Cathay Pacific's policy banning cargoes of non-sustainable shark fin will not come into effect until next year, about 18 months after it was announced, marine experts drawing up the guidelines say.

The experts defended the carrier's drawn-out approach, saying Cathay had "grasped a serious nettle" and that its stance would be more effective for marine conservation in the long term.

Cathay said on September 4 last year it would no longer carry shark fin from unsustainable sources, saying it expected to implement the policy within "approximately three months".

Its lead has since been followed by other airlines including Qantas, Air New Zealand and Emirates, which announced immediate blanket bans.

Cathay, by contrast, still carries shark fin, although it has stopped signing new contracts and volumes have plummeted from 300 tonnes a year to around six tonnes.

The hold-up has baffled some environmentalists who are wary of the airline's intentions and say they cannot understand why it does not impose an immediate ban until it defines sustainability.

But the three global experts working with Cathay - Professor Colin Simpfendorfer, Dr Nick Dulvy and Glenn Sant - insist the airline's approach will help generate demand for sustainable shark fin.

"Unless there is that demand from the consumer for a sustainable product, there is no incentive to transform the fishing industry and that's what I think is very exciting about Cathay's position," said Dulvy, co-chair of the International Union for Conservation of Nature's Shark Specialist Group.

"They are potentially creating this demand for sustainable, traceable shark fin.

"Cathay Pacific has grasped a very serious nettle. It could have chosen to take a prohibitionist stance and said, 'This is too complicated'. I think it is very interesting that Cathay Pacific is trying to throw a bone to the shark fin dealers in Hong Kong, to say, 'We will try to work with you to find your product, but we are going to work on our terms and those terms are sustainability and traceability'."

Sant, global marine programme co-ordinator of Traffic International, said: "I think there's a lot of merit in the way Cathay has gone about this. It is quite a simple argument to say, 'We will just ban everything. It's too difficult to do'."

The complexities of defining sustainable sources meant it would take at least until early next year for Cathay to receive the experts' recommendations, said Simpfendorfer, director of the Centre for Sustainable Tropical Fisheries and Aquaculture and the School of Earth and Environmental Sciences at James Cook University in Australia.

"One of the complicating factors here is that sustainability is a word that means many things to many different people. We need to be very careful how we define sustainability," he said.

Cathay's head of environmental affairs, Dr Mark Watson, admitted the airline had taken a difficult route by seeking to reach its own definition of sustainable sources.

"Perhaps we have chosen a challenging path, but we believe it is the right path," Watson said.

"It's obviously complicated, it's new and it's challenging, but with the expertise and guidance we have, I think we will get there.

"I don't doubt there is a lot of work and effort that needs to be put in, but we have the commitment within the company to do that."