More than 2,800 people have signed an online petition at change.org calling for an end to the flights, citing a China Daily newspaper report about a January 16 delivery of five dolphins from Osaka to Hanoi.

The dolphins are believed to have come from the Japanese town of Taiji, the scene of an annual dolphin slaughter depicted in Oscar Award winning documentary "The Cove", the report said.

"Five Taiji dolphins were transported via cargo flight in 'flying coffins' on January 16, 2012. They spent at least seven hours in this cruel confinement," the petition reads.

"Dolphins are neither cargo, nor commerce, nor entertainment." An internal memo to airline staff made no mention of the animal welfare considerations but described the flight as a success that earned HK$850,000 ($110,000) in cargo revenue.

"The smooth handling of such special cargo which is time sensitive and vulnerable demonstrates that Hong Kong Airlines cargo handling capability has further improved," says the memo cited by the China Daily.

"Based on the experience we have obtained this time, Hong Kong Airlines cargo will develop the business onwards."

It included a photograph of the dolphins lying in shallow, narrow containers inside the belly of a Boeing 733F cargo plane.

Hong Kong Airlines representatives did not immediately respond to requests for comment, however a spokesman told the South China Morning Post website that the company was "committed to the protection of animal welfare".