OSLO - Life in the oceans is at imminent risk of the worst spate of extinctions in millions of years due to threats such as climate change and over-fishing, a study showed on Tuesday.Time was running short to counter hazards such as a collapse of coral reefs or a spread of low-oxygen "dead zones", according to the study led by the International Program on the State of the Ocean (IPSO)."We now face losing marine species and entire marine ecosystems, such as coral reefs, within a single generation," according to the study by 27 experts to be presented to the United Nations."Unless action is taken now, the consequences of our activities are at a high risk of causing, through the combined effects of climate change, over-exploitation, pollution and habitat loss, the next globally significant extinction event in the ocean," it said.Scientists list five mass extinctions over 600 million years -- most recently when the dinosaurs vanished 65 million years ago, apparently after an asteroid struck. Among others, the Permian period abruptly ended 250 million years ago."The findings are shocking," Alex Rogers, scientific director of IPSO, wrote of the conclusions from a 2011 workshop of ocean experts staged by IPSO and the International Union for Conservation of Nature (IUCN) at Oxford University.Fish are the main source of protein for a fifth of the world's population and the seas cycle oxygen and help absorb carbon dioxide, the main greenhouse gas from human activities.Jelle Bijma, of the Alfred Wegener Institute, said the seas faced a "deadly trio" of threats of higher temperatures, acidification and lack of oxygen, known as anoxia, that had featured in several past mass extinctions.A build-up of carbon dioxide, blamed by the UN panel of climate scientists on human use of fossil fuels, is heating the planet. Absorbed into the oceans, it causes acidification, while run-off of fertilizers and pollution stokes anoxia."From a geological point of view, mass extinctions happen overnight, but on human timescales we may not realize that we are in the middle of such an event," Bijma wrote.The study said that over-fishing is the easiest for governments to reverse -- countering global warming means a shift from fossil fuels, for instance, towards cleaner energies such as wind and solar power."Unlike climate change, it can be directly, immediately and effectively tackled by policy change," said William Cheung of the University of East Anglia."Over-fishing is now estimated to account for over 60 percent of the known local and global extinction of marine fishes," he wrote.Among examples of over-fishing are the Chinese bahaba that can grow 2 meters long. Prices per kilo (2.2 lbs) for its swim bladder -- meant to have medicinal properties -- have risen from a few dollars in the 1930s to $20,000-$70,000.