Conservationists fear a falling shark population is prompting Asian chefs to look for manta and devil rays to help meet the voracious demand for shark fin soup.

Found in coastal waters throughout the world, rays present an easy target as they swim slowly near the surface with their huge wings. So far, they have escaped commercial exploitation and have been hunted only by small numbers of subsistence fishermen, who traditionally catch them using harpoons.

But the growing demand for the manta ray (Manta birostris) and its close cousin the devil ray (of the Mobula genus) is turning ray fishing into an export operation. In the eastern Indonesian port of Lamakera, catches of manta have rocketed from a few hundred to about 1,500 a year.

“Mantas and mobulas are being used as shark fin soup filler,” said Tim Clark, a marine biologist at the University of Hawaii. He said the cartilage was being mixed with low-grade shark fins in cheap versions of the soup. “The life history of manta rays makes them highly susceptible to overfishing,” he added.

With a life span thought to be well over 50 years, the fish reach sexual maturity only in their teens, at which time they produce one pup every one to three years.

While the rays, which are distantly related to sharks, are ending up in Hong Kong’s restaurants, their gills are also being used in traditional Chinese medicines. “The big market is for the gill elements,” Mr Clark added. “They are dried, ground to a powder and used in traditional Asian medicines.”

Reaching sizes of up to 7m (23ft) from wing tip to wing tip, the manta’s branchial gill plates, which filter plankton from seawater, constitute a tiny portion of a body that can weigh up to 2½ tonnes. The plates can fetch up to £200 on the street in China. Practitioners of traditional Chinese medicine claim that gill rakers — called peng yu sai — reduce toxins in the body by purifying the blood.

Eli Michael, of the Manta Pacific Research Foundation, said Hawaii is poised to outlaw catching or killing mantas. Until now, getting caught in nets intended for other fish has been the biggest threat to rays, listed as “near threatened” on the International Union for Conservation of Nature (IUCN) Red List.

Overfishing is also a problem in Britain and Europe. Ali Hood, of the Shark Trust, said: “In European waters, particularly the Mediterranean, the giant devil ray is classified by the IUCN Red List as ‘endangered’. The large skate, found in UK waters, has been exploited for decades, leading to alarming declines, and species such as the common skate are now critically endangered.”

The market for shark fin soup is growing at about 5 per cent a year, while shark populations are crashing: 80 per cent of Atlantic sharks have been lost in the past 15 years, according to the trust. Britain is one of only five EU member states that still allows the removal of shark fins at sea. More than 80 tonnes of fins are landed in Britain every year.

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