It's report card time for the world's seas, as a newly released Ocean Health Index has graded the globe's marine expanses in terms of overall health.

The index, produced by a coalition led by scientists at the National Center for Ecological Analysis and Synthesis at the University of California, Santa Barbara and by Conservation International, covers the 15 ocean regions of the world beyond national jurisdiction -- the world's high seas.

The University of British Columbia, the New England Aquarium and other institutions also contributed data to the index.

The average score? On a typical school report card, it would be a "D" with 67 out of a possible 100, based on a "quantifiable assessment of the capacity of our oceans to deliver benefits and resources sustainably," the index's website said.

The oceans were rated on their contribution to maintaining a healthy climate, safeguarding biodiversity and providing sustainable food sources, UCSB researchers say.

To determine the scores, the researchers looked at ecological, economic, social and political factors and combined them all in a computer model for a final grade.

The index identified pollution, overfishing, climate change and the lack of marine protections as key problems.

There was variation around the world, the researchers noted, with the Antarctic and the Southern Ocean both scoring above the 67 average, receiving a grade of 72.

"Antarctica and the Southern Ocean are protected by distance from many of the threats caused by human populations, such as chemicals, excessive nutrients, and pathogens and trash," noted Greg Stone, chief scientist at the Moore Center for Science and Oceans at Conservation International.

The improvement in scores since the previous index released in 2012 was likely due to conservation measures that have slowed the decline in ocean health seen since the beginning of the industrial era, the researchers said.

"The score of 100 that is set as a target for each goal reflects a status that is feasible to achieve and can sustainably produce maximum benefits now and in the future," says Steve Katona, managing director for the Ocean Health Index. "Any score below 100 means there is room for improvement."

Still, he said, given recent bad news of overfishing, deaths of coral reefs, pollution and climate change, the score should be seen as better than many people may have expected, he says.

"If you come home with a paper from school, your parents aren't real happy if it's a 67, but most people expected a score for the ocean that was worse," he said.

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