When lobsters swarm up the beach and octopuses try to clamber up fishing lines to get out of the water, you know that something has gone badly wrong in the ocean.

Oxygen-starved dead zones have been appearing with increasing frequency around the world, with some 400 identified so far. While most are caused by sewage or fertiliser leaching into the ocean, a possible new driver has appeared in the northwest Pacific: climate change.

Just as land animals need oxygen in the air to breathe, those in the sea need the oxygen dissolved in seawater. Dead zones cover up to 20,000 sq miles each and their borders shift according to wind, current and tide, killing all animal life that cannot escape in time.

“Oregon is a little different,” said Jack Barth, of Oregon State University. “We have an open coastline, so the ability to flush the coast is high and there are no rivers carrying fertiliser. All nutrients appear to come from natural sources.”

But for the past four years Professor Barth has been using autonomous underwater robots to monitor worrying developments on a naturally occuring area of low oxygen off the northwest Pacific, on the border of Washington state and Oregon. “We’ve seen various degrees of oxygen deficiency,” he said. “We’ve seen zero oxygen, known as anoxia.”

Camera footage from remotely operated vehicles “showed us just piles of Dungeness crab, dead tube worms. None could flee”.

Off South Africa and Namibia, lobsters swarm to the beach when anoxic waters close in. Professor Barth cites reports that octopuses have tried to climb fishing lines to escape.

Low-oxygen areas occur naturally on the west coast of all continents, where nutrient-rich waters well up to the sunlit surface, causing heavy productivity. The upper waters are well mixed by wind and waves but deeper down there is less opportunity to replenish oxygen. When dead plankton drift into this zone in a marine snowfall, they cause a secondary bloom in microscopic animal life that strips away the oxygen.

If the oxygen-starved water does not reach the seabed, the effects are usually less severe. If it does, the animals there are usually unable to escape.

Usually winds push the de-oxygenated areas off Oregon out to sea but in recent years they have been coming ever closer to land and shallower water. Professor Barth said that climate-change models show coastal winds changing.

“The forecast is for stronger and less persistent winds and for deeper waters becoming less oxygenated as surface layers warm, isolating the deeper layers more. So it’s a double-whammy we’re seeing off Oregon.”

Although Professor Barth does not yet have enough records to prove that climate change is affecting the winds, he is confident that they are not altering as a result of any natural cycles, such as the warm current El Niño.

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