The fish visitors watched through the aquarium's glass panels were also more numerous: their population rose 68 per cent from 6,667 to 11,233 during the year. Correspondingly, the number of deaths also rose, by 113 per cent, from 572 in the last financial year to 1,217.

They included many small species such as goldfish, tetra, razorfish and milkfish with a natural lifespan of two to four years.

"Almost all of the animals lost over the past year were due to natural causes," the report said.

Apart from the fish, four rodents and 42 birds died of old age while 27 baby snakes were lost because of a viral infection before their birth.

A Californian sea lion died during anaesthesia for an operation to desex it.

"The sea lion was a female named Baja which arrived in Hong Kong in 2000," a spokeswoman said. "The animal had suffered cramps due to a brain disorder since 2005, and was deemed not suitable to give birth."

Dr Samuel Hung Ka-yiu, from the Hong Kong Dolphin Conservation Society, said statistics made public fell short of crucial information.

"There is no breakdown in species [for mortality]. Of course we care less about goldfish, but at the same time we have no idea whether a bluefin tuna or a hammerhead shark would be among the deaths," he said.

The park said revenue rose 28 per cent to HK$1.6 billion, while the surplus from operations jumped 49 per cent to reach HK$505.4 million.

However, the overall surplus for the park, a non-profit statutory body, dropped from HK$105.1 million to HK$103.3 million because of higher depreciation and finance costs associated with the debut of new attractions.