Ocean fish can weather marine heatwaves with no major impact on their numbers, according to a new study that offers a rare flash of good news following a summer of record-breaking temperatures.

Weather monitors reported record-warm sea-surface temperatures in July, but the study of thousands of deeper fishery samples from 1993 to 2019 found that ocean heatwaves generally spared fisheries.

"I was surprised by the results," said Alexa Fredston, assistant professor in the department of ocean sciences at the University of California, Santa Cruz, and lead author of the multinational study published in the journal Nature.

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"We know that fish communities have responded to long-term warming of the oceans by moving toward the poles... so I anticipated similar findings – such as the fish community having more warm-affiliated species and fewer cold-affiliated species than usual – following marine heatwaves."

However, the study found such heatwaves did not generally cause cold-water species to decline rapidly or warm-water species to teem. The researchers analysed data on 1,769 species in 82,000 catches by scientific trawlers in the north Atlantic and northeast Pacific, and on 248 deep-sea heatwaves – five days or more of extreme higher-than-average warmth – recorded in the same period.

They noted a 22 percent loss of fish in the Gulf of Alaska after a marine heatwave in 2014-2016, and a gain of 70 percent in the northeastern U.S. after another heatwave in 2012.

Such cases, however, "were the exception, not the rule," the study said.

"Against the highly variable backdrop of ocean ecosystems, marine heatwaves have not driven biomass change or community turnover in fish communities that support many of the world's largest and most productive fisheries."

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