By Helen Briggs
Science reporter, BBC News, Boston
Over-fishing and demand for shark fins, an expensive delicacy, have pushed one of the world's iconic animals towards the brink of extinction, say experts.
Hammerheads are among the most commonly caught sharks for finning
The scalloped hammerhead shark is to be added to the official endangered species list this year, under the heading "globally endangered".
Their plight has been discussed at the American Association for the Advancement of Science annual meeting.
It was told that enforcement of marine reserves would aid shark protection.
The observation takes account of new research that shows hammerhead and great white sharks patrol fixed routes in the ocean, gathering at hotspots to mate or feed.
Dr Julia Baum, a marine ecologist at the Scripps Institution of Oceanography, US, and a member of the World Conservation Union (IUCN), said excessive fishing was putting many of the ocean's "most majestic predators" at risk of extinction.
Speaking at the Boston meeting, she said: "Sharks evolved 400 million years ago, and we could now lose some species in the next few decades - so that would be just a blink of an eye in evolutionary time."
She said conservation concern for sharks had been mounting for several years, and it was now critical that there was effective management action in order to restore and conserve their numbers.
Fishing for sharks in international waters is unrestricted, but conservation groups are calling for urgent measures to set limits on shark catch and fishing quotas.
They say demand for shark fins as an expensive delicacy is greatly increasing the pressure on shark populations.
They want a meaningful ban on the practice of shark finning, which involves a shark's fins being removed before the rest of the animal is thrown back into the ocean to die.
Hammerheads are among the most commonly caught sharks for finning. A large shark fin can fetch over £50 a kilo.
Research presented at the AAAS in Boston is starting to unravel the mysteries of shark behaviour, and how they might best be protected.
Tagging studies show that the scalloped hammerhead gathers at fixed sites around islands in the Eastern Pacific Ocean during its long-distance migrations.
The animals move between a series of "stepping stone" sites off groups of coastal islands between Mexico and Ecuador. They also congregate around mountains rising from the ocean seafloor.
Electronic tagging of 150 great white sharks found off the coast of central California revealed similar findings - the sharks gather in "hotspots".
One site between Hawaii and Mexico attracted so many visitors that researchers dubbed it "the white shark cafe".
Salvador Jorgensen, a researcher at Stanford University's Hopkins Marine Station, US, said it was still not clear why the sharks went there.
"They're going there to feed, or they're going there for meeting, where males and females could meet perhaps away from a feeding area, where there's less competition and more focus on mating behaviour," he suggested.
The scientists say information on shark "superhighway" routes and stepping stone sites can be used to help fisheries managers focus on protecting these areas.
Previous research by Dr Baum's team has found that sharks are declining rapidly in parts of the Atlantic Ocean.
All species they looked at had declined by over 50% since the early 1970s.
For many large coastal shark species, the drop in numbers was much greater: tiger, scalloped hammerhead and dusky shark populations have fallen by more than 95%.
A total of 233 shark species are currently on the IUCN Red List, 12 of which are classified "critically endangered".
Nine, including the scalloped hammerhead, have been added or will be added this year.
Among them are three species of thresher shark and the shortfin mako shark. These are considered "vulnerable to extinction".