A 50% fall in the populations of marine mammals, birds, reptiles and fish has been recorded in the 40 years.
Worst affected have been fish needed to feed humans, according to a new report from WWF which blamed over-fishing, pollution, destruction of marine habitats and climate change for seriously reducing the size of commercial fish stocks between 1970 and 2010.
The conservation agency’s Living Blue Planet report said that the family of fish which includes tuna and mackerel has shrunk 74% in those four decades.
"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," Marco Lambertini, head of WWF International, said in a statement.
The changes “have dire consequences for the entire human population, with the poorest communities that rely on the sea getting hit fastest and hardest," he warned.
At the same time, coral reefs, mangroves and seagrasses which support fish populations were also found to be in steep decline. More than a third of the fish which WWF studied depend on coral reefs as do some 850 million people who rely on them for their livelihoods.
Coral reefs could all vanish by 2050 if temperatures continue to rise.
WWF's analysis was its most ambitious to-date. It just about doubled its research by tracking 5,829 populations of 1,234 species.
This resulted is "a clearer, more troubling picture of ocean health".
WWF called on leaders to put ocean recovery and coastal habitat conditions high on the agenda when the UN's Sustainable Development Goals for the next 15 years are formally approved later this month.
