Populations of marine mammals, birds, reptiles and fish have dropped by about half in the past four decades, with fish critical to human food suffering the greatest declines, the World Wildlife Fund has warned.
In a new report, the conservation group cautioned that over-fishing, pollution and climate change significantly shrunk the size of commercial fish stocks between 1970 and 2010.
WWF's Living Blue Planet Report indicates that species essential to the global food supply could be the hardest hit.
One family of fish that includes tuna and mackerel had declined 74 per cent during the 40-year period.
Overall, WWF found a 49 percent decline in marine populations.
"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.
"Overfishing, destruction of marine habitats and climate change have dire consequences for the entire human population, with the poorest communities that rely on the sea getting hit fastest and hardest."
"Profound changes are needed to ensure abundant ocean life for future generations."
Fish are not the only marine species that are suffering.
The WWF report shows there has also been a steep decline in coral reefs, mangroves and seagrasses that support fish species - with more than one third of fish tracked for the study relying on coral reefs, and some 850 million people around the world relying on them for their livelihoods.
A previous report from the group showed that half of all corals have already vanished, and they are all expected to be gone by 2050 if temperatures continue to rise at the same rate.
WWF's analysis tracked 5829 populations of 1234 species - nearly twice as many as in its past studies - giving "a clearer, more troubling picture of ocean health".
WWF called on global leaders to ensure ocean recovery figures high on the list of priorities when the United Nations' Sustainable Development Goals for the next 15 years are formally approved later this month.
"We must take this opportunity to support the ocean and reverse the damage while we still can," Lambertini said.