Big fish and other large ocean creatures face higher risks of extinction than small ones, overturning a 500 million-year pattern and indicating that human hunting is to blame, scientists said on Wednesday.
Fossils from five mass extinction events, most recently when an asteroid struck the Earth 65 million years ago, showed that small marine animals were slightly more likely to be wiped out than big ones in the pre-historic cataclysms, a study published in the journal Science said.
By contrast, large modern fish such as tuna and sharks, as well as mammals including whales and seals were more likely to be on a global "Red List" of endangered species than small fish and molluscs.
Modern threats "are dominated by large body size. The past extinction events are not," Jonathan Payne, of Stanford University, who led the study told Reuters.
"This kind of effect hasn't happened before in the ocean ... in half a billion years," he said. He said the findings were evidence that human hunting was the dominant threat to marine life.
U.N. reports say the Earth risks a "sixth mass extinction", driven on land by people clearing forests to make way for farms and roads, as well as by pollution and hunting. Climate change will add to the risks, on land and in the oceans.
The Red List, run by scientists at the International Union for Conservation of Nature, says between 24 and 40 percent of ocean creatures are now vulnerable to extinction.
Even so, the oceans have escaped hunting on a scale blamed for wiping out land creatures such as the woolly mammoth, the cattle-like auroch, the Tasmanian tiger and the dodo.
"Until relatively recently, humans were restricted to coastal areas and didn't have the technology to fish in the deep ocean on an industrial scale," co-author Noel Heim of Stanford wrote in a statement.
Of large modern ocean creatures, only Stellers' sea cow, which grew up to 9 metres (30 feet) in length, has been driven to extinction by hunting, in the 18th century.
That meant there was still time to avert extinctions.
"The good news is that it's not going to happen right away," Payne said. "We have the opportunity to change."
Most whaling has been banned since the 1980s. Last month, U.S. President Barack Obama expanded a protected area off Hawaii to ban commercial fishing from 582,500 square miles (1.5 million sqkms) of the Pacific Ocean.
(Reporting by Alister Doyle; editing by Ralph Boulton)