Depleted by over-harvesting and pollution, the world's major fishing grounds are now severely threatened by climate change as well, according to a UN report released Friday.
Warmer water and acidification caused by the seas' absorption of atmospheric carbon dioxide (CO2) are disrupting fragile natural cycles and threaten a dramatic collapse of fish stocks, the report said.
"What we do over the next decades has the potential to affect ocean chemistry for tens of thousands of years, and marine life for millions of years," said one of the authors, marine scientist Ken Caldeira of Stanford University.
The report was unveiled at an international meeting of environment ministers, gathered in Monaco for a special session of the United Nations Environment Programme (UNEP) focussing on global warming.
Over-fishing, pollution and now climate change have had catastrophic impacts on the world's wild fish populations, it said.
Previous studies have shown that 90 percent of many of the ocean's big fishes -- including tuna, marlin, swordfish, some sharks, cod and halibut -- have disappeared from the seas due to industrial exploitation.
Virtually all of commercially-fished wild species are in decline.
"We are fishing in deeper waters, farther from shore and with much more advanced technology, but are no long able to catch more fish," said lead scientist Christian Nellemann.
Stocks are also threatened by pollution -- 80 percent of it coming from land -- precisely in these areas that were once thick with fish.
Invasive species, often carried in the ballast of ocean vessels, have also disrupted natural food chains.
More than 50 percent of the fish extracted from the sea comes from only seven percent of the planet's oceans.
The scientists also sounded the alarm over a discovery that global warming could imperil an ocean circulation system that has allowed fish stocks to replenish despite intensified industrial fishing.
These natural pumps, dotted across the world including the Arctic and the Mediterranean, bring nutrients to fisheries and keep them healthy by flushing out wastes and pollution.
"We seriously fear that if this mechanism stops, we may risk a potential collapse in major fishing grounds in the world and that they will not be able to recover as we have seen in the past," said Nellemann.
A dramatic fall in fish yields, he pointed out, has more than an economic impact: over 2.6 billion people depend on them as their main source of protein.
Rising acidity levels are also a growing concern, said the report.
Shell-forming marine life, such as shrimp, and coral are both extremely sensitive to tiny changes in temperature and acidification, as are plankton, the bottom rung of the ocean's food chain.
Other key findings of the report, entitled "In Dead Water", include:
-- An area of 10-15 percent of the world’s seas and oceans cover most of the commercial fishing grounds
-- Eighty percent to 100 percent of the world’s coral reefs may suffer annual bleaching events by 2080
-- Over 80 percent of marine pollution comes from the land
-- There are an estimated 200 temporary or permanent "dead zones," or areas of de-oxygenated water caused by pollution, up from around 150 in 2003
-- Up to 80 percent of the world’s primary fish catch species are exploited beyond or close to their harvesting capacity