June 25 (Bloomberg) -- Great whites, hammerheads and a third of deep-sea sharks and rays face extinction as the world’s fishing fleets haul them in for their meat and fins.

Twenty species of sharks and rays are threatened, four more than a year ago, the Gland, Switzerland-based International Union for the Conservation of Nature said today in a report. Ocean predators most at risk are those that reproduce infrequently or recover slowly from commercial harvests, the organization said.

“Sharks are very vulnerable to over-fishing because they tend to grow and mature slowly and have a small number of young,” Sonja Fordham, co-author of the study, said in a phone interview. “Demand for shark fins is a driving force in over- fishing of sharks worldwide.” Fins are a delicacy in Asia.

Shark populations may collapse in coming decades along with commercial fisheries for salmon, tuna and mackerel because of scant protections. Excessive catches already led to the extinction of Atlantic salmon and European oysters in the Wadden Sea off the Netherlands and the Atlantic sturgeon in the U.S.’s Chesapeake Bay, the conservation group WWF says.

Sharks are often snared in nets meant for tuna. An estimated 10.7 million blue sharks are killed annually for their fins, many of which are sold at the Hong Kong shark fin market, according to the report. Basking shark fins are used as signboards for restaurants serving shark-fin soup, Fordham said.

While governments have tried to halt the extinction of marine species by limiting open seasons and banning nets that catch unwanted species, many sharks are reeled in over international waters where national governments have no right to control catches, the IUCN said.

“No Limits”

“We have a growing awareness and growing evidence of a serious declines in these species, and yet there are still no international limits on shark catches,” said Fordham, policy director in Brussels for the Shark Alliance group.

Two hammerhead species -- scalloped and great -- are deemed “endangered,” the second-highest degree of threat, according to the survey. Among rays, a cousin of sharks with flattened bodies and wing-like fins, the giant devilray is endangered as well, the study said. Sixteen other species are vulnerable to extinction, including the porbeagle, thresher, whales shark, and great white, which gained notoriety as a vicious killer in the Steven Spielberg movie “Jaws,”

Some populations of hammerhead sharks have declined as much as 99 percent in heavily-fished regions such as the Northwest Atlantic and Mediterranean Sea, according to today’s report. While demand for shark meat is “strong” in Europe, in China and other parts of Asia, fins are sought for soup, Fordham said.

“Good for Vitality”

“Shark-fin soup is supposed to be good for your vitality and also a status symbol served at weddings and celebrations,” she said. “Blue sharks really dominate the market because they’re the most abundant.”

Hammerheads are endangered and known to have “exceptionally high-value fins and exceptionally low-value meat, so they often fall victims to shark-finning,” the practice of cutting off the animal’s fin and discarding the carcass, Fordham said.

To help restore shark populations, governments should establish catch limits and end the practice of shark-finning, the IUCN recommended. The Swiss group advises world governments, non-governmental organizations and United Nations bodies on conservation policy.

“It’s up to countries to really step up actions to conserve these species, to adopt precautionary limits within their own waters and work with other countries to conserve populations that are shared,” Fordham said. “The tools are there, but what’s been missing is the political will.”

To contact the reporter on this story: Jeremy van Loon in Berlin at jvanloon@bloomberg.net; Alex Morales in London at amorales2@bloomberg.net.

Last Updated: June 24, 2009 16:00 EDT