Urgent action is needed to protect the world's oceans from human exploitation, according to conservationists.
Fish with high commercial value are in decline
They say over-fishing, pollution and climate change are pushing marine areas to the point of no return.
The warning comes from the United Nations Environment Programme (Unep) and World Conservation Union (IUCN).
In a report launched on Friday in New York, they are calling for new marine protected areas to be set up in deep seas and open oceans.
Dr Stefan Hain from Unep said it was a crucial moment for the marine environment.
"Very often, it's the case that people go out there and fish without considering what effect this has on the fish stocks," he told BBC News.
"We know now that the environments and the eco-systems in the deep water are very fragile.
"The impact is that these stocks are being reduced at an alarming rate, and, simply, these ecosystems are not designed to cope with drastic impact by human activity."
The report, Ecosystems and Biodiversity in Deep Waters and High Seas, is the centrepiece of talks on the law of the sea at the UN.
The report makes the case for marine protected areas in parts of the sea that are not included in the territorial sea or in the internal waters of a State.
Illegal long-line fishing kills more than 300,000 seabirds each year
Large fish like tuna, cod and swordfish have declined by as much as 90% in the past century
Over 46,000 pieces of plastic litter are floating on every square mile of the ocean
Ibrahim Thiaw, acting director general of the IUCN, said well over 60% of the marine world and its rich biodiversity, found beyond the limits of national jurisdiction, was vulnerable and at increasing risk.
"Governments must urgently develop the guidelines, rules and actions needed to bridge this gulf," he said.
"Otherwise we stand to lose and to irrevocably damage unique wildlife and critical ecosystems many of which moderate our very existence on the planet."