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The world's oceans have warmed over the past several decades, and that warming from climate change has significantly affected where fish live. That's according to a study in the journal Nature. Fish are gradually migrating away from the equator and toward cooler water. As NPR's Richard Harris reports, that's influencing which fish end up on dining tables around the world.

RICHARD HARRIS, BYLINE: There's no shortage of anecdotes about this phenomenon. People in Denmark are encountering swordfish, which you'd normally find in the Mediterranean and off the coast of Africa. Fishermen have scuffled over Atlantic mackerel quotas, as the fish moves north to new grounds around Iceland.

DANIEL PAULY: In British Columbia, where I live, we have Humboldt squid, giant squid from Mexico.

HARRIS: Daniel Pauly is a fisheries professor at the University of British Columbia.

PAULY: They eat all the herrings and stuff, and people don't know them. They are stranded on the beach, and people think they are sea monsters.

HARRIS: But anecdotes are, well, anecdotes. So Pauly and his colleagues set out to look at trends in global fishing to see whether this trend is truly a worldwide phenomenon. Their answer is yes indeed.

PAULY: The composition of the fish catch includes more and more fish from the warmer areas, and coldwater fish are getting more rare because the temperatures are increasing.

HARRIS: And as this trend continues, Pauly says it will have some disturbing effects.

PAULY: Imagine a reef fish that is driven by temperature into North Carolina or Delaware coast. Well, that reef fish will not find reefs. It's like you're having to move, but you cannot take your furniture with you or your house. That is the problem.

HARRIS: This trend isn't so obvious at American fish counters. That's because 80 percent of our seafood is imported, and we don't know whether fishermen are catching our swordfish in the tropics or the North Atlantic. But the lead author of the study, William Cheung, says this is potentially a serious problem for areas that depend on their local fishermen for protein. This is the story in the tropics.

WILLIAM CHEUNG: In the tropics, there are lots of developing countries' fisheries where their ability to adapt to changes in the resource is much lower.

HARRIS: And unfortunately, that's where the change is the most serious. That's because there are no fish to replace the ones that are heading toward cooler waters.

CHEUNG: These fisheries in the tropics will be most vulnerable to climate change impacts.

HARRIS: Cheung, who is also at the University of British Columbia, says he was surprised by his result. He thought this would only be evident in the years to come, but no.

CHEUNG: The changes have already been happening in the last few decades. That means that there is an even more urgent need to deal with this problem.

HARRIS: That sentiment is echoed by Mark Payne at the National Institute for Aquatic Resources in Denmark.

MARK PAYNE: This is suddenly a wake-up call. It's a strong suggestion that climate change is here, that it's real, and it's really starting to affect what we catch and, therefore, what we eat.

HARRIS: Payne was not involved in the latest study, but he's impressed by the result.

PAYNE: Fisheries scientists have been very focused on the overfishing crisis and rightly so. This has been the most important thing in global fisheries for the last 15 to 20 years. But what we've actually suddenly realized and what this article shows is that at the same time, under our very noses, the composition of our fisheries have also been changing, and they're correlated with the warming trend.

HARRIS: It won't be easy to prevent the oceans from warming up still more, but Payne says the people who manage fisheries can at least start taking into account that many fish are moving away from the equator and toward the poles. That not only changes where they'll be caught, but relocating fish populations are facing additional stress, and that means it's even more important to rein in overfishing. Richard Harris, NPR News.

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