Pacific stocks of the oceanic whitetip shark, a favourite of fin soup enthusiasts, sank by as much as 17 percent a year between 1995 and 2010 despite catch and finning limits, a study said Wednesday.

And the north Pacific blue shark, also sought after by Asian chefs, showed a worrying population decline of about five percent per year, according to the research published in the journal Conservation Biology.

Using data collected by onboard observers of catches in the western and central Pacific over a 15-year period, the study also revealed a decline in shark size -- a key indicator of overfishing.

"These results... heighten concerns for the sustainability of Pacific shark populations," said a statement.

The drop in whitetip sharks represented a "severe decline" for a species that does not reproduce quickly, study co-author Shelley Clarke told AFP.

"It was also very surprising to see a sharp decline for North Pacific blue fin sharks which are relatively much more productive than the oceanic whitetip sharks".

The research was conducted by the oceanic fisheries programme of the Secretariat of the Pacific Community (SPC) -- a regional intergovernmental body.

The findings suggest that bans on finning -- the practice of slicing the fins off a living shark and dumping it back into the water to die -- were not helping to reduce the numbers being killed.

This was "likely due to a combination of poor enforcement and increasing markets for shark meat," said the statement.

The oceanic whitetip, which lives in tropical waters, is the only shark species subject to catch limits in the Pacific.

According to conservation group WWF, about 73 million sharks are killed every year, mainly for their fins.

Hong Kong imports about 10,000 tonnes per year, most of which is re-exported to mainland China as the demand for shark fin soup continues to grow and the number of threatened species has soared from 15 in 1996 to more than 180 in 2010.

Shark fin soup is viewed as a delicacy by many in Asia and is traditionally served at wedding parties and business banquets in Hong Kong, which handles around 50 percent of the global fin trade.

Sharks are slow-growing and maturing animals, and produce relatively few young, making them particularly vulnerable to overfishing.

"These findings underscore conservationists' messages that most finning bans are not properly enforced, and alone are not sufficient to reverse shark population declines," said Sonja Fordham, president of Shark Advocates International.

The United States banned finning in its waters in 2000 and several American states have banned the trade in shark fins.

The European Union has had a finning ban since 2003, but in March endorsed even tighter shark fishing rules that would force fishermen to bring sharks to port intact.