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A shark swims on a reef in Ailinginae Atoll in the Marshall Islands.

Greg Asner/The Associated Press

A Canadian-led team of scientists has concluded that tropical coral reefs that feed millions around the world have lost about half their ability to support human communities since 1950.

“I don’t think any of us expected the declines to be as big as we found,” said Tyler Eddy of Memorial University, lead author of the paper published Friday in the journal One Earth.

The paper is the first to tote up the cumulative effects of threats faced by tropical coral reefs from overfishing to pollution to climate change, the authors say.

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“This is the first study that puts it all together,” Eddy said.

It combines data from myriad sources.

Just the figures on reef extent drew on 14,705 surveys from 3,582 reefs in 87 countries. The conclusions on biodiversity were made from a database with more than a billion records on 100,000 species from plankton to mammals.

“We had to spend a lot of time standardizing the data,” said co-author William Cheung of the University of British Columbia. “That was a big part of the analysis.”

The findings are sobering.

Globally, reefs cover about half the sea floor they did in 1950. Catches of reef-associated fish peaked in 2002 at about 2.2 million tonnes and have declined about 10 per cent since then.

That’s despite increased fishing efforts. The paper finds that what it calls “catch per unit effort” has declined about 60 per cent.

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Imagine a fisherman with one hook casting it for an hour, Cheung said.

“Fifty years ago, they would have caught 10 fish. With 60 per cent decline, it means the fisherman would have caught five or fewer fish with the same amount of effort.”

That’s a powerful clue as to what’s happening under the surface, said Eddy.

“That’s an indication of how much fish is available to catch.”

Finally, reefs around the world have lost almost two-thirds of their biodiversity. That not only affects the reefs but the other oceanic ecosystems they are connected to.

“It’s an important part of the global system,” Cheung said.

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The study reports that coral reef fisheries are worth about $7.6 billion worldwide. It says about 6 million people — a high number of them Indigenous groups from small island states – depend on them.

But that’s not the only cost.

Reefs protect shorelines from heavy storms by breaking and weakening waves produced by hurricanes, Eddy said. Destructive storms are expected to increase as the globe heats up.

Reefs also nurture tourism as well as the cultures of many coastal peoples.

“Our results highlight the sensitivity of coral reef ecosystems … as well as the high dependence on them by human communities,” the study concludes.

The threats include pollution from agricultural runoff, ocean acidification from greenhouse gases and damaging fishing practices such as trawling, as well as climate change.

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Canada may not have any tropical coral reefs. But that doesn’t mean the country isn’t affected by their decline.

“Canadians are implicated in terms of our contribution to climate change,” Eddy said. “This is really a global responsibility.”

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