Fewer than 10 percent of the 500-plus near-shore hypoxic zones the study recorded were known to have such dead zones before 1950. And the spread of those zones affects not only marine organisms but the people who depend on them for food and money.
“Low oxygen effectively become a form of habitat loss for those fish who don’t tolerate it,” Levin said. The dead zones squeeze the range where big commercial species like tuna can live and can force other species closer to the surface, where they’re easier to catch — leaving them vulnerable to overfishing.
The study is the biggest survey of low-oxygen conditions across the world’s oceans. It’s also the first publication by the Global Ocean Oxygen Network, an international coalition of scientists assembled by the United Nations. Other co-authors come from Europe, Peru, South Africa, China, Kuwait, and the Philippines, as well as the United States and Canada.
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The review ends with a list of recommendations for policymakers that can help address the problem. Some, like reducing farm runoff, are already well-known; others, such as creating more marine preserves or eating more hypoxia-tolerant seafood, aren’t things that are at the top of the public mind, Levin said.
But the biggest step is one that the world is already struggling to accomplish — cutting emissions of carbon dioxide and other planet-warming greenhouse gases.
“I think it’s really important that people be aware of what’s happening, and that it’s another reason we should be reducing our carbon dioxide emissions,” Levin said. “We have a lot of reasons already, but here is another very important one that people don’t know about.”