Overfishing partly caused by booming demand for shark fin soup, a delicacy in some Asian countries, is threatening the existence of 11 kinds of ocean sharks, an international study showed.

The fish, often seen as ferocious sea predators, suffer from largely unregulated fishing for their valuable fins, said the report into 21 species of sharks and rays living in the open oceans.

The experts who wrote the study, organised by the International Union for Conservation of Nature, also urged governments to quickly impose catch limits.

"The traditional view of oceanic sharks and rays as fast and powerful too often leads to a misperception that they are resilient to fishing pressure," Sonja Fordham, report co-author and deputy head of the IUCN's shark specialist group, said.

The sharks, all "pelagic" or living in the open ocean, include large species such as the whale shark and great white shark. Although relatively few compared to coastal and deep sea sharks, a greater number of pelagic species is under threat.

"The increase in demand for shark fin soup in countries like China is a major driver of the problem," Ms Fordham told Reuters, noting that growing affluence in China, where the soup is served as a treat at celebrations, is behind its increasing popularity.

Fishers from all over the world catch and trade sharks for their lucrative fins, often discarding their carcasses, said Ms Fordham, noting Indonesia and Spain are among the top culprits.

Seven ocean pelagic shark species will be added to the IUCN 2008 Red List of endangered species, bringing the total to 21.

Sharks and rays are especially vulnerable as they take many years to reach sexual maturity and have few offspring.

Research shows the disappearance of shark species could lead to the demise of other species by upsetting the natural balance in the world's oceans.

Governments should set up catch limits for sharks and rays and ensure an end to shark finning, said the report. It also recommended a better monitoring of fisheries, more investment in research and closer international cooperation.

"Humans are making increasing use of ocean resources so many more aquatic species, particularly sharks, are coming under threat," said Nicholas Dulvy, lead author of the study published in Aquatic Conservation: Marine and Freshwater Ecosystems.