One day more than a decade ago, Eric Prince was studying the tracks of tagged fish when he noticed something odd. Blue marlin off the southeastern United States would dive a half-mile deep chasing prey. The same species off Costa Rica and Guatemala stayed near the surface, rarely dropping more than a few hundred feet.

Prince, a billfish expert who has since retired from the National Oceanic and Atmospheric Administration, was stumped. He’d studied blue marlin off the Ivory Coast and Ghana, Jamaica and Brazil, and he’d never seen anything like it. Why wouldn't these expert divers dive?

The billfish, it turns out, were trying to avoid suffocation. The marlin near Guatemala and Costa Rica wouldn't plunge into the murky depths because they were avoiding a deep, gigantic and expanding swath of water that contained too little oxygen. The discovery was among the first examples of the many ways sea life is already shifting in response to a new reality that hasn't gotten much attention: Marine waters, even far out in the high seas, are losing oxygen thanks to climate change, upending where and how sea creatures live.

"This is a global problem, and global warming is making it worse," says Denise Breitburg, senior scientist at the Smithsonian Research Center. "It requires global solutions," she says.

Breitburg is the lead author of a new study published today in Science that examined all the major research on ocean oxygen loss. The authors conclude that it’s emptying vast regions of the ocean, changing what and where creatures live and eat, threatening to shrink fish populations and individual fish, and making overfishing more likely. Like warming seas and ocean acidification, oxygen loss is one of the most important byproducts of climate change, but one that few people understand.

"Loss of oxygen in many ways is the destruction of an ecosystem," Breitburg says. "If we were creating vast areas on land that were uninhabitable by most animals, we'd notice. But we don't always see things like this when they are happening in the water."

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Bigger Than Coastal Dead Zones

Breitburg's research isn't focused just on coastal dead zones, such as the polluted runoff-fueled area in the Gulf of Mexico, but enormous stretches of deep water in the open ocean that can extend for thousands of miles.

These low-oxygen zones occur naturally, but have grown by more than 4.5 million square kilometers—an area roughly as large as the entire European Union—just since the mid-20th century. In part that's because of rising temperatures.

Warm water simply carries less oxygen. It also stokes the metabolism of both microbes and larger creatures, causing them to use more of whatever oxygen there is. Finally, as climate change warms the ocean from the surface down, making the surface layer more buoyant—warm water is lighter than cold water—it makes it harder for fresh oxygen from the air to mix down into the deep layers where the oxygen-poor zones are located.

Today, those low-oxygen zones are expanding toward the surface by as much as a meter a year. That includes major areas in the eastern Pacific and Baltic Sea. One area down deep off southern California has seen an oxygen decline of 30 percent in just a quarter century. A low-oxygen area of the Atlantic Ocean near the coast of Africa is broader than the continental United States, and has grown 15 percent since the 1960s

In fact, the world's oceans have lost about two percent of their oxygen in just 50 years, while the amount of water that's completely free of oxygen has increased fourfold, according to the new study. Scientists now can identify 500 sites along the coasts where oxygen is exceedingly low. Fewer than 10 percent of those were known before the mid-20th century.

A Threat Multiplier

For some marine creatures, low oxygen waters can impair reproduction, shorten lifespans, and change behavior. Even brief exposure can change immune systems and increase disease. Low-oxygen waters can even affect future generations by altering gene expression in fish and other marine creatures.

The change is already forcing everything from tuna and sharks, to herring, shad, mackerel, Pacific cod, and swordfish into ever-smaller bands of oxygen-rich water near the surface. Concentrating all these creatures makes them easier pickings for turtles, birds, and other surface predators—including fishing fleets.

Breitburg's team also found that oxygen loss is affecting some of the world's most productive areas—"upwelling zones," such as the ones off the west coast of the U.S. and South America, where winds draw nutrient-rich cold water to the surface. Warming is expected to increase such upwelling, but increasingly it’s bringing low-oxygen waters to the surface. In places such as central Oregon, that has helped create new dead zones.

Of course, declining oxygen isn't happening in isolation, Breitburg says. Warming itself threatens marine food webs, as does the acidification caused by increased carbon dioxide in the water. But the threats are worse when combined.

"We've been doing work in Chesapeake Bay and we've found that acidification actually makes some fish more sensitive to low oxygen," she says.

What’s more, areas with extremely low oxygen also seem to produce their own greenhouse gas, which could further worsen climate change.