Researchers have discovered dead zones in the Atlantic Ocean with oxygen levels low enough to kill most marine life, according to a report published Thursday. It added that it was the first time such zones, which usually occur near coastlines, were observed in the open ocean.

Researchers found the dead zones in eddies, large areas of swirling water, that were slowly moving westward — raising fears that the low-oxygen zones could encounter an island and potentially lead to mass fish kills in shallower water, the research report said.

“The few eddies we observed in greater detail may be thought of as rotating cylinders of 100 to 150 km (60-94 miles) in diameter and a height of several hundred meters, with the dead zones taking up the upper 100 meters (328 feet) or so,” Johannes Karstensen, a researcher at GEOMAR, the Helmholtz Center for Ocean Research Kiel, in Kiel, Germany, said in a press release. Karstensen was part of the team of German and Canadian researchers behind the report.

Dead zones normally occur near coastlines where river runoff carries fertilizers and other chemical nutrients into the ocean, causing algae blooms that rapidly consume the water’s oxygen, the press release said. Though ocean currents can carry these waters away from the coast, a dead zone forming in the open ocean had not yet been discovered, according to the press release for the report, titled “Open ocean dead zones in the tropical North Atlantic Ocean.” It was published in the European Geosciences Union’s open access journal Biogeosciences.

It is unclear what caused the dead zones to form in the open ocean, the report said.

“Quantifying and fully understanding processes that control the [oxygen] supply and consumption balance, and any possible alterations over time, remain challenges in current research,” it said.

Whatever the cause, scientists believe such dead zones limit the biodiversity in an ecosystem by causing marine life to avoid such areas or by killing organisms that do not avoid them, according to the report.

One kind of impact observed by researchers involved zooplankton — tiny marine animals that play an important role in the ocean’s food chain. Zooplankton usually stay in the lower levels of the ocean during the day to avoid predators. But researchers found that the animals present in the dead zones remained at the surface, where oxygen levels were higher even during daytime.

The report said the open-ocean dead zones have a shallow top layer of water rich in oxygen that supports intense plant growth like the algae blooms that occur in coastal dead zones, the report said. The blooms' rapid consumption of oxygen depletes the supply of oxygen needed by life at lower levels, the researchers said.

Given the negative impact on most marine life, the researchers said any encounter between a dead zone eddy and an island could lead to a mass die-off. The scientists said they observed such eddies less than 60 miles north of the Cape Verde islands, making such an encounter a possibility.

“Given the shallow depth of a few tens of meters where the lowest [oxygen] concentrations are found a sudden flooding of a coastal area with low-[oxygen] waters may occur. A dramatic impact on the local ecosystems and sudden fish or crustacean death may be the consequence,” the report said.

“In retrospect, such eddy-island interactions may explain events that have been reported in the past,” the report added, referring to mysterious deaths of marine life.

Researchers at the University of California, Davis, who were looking into connections between global warming and increasing dead zones for a separate report published in February, spoke to crab fishermen on the U.S. West Coast, where lower oxygen levels have been observed.

The fishermen said that their catches were lower and that they had pulled up more dead crabs than usual. Other anecdotal evidence researchers came across indicated that fish are moving farther up the water column than is natural — perhaps because of depleted oxygen supplies, according to the UC Davis analysis.

Also, mass sea star die-offs along western parts of North America have been observed in recent years, leading some scientists to speculate that dead zones have made the sea stars more vulnerable to a virus ravaging the animals’ population.

The number of dead zones across the globe has roughly doubled every decade since the 1960s, a report from the Smithsonian Tropical Research Institute and the Smithsonian Environmental Research Center said last year. The largest dead zones were observed in the Gulf of Mexico and the Baltic Sea, it said.

At least 94 percent of the world’s coastal ocean dead zones occur in regions expected to warm at least 2 degrees Celsius by the end of the century, the Smithsonian report said, adding that warmer waters and other climate change-related factors could lead to expanded dead zones.