Sharks may strike terror among swimmers at the beach in Hollywood movies, but the predators are increasingly ending up as prey, served up in fish-and-chips shops, sparking concern among environmentalists.

The great white shark, for example, is a protected species -- though that hasn't prevented its stocks from declining -- but tens of millions of other sharks are caught each year by fishermen.

Why are they in such demand? Their fins are the main ingredient in shark fin soup, a prestigious dish in traditional Chinese cuisine, and even in Europe shark meat is often served to consumers, usually without their knowledge.

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"People don't realize they're eating shark because it's not called shark, but they are," Sonja Fjordham, the head of Shark Advocates International, told AFP on the sidelines of an international conference this week on migratory species in Bergen, Norway.

The name can be misleading: "rock salmon" often sold in fish and chips shops in Britain, Australia and elsewhere is actually a small type of shark called spiny dogfish.

Ecologists' main concern is the practice known as "finning", when fishermen cut the fins off of sharks and then throw the fish back in the water, usually still alive and leaving them to a certain death by drowning, suffocation, blood loss or to be devoured by other fish.

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In Asia, where shark fin soup is a sign of status and social standing, a fin can cost several hundred dollars.

"It's as if you cut the arms and the legs off of a person. It's just a torso. Without fins, they can't swim, they can't breathe, they can't eat, they just sink to the bottom," explains Rebecca Regnery, the deputy director of the Humane Society International.

Finning, which is often carried out on by-catches but also targeted ones, weighs heavily on species that have slow reproduction patterns.

Bans on the practice exist in many countries but are often ignored.

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In a bid to help put an end to the practice, the European Commission recently proposed to tighten its legislation by requiring boats fishing in EU waters and EU-based ships fishing anywhere in the world to "unload sharks with the fins attached to their bodies."

"Banning finning is a no-brainer because it's such a huge waste," said Fjordham.

However, she added, "not finning alone is not going to save the sharks. We need to reduce the catches."

At the top of the food chain, sharks are indispensable for keeping the oceans' ecosystems in check. Scientists don't even dare consider the consequences if the animals were to disappear entirely, a fate faced by 20 percent of shark species, according to Fjordham.

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Palau, a small archipelago in the Pacific, has set a good example, making the most of its shark population by creating a sanctuary that has become a popular ecotourist site.

Diving with sharks now accounts for eight percent of Palau's gross domestic product (GDP), each animal bringing in 1.9 million dollars throughout its lifetime, according to the Australian Institute of Marine Sciences.

Palau remains however an isolated example. Elsewhere, the trend is alarming.

The giant manta ray, a cousin of the shark, has also fallen victim to a similar fate: prized in Asia, it is turned into a powder used in traditional Chinese medicine.

According to the UN Food and Agriculture Association, catches of manta rays have more than tripled in recent years, from 900 tonnes in 2000 to 3,300 tonnes in 2007.

In Bergen on Friday, the UN Convention on Migratory Species added giant manta rays to its lists of protected species.

Manta rays could generate some 100 million dollars in ecotourism revenue worldwide each year, according to experts.