By Victoria Gill
Science reporter, BBC News
Endangered hammerhead sharks are often caught for their valuable fins
Many species of open ocean shark are under serious threat, according to an assessment by the International Union for Conservation of Nature (IUCN).
The Red list gives the status of 64 types of shark and ray, over 30% of which are threatened with extinction.
The authors, IUCN's Shark Specialist Group, say a main cause is overfishing.
Listed as endangered are two species of hammerhead shark, often subject to "finning" - a practice of removing the fins and throwing away the body.
This is the first time that IUCN Red List criteria, considered the world's most comprehensive inventory of the conservation status of plants and animals, have been used to classify open ocean, or pelagic, sharks and rays.
The list is part of an ongoing international scientific project to monitor the animals.
The authors classified a further 24% of the examined species as Near Threatened.
Sharks are "profoundly vulnerable" to overfishing, they say. This is principally because many species take several years to mature and have relatively few young.
Open ocean sharks are caught in high seas tuna and swordfish fisheries
"[But] despite mounting threats, sharks remain virtually unprotected on the high seas," said Sonja Fordham, deputy chair of the IUCN Shark Specialist Group and one of the editors of the report.
"[We have] documented serious overfishing of these species, in national and international waters. This demonstrates a clear need for immediate action on a global scale."
The UN Food and Agriculture Organization (FAO) recognised the potential threat to sharks over a decade ago, when it launched its "International Plan of Action for the Conservation and Management of Sharks" in 1999.
But the "requested improvements fisheries data from member states... have been painfully slow and simply inadequate", according to this report by the IUCN.
"There have been improvements here and there but overall progress hasn't been as swift as we might like to see," said Jorge Csirke, director of the FAO's Fisheries and Aquaculture Management Division.
"Very often the reason is not a lack of will - although that can be a problem - but a lack of know-how and, most critically, a lack of budgetary resources for improving fisheries data collection.
Many pelagic sharks are caught in high seas tuna and swordfish fisheries.
Although some are accidentally caught in nets meant for these other fish, they are increasingly targeted for their meat, teeth and liver oil, and because of high demand, particularly in Asia, for their fins.
"The hammerheads are special because they have very high quality fins but quite low quality meat," explained Ms Fordham. "They often fall victim to finning."
She told BBC News that, although finning is widely banned, this ban is not always well enforced.
"The EU finning ban is one of the weakest in the world," she said.
"The best, most sure-fire way to enforce a ban is to prohibit the removal of fins at sea.
"But in the EU, you can remove them, providing the fins you bring ashore weigh less than 5% of the weight of the bodies."
This rule was designed to prevent finning, but it provided "wiggle room", said Ms Fordham.
"The IUCN has estimated that, under these rules, you could fin and discard two to three sharks for every shark you keep, " she explained.
The European Commission agreed that EU policy on finning "contained a number of loopholes".
"That's why in February we proposed to strengthen the finning ban and close these loopholes," the commission said in a statement.
Species listed as Vulnerable included the smooth hammerhead shark, the porbeagle shark and the common, bigeye and pelagic thresher sharks.
Fisheries have fought to keep their right to fish porbeagle sharks because their meat is so valuable, according to Ms Fordham.
"Yet we've already had recommendations from scientists that there should be no fishing of these sharks."
For certain species - that are considered particularly vulnerable - the authors have recommended their complete protection.
"The big-eyed thresher shark, for example, is very slow growing," explained Ms Fordham.
"Fishermen can very easily identify it, because it has a very big eye. So if they catch it accidentally, they can throw it back.
"These sharks tend to survive well when they're thrown back."
By the end of this year, the Shark Specialist Group will publish a complete report, outlining the status of all 400 species of shark, and closely-related skates and rays.