean Park we knew from childhood.

'Driven by a business mentality, they aim for the rarest species for gimmicky effect. Bluefin tuna swim very fast and long distances. They can only make circles inside the aquarium. Their policy on animal acquisition, which states that acquisition of animals from the wild is pursued only if the wild population is sustainable, is a joke.'

Zeman said green groups always protested at the opening of aquariums. 'It's a good time to protest. It's good that green groups keep us on our toes, but we can't always do what they want. Some animal conservationists say we should not have animals in captivity ... [Ocean] pollution is serious [in Hong Kong]. People need to know about conservation. Criticising us is always easy ... if we are just about rides, we will just be Disney.'

The aquarium is double the size of the old one named Atoll Reef, which was closed last month for renovation and will house sharks in future.

At the entrance to the aquarium is a man-made blowhole, where ocean waves are simulated, and a touch pool containing starfish. An artificial lagoon in front of the aquarium is another highlight, where a show featuring pyrotechnics, water jets and laser lights will be put on every night.

Zeman points out another benefit of the new aquarium.

'People can get married in the tank if they have diving licence.'

It's not just the acquisition of hammerheads and bluefin tuna for Aqua City that greens are criticising. They don't like the park bringing in beluga whales for Polar Adventure, an attraction to open next year.

Hung said: 'Due to the big demand for the whales from the mainland, there are fewer numbers of the species in Russian waters. Russia doesn't know much about conservation. You can buy a permit for acquisition easily. The park is soon to be open, but they have yet to make public the research findings.'

The theme park earlier confirmed it had been funding Russian research on beluga whales in the Okhotsk Sea since 2007.

Sun Ho-yan, senior project officer with Green Sense, accused the park of causing more global warming by building a simulated polar area that will consume a lot of power to maintain the freezing conditions required.

Zeman said he expected a 15 per cent increase in visitor numbers once the new attraction opened. 'The Rainforest will open in May. This is a big year for Ocean Park.'