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Rising ocean heat leaves fish gasping for oxygen

Lack of oxygen will leave some fish gasping as the thermometer rises. Deep time offers a guide to those at greatest risk.

LONDON, 2 December, 2020 − As global temperatures soar, the planetary menu could start to dwindle. Cod, sea bass and haddock will move to cooler and more distant waters. Tropical species relying on the shelter of coral reefs could simply disappear. Fish gasping for oxygen will struggle to survive.

And although the world’s marine catch is already under pressure from pollution, ocean acidification and overfishing, the real threat is now clear. As ocean temperatures rise, oxygen levels in the world’s seas will fall, and the most active fish could start to stifle.

Some sea creatures will survive: sharks, rays and other cartilaginous fish will do better than the bony ones. Bivalves that cling to rocks will also cling on to life.

But some types of fish could be pushed to their tolerance limits, says a new study in the journal Global Change Biology, and global heating driven by ever-higher carbon dioxide levels in the atmosphere will be the primary cause.

“Warm water contains less oxygen than cool water. This tends to affect organisms that consume the most oxygen, which can mean that actively mobile animals are particularly affected,” said Carl Reddin of the Museum of Natural History in Berlin, who headed the research.

Heading for 3°C

He and his colleagues set themselves a simple challenge: why do some groups of marine creatures go extinct more often than others? The steady decline in fish catches on traditional grounds already has one obvious explanation: humans have overfished, and polluted. So the scientists decided to take a long cool look at the past.

“The deep time fossil record, conversely, is free from human impacts, and documents extinctions during ancient episodes of rapid climate warming, or hyperthermals,” they write.

They looked back across the evidence preserved in the rocks over the last 300 million years and identified what they call “six global hyperthermal events that shared a rapid increase in tropical sea surface temperatures, generally greater than 2°C, with an onset duration less than 100,000 years.”

In effect, they were looking for global conditions that matched those now happening. In the last 100 years, planetary average temperatures have risen by 1°C, and although almost all the world’s nations met in 2015 and vowed to try to contain global heating by 2100 to “well below” 2°C, the planet is heading towards a rise of more than 3°C above the long-term average in the next eight decades.

The Berlin team found that those groups of marine animals that − on the evidence of the fossil record − were most vulnerable to global warming in the deep past looked very like those that seem most in trouble today, among them the bony fishes.

“The deep time fossil record documents extinctions during ancient episodes of rapid climate warming”

The idea is not new. Other marine biologists have repeatedly warned of oxygen depletion in and beyond the fishing grounds.

Separately, there has been evidence that higher temperatures have begun to change the nature of the oceans, and fishermen have begun to count the cost as their catch migrates to waters that are cooler.

What this latest study does is clear up the uncertainty. Overfishing remains a problem. Ocean acidification will certainly affect some shellfish and possibly also fish behaviour. Pollution has already increased the number of marine dead zones.

But beyond that, the problem is simply one of temperature, and the latest study identifies those groups or classes of marine creature most at risk from another rise in the planetary thermometer: those sensitive to “warming-induced seawater de-oxygenation,” the researchers report.

And they add: “In anticipation of modern warming-driven marine extinctions, the trends illustrated in the fossil record offer an expedient preview.” − Climate News Network

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