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As Ocean Oxygen Levels Dip, Fish Face an Uncertain Future

As Ocean Oxygen Levels Dip, Fish Face an Uncertain Future

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This story was originally published by Yale Environment 360 and is reproduced here as part of the Climate Desk collaboration.

Off the coast of southeastern China, one particular fish species is booming: the oddly named Bombay duck, a long, slim fish with a distinctive, gaping jaw and a texture like jelly. When research ships trawl the seafloor off that coast, they now catch upwards of 440 pounds of the gelatinous fish per hour—a more than tenfold increase over a decade ago. “It’s monstrous,” says University of British Columbia fisheries researcher Daniel Pauly of the explosion in numbers.

The reason for this mass invasion, says Pauly, is extremely low oxygen levels in these polluted waters. Fish species that can’t cope with less oxygen have fled, while the Bombay duck, part of a small subset of species that is physiologically better able to deal with less oxygen, has moved in.

The boom is making some people happy, since Bombay duck is perfectly edible. But the influx provides a peek at a bleak future for China and for the planet as a whole. As the atmosphere warms, oceans around the world are becoming ever more deprived of oxygen, forcing many species to migrate from their usual homes. Researchers expect many places to experience a decline in species diversity, ending up with just those few species that can cope with the harsher conditions. Lack of ecosystem diversity means lack of resilience. “Deoxygenation is a big problem,” Pauly summarizes.

Our future ocean—warmer and oxygen-deprived—will not only hold fewer kinds of fish, but also smaller, stunted fish and, to add insult to injury, more greenhouse-gas producing bacteria, scientists say. The tropics will empty as fish move to more oxygenated waters, says Pauly, and those specialist fish already living at the poles will face extinction.

Researchers complain that the oxygen problem doesn’t get the attention it deserves, with ocean acidification and warming grabbing the bulk of both news headlines and academic research. Just this April, for example, headlines screamed that global surface waters were hotter than they have ever been—a shockingly balmy average of 70 degrees F. That’s obviously not good for marine life. But when researchers take the time to compare the three effects—warming, acidification, and deoxygenation—the impacts of low oxygen are the worst.

“That’s not so surprising,” says Wilco Verberk, an eco-physiologist at Radboud University in the Netherlands. “If you run out of oxygen, the other problems are inconsequential.” Fish, like other animals, need to breathe.

Oxygen levels in the world’s oceans have already dropped more than 2 percent between 1960 and 2010, and they are expected to decline up to 7 percent below the 1960 level over the next century. Some patches are worse than others—the top of the northeast Pacific has lost more than 15 percent of its oxygen. According to the IPCC’s 2019 special report on the oceans, from 1970 to 2010, the volume of “oxygen minimum zones” in the global oceans—where big fish can’t thrive but jellyfish can—increased by between 3 and 8 percent.

The oxygen drop is driven by a few factors. First, the laws of physics dictate that warmer water can hold less dissolved gas than cooler water (this is why a warm soda is less fizzy than a cold one). As our world warms, the surface waters of our oceans lose oxygen, in addition to other dissolved gases. This simple solubility effect accounts for about half of the observed oxygen loss seen so far in the upper 1,000 meters of the ocean.

Percent change in dissolved ocean oxygen per decade since 1960.

Schmidtko et al/Geomar

Deeper down, oxygen levels are largely governed by currents that mix surface waters downward, and this too is being affected by climate change. Melting ice adds fresh, less-dense water that resists downward mixing in key regions, and the high rate of atmospheric warming at the poles, as compared to the equator, also dampens winds that drive ocean currents.

Finally, bacteria living in the water, which feed off phytoplankton and other organic gunk as it falls to the seafloor, consume oxygen. This effect can be massive along coastlines, where fertilizer runoff feeds algae blooms, which in turn feed oxygen-gobbling bacteria. This creates ever more “dead zones,” including the infamous one in the Gulf of Mexico.

Researchers have even suggested that the rise of microplastics pollution has the potential to exacerbate the low-oxygen problem. This theory predicts that if zooplankton fill up on microplastics instead of phytoplankton—their usual prey—phytoplankton will proliferate, again feeding all those oxygen-gobbling bacteria on their way to the seafloor.

The Global Ocean Oxygen Network—a scientific group set up as part of the United Nation’s Decade of Ocean Science for Sustainable Development, 2021-2030—reports that since the 1960s, the area of low-oxygen water in the open ocean has increased by 1.7 million square miles. That’s an area a little more than half the size of Canada. By 2080, a 2021study reported, more than 70 percent of the global oceans will experience noticeable deoxygenation.


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