It’s Getting Harder for Fish in the Sea to Breathe

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https://thetyee.ca/News/2023/07/07/Harder-Fish-Sea-Breathe/

Nicola Jones7 Jul 2023Yale Environment 360
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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.
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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.
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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.

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Oxygen levels in the world’s oceans have already dropped more than 2 per cent between 1960 and 2010, and they are expected to decline up to seven per cent 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 per cent of its oxygen.

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The Global Ocean Oxygen Network — a scientific group set up as part of the United Nations’ Decade of Ocean Science for Sustainable Development, 2021-30 — reports that since the 1960s, the area of low-oxygen water in the open ocean has increased by 4.4 million square kilometres. That’s an area a little more than half the size of Canada. By 2080, a 2021 study reported, more than 70 per cent of the global oceans will experience noticeable deoxygenation.

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One issue, he notes, is that low-oxygen conditions tend to host a class of anoxic bacteria that produce methane or nitrous oxide — potent greenhouse gases.

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In general, a hot fish has a higher metabolism and needs more oxygen. Trout, for example, need five to six times more dissolved oxygen when waters are a balmy 24 C than when they are a chilly 5 C. So as waters warm and the oxygen seeps out, many marine creatures take a double hit. “Fish require a lot of oxygen, particularly the large

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A 2021 paper showed that the oceans are already committed to a fourfold greater oxygen loss, even if CO2 emissions stop immediately.

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