A United Nations agency has warned that the number of these "dead zones", caused mainly by the run-off of nitrogen fertilisers from intensive farming and sewage from large cities, had doubled in the past 15 years and was increasing all over the world.
In its first Global Environment Outlook Year Book report, published recently, the UN environment programme (UNEP) said 150 sea areas were now regularly starved of oxygen and were becoming major threats to already declining fish stocks, including those in Europe, where areas of the Baltic Sea were lifeless for several months, as were parts of the Irish Sea and the Adriatic.
The Black Sea - the largest and oldest "dead zone" in the world - supported only a few bacteria to a depth of 150m.
"Humankind is engaged in a gigantic, global experiment as a result of the inefficient and often overuse of fertilisers, the discharge of untreated sewage and the ever-rising emissions from vehicles and factories," said Klaus Toepfer, the UNEP director. "The nitrogen and phosphorus from these sources are being discharged into rivers and the coastal environment or being deposited from the atmosphere, triggering these alarming and sometimes irreversible effects."
In some parts of the world, such as large swaths of Africa, nitrogen shortages are reducing farmers' chances of meeting food demands. Such areas desperately need more fertilisers. However, in many other parts of the globe, excessive use of fertilisers is contributing to the escalating problem of dead zones.
Some of the dead zones are less than one square kilometre, whereas others are up to 70,000 sq km. Many have been found near the outlets of big rivers such as the Mississippi and the Yangtze, which drain huge industrial areas. Most lie off countries that heavily subsidise their agriculture.
"What is clear is that unless urgent action is taken to tackle the sources of the problem, it is likely to escalate rapidly," he said.
"Dead zones are especially dangerous to fisheries because they afflict coastal areas where many fish spawn and spend most of their lives before moving to deeper water," said a UNEP officer, Marion Cheatle. "It is getting noticeably worse."
She advised countries that often share water basins to cooperate in reducing nitrogen discharges by cutting fertiliser use or planting forests along rivers to soak up excess nitrogen. The "creeping dead zones" have been noted since the 1970s but the speed of their growth has surprised scientists, who are only now beginning to understand their mechanism.
There is concern that more oxygen-starved areas will emerge in coastal waters off parts of Asia, Latin America and Africa as industrialisation and more intensive agriculture increase the discharge of nutrients.
Robert Diaz, professor of marine science at Maryland University and author of the marine section of the report, said dead zones were fast becoming a bigger threat to fish stocks than overfishing.
He warned that global warming, with its likely increase in rainfall, was likely to aggravate the problem, because it would increase significantly the discharge of polluted water from rivers into oceans.
The report ranks dead zones as one of the top 20 threats to the global environment. Others included duststorms and sandstorms, more frequent around the world as land is degraded, and impending global water shortages.
More than one in three of the world's population is likely to suffer chronic water shortages in the next few decades, according to the report, while more than 2.4 billion people lack access to basic sanitation.