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 on Friday.
Dead zones now comprise around 245,000 square kilometers (95,000 square miles), 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) ... fueled by riverine runoff of fertilizers 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 study said.
The researchers said the expansion of dead zones in these areas threatens commercial fishing and shrimping near the coastlines.
The phenomenon was first noted along the Adriatic Coast in the 1950s.
Seasonal dead zones affect the Gulf of Mexico, Chesapeake Bay and Scandinavian waters.
It can take years to treat severe hypoxia in a coastal region, and only four percent of treated areas have shown any signs of improvement, though the trend is reversible, the study said.
"From 1970 to 1990, the hypoxic zone on the northwestern continental shelf of the Black Sea has expanded to 40,000 square kilometers (15,500 square miles)," the study noted.
"However, since 1989, the loss of fertilizer subsidies from the former Soviet Union reduced nutrient loading by a factor of two to four, with the result that by 1995 the hypoxic zone had gone."
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.
"Climate change also has the potential to expand naturally occurring OMZs (oxygen minimum zones) into shallower coastal waters, damaging fisheries and affecting energy flows in the same way that eutrophication-driven hypoxia does," the authors wrote.
The researchers noted that any return to preindustrial levels of nutrient input into global waters would be "unrealistic."
However, they said "an appropriate management goal would be to reduce nutrient inputs to levels that occurred in the middle of the past century, before eutrophication began to spread dead zones globally."