"Sea areas starved of oxygen will soon damage fish stocks even more than unsustainable catches, the United Nations believes. The UN Environment Programme says excessive nutrients, mainly nitrogen from human activities, are causing these "dead zones" by stimulating huge growths of algae.
Since the 1960s the number of oxygen-starved areas has doubled every decade, as human nitrogen production has outstripped natural sources.
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About 75% of the world's fish stocks are already being overexploited, but Unep says the dead zones, which now number nearly 150 worldwide, will probably prove a greater menace.
It quotes research by a team of scientists at the Virginia Institute of Marine Science in the US. They concluded: "The history and pattern of human disturbance in terrestrial, aquatic, coastal and oceanic ecosystems have brought us to a point at which oxygen depletion is likely to become the keystone impact for the 21st Century, replacing the 20th Century keystone of overfishing."
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http://news.bbc.co.uk/1/hi/sci/tech/3577711.stm