SYDNEY, Australia (Reuters) -- Scientists are mapping out a plan for a network of marine parks to save the world's oceans from fish stock depletion and growing pollution.Achim Steiner, director-general of the World Conservation Union (IUCN), said a conservation plan for the unregulated high seas -- part of a U.N.-backed plan -- would be produced by 2008, for adoption by world governments by 2012."We've had a good century of developing terrestrial protected areas, national parks on land," Steiner told Reuters late on Monday at the world's first conference on marine protected areas."But in the face of big challenges such as habitat loss, pollution of coastal zones, and species loss, and the high seas collapse of fish stocks, the whole marine realm is becoming rapidly more important," Steiner said by telephone from Geelong, a southern Australian city where 700 scientists from 70 countries gathered for the conference.An IUCN report released on Tuesday said that up to half of the world's coral reefs might be lost in the next 40 years unless urgent measures were taken to protect them against climate change and other threats.As much as 20 percent of the Earth's coral reefs have been effectively destroyed, Carl Gustaf Lundin, head of IUCN Marine Program, told the conference on Tuesday.Another 30 percent would be seriously depleted if no action was taken in the next 20-40 years.In some cases coral reefs have been 90 percent lost. Warming sea surface temperatures can cause bleaching of some reefs and coral can take years to recover. Bleaching is the whitening of coral colonies due to the loss of symbiotic zooxanthellae -- microscopic plants -- from the tissues of coral polyps.Sediment run-off from farming is also harming reefs.Scientists say marine protected areas could help save coral reefs, for example by preventing the overfishing that can decrease coral cover or deplete fish populations important for the coral reef ecosystem.The attempt to bring the seas under greater control follows increasingly lawless acts, epitomized by Australia's dramatic 21-day chase in 2003 of a Uruguayan-flagged boat which had been poaching the Patagonian toothfish -- prized as a delicacy -- in treacherous Antarctic seas.While the world's first marine park was established almost 100 years ago, the hundreds that now exist around the globe were mostly set up in the past 15-20 years.Australia established the World Heritage-listed Great Barrier Marine Park in 1975 over an area of 35 million hectares (87 million acres) -- bigger than Italy.Marine parks also exist in the United States, Europe, Africa and elsewhere, but they could be just the start."The situation in oceans around the world is deteriorating, and at an escalating pace," Steiner said.Of the 17 largest fisheries around the world, 15 are at either maximum exploitation levels or are depleting the level of their fish resource base."The offtake is unsustainable," Steiner said.Copyright 2005 Reuters. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.