An ambitious plan to create the world's largest shark sanctuary in the Pacific Ocean will struggle to succeed without support from locals, a conservationist warned on Tuesday. Five Micronesian states voted last month to create a shark sanctuary by the end of next year covering more than five million square kilometres (two million square miles) -- an area two-thirds the size of the United States. The sanctuary would make it illegal to fish for sharks and outlaw the trade in shark fins, which has boomed in recent years to meet growing demand from Asia's burgeoning middle classes for the delicacy shark fin soup. Marshall Islands Conservation Society director Albon Ishoda welcomed the proposal but said there was likely to be resistance from islanders if the sanctuary was imposed on them without consultation. He said many locals supplemented their meagre incomes by selling shark fins and also regarded the predators as competition for fish without realising their importance to the marine ecosystem. "They don't see the value of (living) sharks, we need a lot of education on the ground," Ishoda said. "Once people understand the issue better, they become advocates for a ban." The Pew Environment Group estimates more than 70 million sharks are killed annually for their fins, leaving up to a third of open-water species on the brink of extinction. And Pew shark conservation campaigner Matt Rand hailed the Pacific plan -- endorsed by Guam, the Marshall Islands, Palau, the Federated States of Micronesia and Northern Marianas -- as a major breakthrough. "(It) should serve as a model for other coastal nations to safeguard these important keystone species which are rapidly disappearing from the world?s oceans," he said. Palau led the way by declaring its waters a shark sanctuary in 2009, but leaders of the five Micronesian nations said unified action was needed to save the animals. "Effective protection of sharks in our Pacific region requires a joint and concerted effort by all jurisdictions to develop a single shark sanctuary where it is prohibited to possess, sell or trade shark fins or to commercially harvest sharks," they said in a statement.