Free to the Public, the “ World Ocean Festival Speaker Forum sponsored by National Geographic Encounter: Ocean Odyssey” will feature dialogue with leading explorers, scientists and entrepreneurs on NYC’s Governors Island on June 4

Marlin and sailfish are the oceans’ perfect athletes. A marlin can outweigh a polar bear, leap through the air, and traverse the sea from Delaware to Madagascar. Sailfish can outrace nearly every fish in the sea. Marlin can hunt in waters a half mile down, and sailfish often head to deep waters too.

Yet in more and more places around the world, these predators are sticking near the surface, rarely using their formidable power to plunge into the depths to chase prey.

The discovery of this behavioral quirk in fish built for diving offers some of the most tangible evidence of a disturbing trend: Warming temperatures are sucking oxygen out of waters even far out at sea, making enormous stretches of deep ocean hostile to marine life.

View Images Sharks, particularly great whites and mako sharks like this one near San Diego, tend to avoid marine waters that are low in oxygen. The expansion of low-oxygen zones may change what they can eat. Photograph by Andy Murch, Visuals Unlimited/Corbis

“Two hundred meters down, there is a freight train of low-oxygen water barreling toward the surface,” says William Gilly, a marine biologist with Stanford University’s Hopkins Marine Station, in Pacific Grove, California. Yet, “with all the ballyhoo about ocean issues, this one hasn’t gotten much attention.”

These are not coastal dead zones, like the one that sprawls across the Gulf of Mexico, but great swaths of deep water that can reach thousands of miles offshore. Already naturally low in oxygen, these regions keep growing, spreading horizontally and vertically. Included are vast portions of the eastern Pacific, almost all of the Bay of Bengal, and an area of the Atlantic off West Africa as broad as the United States.

Globally, these low-oxygen areas have expanded by more than 1.7 million square miles (4.5 million square kilometers) in the past 50 years.

This phenomenon could transform the seas as much as global warming or ocean acidification will, rearranging where and what creatures eat and altering which species live or die. It already is starting to scramble ocean food chains and threatens to compound almost every other problem in the sea.

Scientists are debating how much oxygen loss is spurred by global warming, and how much is driven by natural cycles. But they agree that climate change will make the losses spread and perhaps even accelerate.

“I don’t think people realize this is happening right now,” says Lisa Levin, an oxygen expert with the Scripps Institution of Oceanography, in San Diego.

Bad Water Rising

Few understand marlin and sailfish better than biologist Eric Prince. He has studied them in Jamaica, Brazil, the Ivory Coast, and Ghana. He has examined their ear bones in Bermuda, taken tissue samples in Panama, and gathered their heads—with bayonet-like bills still attached—during fishing contests in Puerto Rico.

One day a decade ago, while tracking satellite tags attached to these fish, Prince saw something bizarre: Marlin off North Carolina fed in waters as deep as 2,600 feet (800 meters). But marlin off Guatemala and Costa Rica hovered high in the water, almost never descending beyond a few hundred feet. Sailfish followed a similar pattern.

These billfish have special tissues in their heads that keep their brains warm in deep water. So why were they bunching up at the ocean’s surface?

View Images Low-oxygen waters drawn from the deep sea onto the continental shelf can be deadly for invertebrates like this fish-eating anemone. Photograph by Brandon D. Cole, Corbis

The culprit, it turned out, was a gigantic pool of low-oxygen water deep off Central America. These fish were staying up high, trying to avoid suffocating below.

Prince’s discovery came just as other scientists were figuring out that rising temperatures were expanding natural low-oxygen zones in the deep ocean, pushing them skyward by as much as a meter (three feet) per year.

Over the next decade, researchers figured out that this change already was driving marine creatures—sailfish, sharks, tuna, swordfish, and Pacific cod, as well as the smaller sardines, herring, shad, and mackerel they eat—into ever narrower bands of oxygen-rich water near the surface.

“It leaves just a very thin lens on the top of the ocean where most organisms can live,” says Sarah Moffitt, of the Bodega Marine Laboratory at the University of California, Davis.

View Images Dungeness crabs can suffocate when low-oxygen waters from the deep ocean are swept near coastal Oregon. Photograph by Paul Nicklen, National Geographic

Congregating alongside their prey appears to be making some bigger fish fatter, as they burn less energy hunting. But living in such a compressed area also may be speeding the decline of top predators such as tuna, sailfish, and marlin by making them more accessible to fishing fleets.

“It makes the predators much more likely to be caught by the longline fleet,” says Prince, of the National Oceanic and Atmospheric Administration’s Southeast Fisheries Science Center in Florida. “Very slightly, every year, they become more and more susceptible to overfishing.”

Oxygen is so central to life, even in the marine world, that its loss is harming animals in countless other ways, too.

Warming Waters Deplete Oxygen

View Images Sea stars are often early victims when low-oxygen waters get drawn onto the continental shelf from the deep sea. Photograph by Tom Neves, Canada Photos/Corbis

Fish, squid, octopus, and crab all draw dissolved oxygen from the water. And just as oxygen levels shift with elevation, oxygen at sea varies with depth. But in the ocean, oxygen is also dynamic, changing daily and seasonally with weather and tides or over years with cycles of warming and cooling.

Oxygen gets into the sea in two ways: through photosynthesis, which takes place only near the top where light penetrates, or through the mixing of air and water at the surface by wind and waves.

Deep ocean waters hold far less oxygen than surface waters because they haven’t been in contact with air for centuries. And in many places, decomposing organic matter raining down from the surface uses up what little oxygen remains. These natural deep-water “oxygen minimum zones” cover great swaths of ocean interior.

They are far different from hypoxic coastal dead zones, which are multiplying, too, with more than 400 now reported worldwide. Dead zones are caused by nitrogen and other nutrients as rivers and storms flush pollution from farms and cities into nearshore waters.

Breathless seas Oxygen is as essential for life in the sea as it is on land. Oxygen levels normally vary with depth. But deep ocean areas already low in oxygen are losing more as seas warm, wreaking havoc on marine life. Here are four elements of that change. Introduction Ocean mixing Chemistry Shoaling Consequences Euphotic zone Oxygen can get into the sea as wind and waves stir the surface or through photosynthesis, which takes place in surface regions where light penetrates. Warm water is lighter. As the upper ocean heats up, it gets harder for that water to penetrate cold layers below through ocean circulation. Such mixing is how oxygen gets down from the surface. Warm water holds less oxygen. As temperatures rise at the surface, that water loses its ability to carry oxygen. Oxygen minimum zones (OMZs) OMZs are not dead zones, but are vast midwater areas far out at sea that hold less oxygen than surface waters do. Organic matter decomposes as it rains down from the surface, robbing OMZs of O2. In some regions, such as the U.S. West Coast, wind draws oxygen-poor water up from the deep and blows it into shallow areas over continental shelves. Low-oxygen zones expanding out and up kill some animals and drive others to thinner bands of oxygen-rich water near the surface. That alters the food chain and makes large predator fish easier to catch. The bottom Waters below OMZs are less depleted of oxygen. Most organic matter sinking in the ocean decomposes before it reaches the bottom. Zones' depths vary with location. MATT TWOMBLY, CHIQUI ESTEBAN, NG STAFFSOURCES: Francis Chan, Oregon State University; William Gilly, Stanford University's Hopkins Marine Station

The expansion of deep-sea low-oxygen zones, on the other hand, is driven by temperature. Warm water carries less dissolved oxygen. It’s also lighter than cold water. That leaves the ocean segregated in layers, restricting delivery of fresh oxygen to the deep and making these oxygen-poor zones much bigger.

“The natural thing to expect is that as the ocean gets warmer, circulation will slow down and get more sluggish and the waters going into the deep ocean will hang around longer,” says Curtis Deutsch, a chemical oceanography professor at the University of Washington, in Seattle. “And indeed, oxygen seems to be declining.”

The zone off West Africa that’s as big as the continental United States has grown by 15 percent since 1960—and by 10 percent just since 1995. At 650 feet (200 meters) deep in the Pacific off southern California, oxygen has dropped 30 percent in some places in a quarter century.

Many scientists already suspect global warming is partly to blame for this transformation. Deutsch and others, however, think oxygen declines so far have been driven by complicated natural factors. Ocean conditions vary so much normally that they might be experiencing an unusual period of depletion—one that could moderate soon.

But Deutsch called that “a very, very thin silver lining.”

View Images The larvae of strange but important midwater fish like the black sea dragon have declined off California as low-oxygen regions have expanded. Photograph by Norbert Wu, Minden/Corbis

“Right now in the ocean, there is incredibly strong internal variability and a very tiny climate trend on top of it,” he says. “But my sense from all the model simulations we’ve done is that we’re on the verge of having that trend emerge from the noise.”

Some species, such as Dover sole, may be unaffected, but many areas could be left with far fewer higher life forms.

Most researchers project that oxygen loss will keep driving many species toward the surface, reducing habitat for some and concentrating prey for birds, turtles, and other surface predators.

Winds in some regions will draw the oxygen-depleted water to the surface and push it onto shallower continental shelves. When oxygen drops there, some sensitive species that can’t move die. Even survivors experience stress, which can make them vulnerable to predators, disease, or overfishing.

View Images Many midwater fish, such as the odd-looking Pacific hatchetfish, hide in deep, dark water during the day, rising only at night to feed. But changes in the ocean’s oxygen level can alter how high in the water they go. Photograph by Solvin Zankl, Nature Picture Library

This has already begun. The waters of the Pacific Northwest, starting in 2002, intermittently have gotten so low in oxygen that at times they’ve smothered sea cucumbers, sea stars, anemones, and Dungeness crabs. This biologically rich region—where winds draw waters from the deep 50 miles (80 kilometers) offshore and push them to the beach—is temporarily transformed into a lifeless wasteland.

“I look at it as a major reshaping of the ecosystem,” says Jack Barth, a chemical oceanographer at Oregon State University, in Corvallis.

Localized die-offs aren’t even the most disruptive effect of depleted oxygen.

“Changes in oxygen turn out to be really important in determining where organisms are and what they do,” says marine biologist Francis Chan, also at Oregon State University.

The fate of some odd little fish suggests the consequences can be enormous.

Into the Light

Since the 1950s, researchers every year have dropped nets 1,000 feet (300 meters) down to catalog marine life many miles off California. Most track commercially important species caught by the fishing industry. But J. Anthony Koslow tallies fish often credited with keeping marine systems functioning soundly—tiny midwater bristlemouths, the region’s most abundant marine species, as well as viperfish, hatchetfish, razor-mouthed dragonfish, and even minnow-like lampfish.

All are significant parts of the seafood buffet that supports life in the eastern Pacific, and all are declining dramatically with the vertical rise of low-oxygen water.

“If it was a 10 percent change, it wouldn’t have been worth noting, but they’ve declined by 63 percent,” says Koslow, of the Scripps Institution of Oceanography. And “what’s been amazing is it’s across the board—eight major groups of deep-sea fishes declining together—and it’s strongly correlated with declining oxygen.”

View Images As lower oxygen levels drive fish closer to the surface, many, including this viperfish and the hatchetfish it is chasing, may spend more time in areas where light penetrates. That can make them far more vulnerable to predators. Photograph by Norber Wu, Minden/Corbis

Most of these fish spend their days swimming hundreds of feet down, just above low-oxygen water. Many are black, camouflaged by the dark, deep waters where light never reaches. They rise at night to feed on plankton.

Koslow can’t say precisely why these fish populations have collapsed. But he suspects they, too, now spend more time closer to the surface seeking oxygen. That puts these fish during the day in a region where light penetrates, making them easier pickings for birds, marine mammals, rockfish, and other sight-feeders.

If that’s the case, Koslow says, “the ramifications would be huge.”

Such tiny fish are a massive food source around the world. Globally, they account for far more mass in the sea than the entire world’s catch of fish combined. But there isn’t enough historical data in other parts of the world to determine if the trend is unique to California.

“They are central to the ecology of the world’s oceans,” Koslow says.

Scientists suspect these fish already may be partly responsible for at least one surprising change—a massive northward expansion between 1997 and 2010 of the northern Pacific Ocean’s most ravenous visitor, the Humboldt squid.

Once found from South America to Mexico, with occasional forays into California, the Humboldt squid has moved so far north that in recent years it has been seen off Alaska. Researchers tested squid in tanks and found low oxygen was hard on them, too, even though the jumbo squid could slow its metabolism. Yet here they were, faring so well at the edge of low-oxygen areas they had become a master predator of midwater fish.

“These squid are out-competing all the tunas and sharks and marine mammals that may want to feed in this zone,” Stanford's Gilly says.

Researchers did not directly connect the expansion of the squid's feeding area to rising oxygen-poor water. But Koslow linked low-oxygen water to shifts in where the midwater fish on the squid's menu live. And scientists now can draw a direct line between where those fish went and the squid’s northward march, Gilly says.

“I think there might be a sweet spot for Humboldt squid, where low oxygen, food, and light are in perfect balance—and that’s accounting for their expansion,” Gilly says.

View Images From 1997 to 2010, Humboldt squid expanded their range in the eastern Pacific Ocean. Scientists suspect changing oxygen levels may have played an important role. Photograph by Carrie Vonderhaar, Ocean Futures Society/National Geographic

Still, the squid’s expansion was not subtle. Tracking its causes almost certainly is simpler than unspooling other impacts. And oxygen loss exacerbates other issues. Marine creatures need more oxygen in warmer waters, for example. Climate change means they increasingly will have less.

“I think we are changing the world; I just don’t think the responses are going to be as predictable as we think,” says Francisco Chavez, senior scientist with California's Monterey Bay Aquarium Research Institute. “I think there are a slew of surprises ahead.”

And how low-oxygen areas will affect everything else depends on how much they spread.

Looking Back to See Ahead

To answer that question, scientists recently examined marine sediment cores from a period of glacial melt 17,000 to 11,000 years ago.

During that time, global average air temperatures rose 3 to 4 degrees Celsius, the closest historical analog for the projected future, says study co-author Tessa Hill, of the Bodega Marine Laboratory. “The idea here is … let’s take an interval with somewhat analogous warming and see how low-oxygen zones responded,” Hill says.

The results: Low-oxygen areas exploded around the world.

“What we found is that their expansion was just extremely large and abrupt,” says lead author Moffitt. “Their footprint across ocean basins grew much more than we had anticipated.”

One low-oxygen region off Chile and Peru—combined, the two countries now have an anchovy fleet that makes up the world’s largest single-species fishery—was much larger then, thousands of years ago. It stretched from 9,800 feet (3,000 meters) deep to within 490 feet (150 meters) of the surface. And off California, low-oxygen waters came far closer to the surface than they do today.

Their research showed that “environments we might think of as stable, like the deep ocean, may not be so stable at all,” Moffitt says.

In the blink of an eye, geologically speaking, entire ocean basins changed. And many scientists suspect they are doing so once again, at a cost they can’t yet quantify.