The scientists place the problem on runoff of chemical fertilizers in rivers and fallout from burning fossil fuels, and they estimate there are now more than 400 dead zones along 95,000 square miles of the seas - an area more than half the size of California.

The number of those areas has nearly doubled every decade since the 1960s, said Robert J. Diaz, a biological oceanographer at the Virginia Institute of Marine Science.

"Dead zones were once rare, but now they're commonplace, and there are more of them in more places," he said.

Diaz and Rutger Rosenberg, a marine ecologist at Sweden's Göteborg University, have just completed a global survey of the imperiled areas, and their report appears today in the journal Science.

The phenomenon that drives life away from so many coastal habitats is called hypoxia - the lack of enough oxygen in bottom waters for fish and other valuable marine life to thrive, the report notes.

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