Such zones are found from the Gulf of Mexico to the Baltic Sea in areas where algae bloom and suck oxygen from the water, feeding on fertilisers washed from fields, sewage, animal wastes and pollutants from the burning of fossil fuels.

"Marine organisms are more vulnerable to low oxygen content than currently recognised, with fish and crustaceans being the most vulnerable," said Raquel Vaquer Suner of the Mediterranean Institute for Advanced Studies in Spain.

"The number of reported hypoxic (low oxygen) zones is growing globally at a rate of 5 percent a year," she told Reuters.

Her study with a colleague showed that the number of "dead zones" had risen to more than 140 in 2004 from almost none until the late 1970s.

Hundreds of millions of people depend on coastal fisheries for food. Crustaceans such as crabs, lobsters and shrimps are less able to escape from low-oxygen waters than fish.

WARMING

Higher temperatures tied to global warming, blamed by the UN Climate Panel on human use of fossil fuels, may aggravate the problem of "dead zones", partly because oxygen dissolves less readily in warmer water, the study said.

The first "dead zones" were found in northern latitudes such as Chesapeake Bay on the US east coast and Scandinavian fjords. Others have been appearing off South America, Ghana, China, Japan, Australia, New Zealand, Portugal and Britain.

The study said that most scientists had until now reckoned that oxygen levels could fall to 2 milligrams per litre of sea water before the water was considered starved of oxygen.

But many creatures were far more sensitive. Larvae of one type of crab found off eastern Canada and the United States started suffering at oxygen levels of 8.6 mg per litre, just below normal levels.

"Currently used thresholds ... are not conservative enough to avoid widespread mortality losses," the scientists wrote. They urged a revised minimum of 4.6 mg of oxygen per litre as the lowest before water was considered hostile to life. -- For Reuters latest environment blogs click on: http://blogs.reuters.com/environment/ (Editing by Tim Pearce)