OCEANIC "dead zones" where marine life cannot survive have been steadily increasing over the past five decades and now encompass 400 coastal areas of the world, a US-Swedish study.

The number of these areas, in which aquatic ecosystems disappear due to lack of oxygen in the water, have "approximately doubled each decade since the 1960s," said the study in the journal Science last month.

Dead zones now comprise around 245,000 square kilometers, according to researchers Robert Diaz of the Marine Sciences Institute at the College of William and Mary in Virginia, and Rutger Rosenberg, a marine scientist at Gothenburg University in Sweden.

"The formation of dead zones has been exacerbated by the increase in (pollution) ... fuelled by riverine runoff of fertilisers and the burning of fossil fuels," the study said.

The phenomenon, called eutrophication, is caused by industrial pollution as well as runoff of water containing phosphates and nitrates into the oceans.
Oceans react to the boost in pollution by growing more algae and vegetation in coastal areas.

When the algae dies and sinks to the bottom, it decreases the amount of oxygen available in the bottom waters, a process called hypoxia, eventually wiping out fish and crustaceans that live there, as well as the foods they eat.

Dead zones tend to creep up in calm waters that see lower water exchange, but have more recently been affecting major fishery areas in the Baltic, Kattegat, and Black Seas as well as the Gulf of Mexico and East China Sea.

The researchers said the expansion of dead zones in these areas threatened commercial fishing near the coastlines.

It can take years to treat severe hypoxia in a coastal region, and only four per cent of treated areas have shown any signs of improvement.

The study authors said the global warming trend alone was likely to increase oceanic dead zones by increasing temperature, causing changes in rainfall patterns and changing discharges of fresh water and agricultural nutrients into the oceans. -- AFP