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A study by the World Wildlife Fund has found that populations of marine species fell by about half over four decades. Among the hardest hit were those essential to global food supply.
The WWF warned in its Living Blue Planet Report that over-fishing, pollution and climate change had reduced the size of fish stocks significantly between 1970 and 2010.
One family of fish, which includes tuna and mackerel had shown a decline of 74 percent. In large part, the WWF blamed unsustainable fishing practices.
"In the space of a single generation, human activity has severely damaged the ocean by catching fish faster than they can reproduce while also destroying their nurseries," said Marco Lambertini, head of WWF International.
"The picture is now clearer than ever: humanity is collectively mismanaging the ocean to the brink of collapse," said Lambertini. "Considering the ocean's vital role in our economies and its essential contribution to food security – particularly for poor, coastal communities – that's simply unacceptable."
The WWF analysis tracked 5,829 populations of 1,234 species - almost twice as many as in its past studies, to give "a clearer, more troubling picture of ocean health."
Turning the tide
The group highlighted sharks and rays, sea cucumbers, and marine turtles - all of which have suffered drastic population reductions - as indicators of current stress levels on biodiversity.
Without underplaying the severity of the crisis, the WWF stressed that the ocean was a renewable resource and that marine life could be restored if the human population lives within "sustainable limits."
Among the solutions that the WWF recommended to reverse the trend were "smart fishing" that would eliminate waste, a crackdown on unregulated fishing and the protection of marine environments.
The report noted there was a steep decline in coral reefs, mangroves and seagrasses that support fish species. A previous report by the group showed half of all corals had vanished, and they could disappear entirely by 2050 should temperatures continue to rise.