Fish populations are declining as oceans warm, putting a key source of food and income at risk for millions of people around the world, according to new research published Thursday.
The study found that the amount of seafood that humans could sustainably harvest from a wide range of species shrank by 4.1 percent from 1930 to 2010, a casualty of human-caused climate change.
“That 4 percent decline sounds small, but it’s 1.4 million metric tons of fish from 1930 to 2010,” said Chris Free, the lead author of the study, which appears in the journal Science.
Scientists have warned that global warming will put pressure on the world’s food supplies in coming decades. But the new findings — which separate the effects of warming waters from other factors, like overfishing — suggest that climate change is already having a serious impact on seafood.
Fish make up 17 percent of the global population’s intake of protein, and as much as 70 percent for people living in some coastal and island countries, according to the Food and Agriculture Organization of the United Nations.
“Fish provide a vital source of protein for over half of the global population, and some 56 million people worldwide are supported in some way by marine fisheries,” Dr. Free said.
As the oceans have warmed, some regions have been particularly hard-hit. In the northeast Atlantic Ocean and the Sea of Japan, fish populations declined by as much as 35 percent over the period of the study.
“The ecosystems in East Asia have seen some of the largest decline in fisheries productivity,” Dr. Free said. “And that region is home to some of the largest growing human populations and populations that are highly dependent on seafood.”
Now a postdoctoral researcher at the University of California, Santa Barbara, Dr. Free began the research while a Ph.D. student at Rutgers University.
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Marine life has been subjected to some of the most drastic effects of climate change. The oceans have absorbed 93 percent of the heat that is trapped by the greenhouse gases that humans pump into the atmosphere.
A study published in January, also in Science, found that ocean temperatures were increasing far faster than previous estimates.
Amid these changing conditions, fish are shifting where they live, in search of their preferred temperatures. High ocean temperatures can kill off both the fish themselves and the sources of food they depend on.
“Fish are like Goldilocks: They don’t like their water too hot or too cold,” said Malin L. Pinsky, an associate professor in the School of Environmental and Biological Sciences at Rutgers University and a co-author of the new study.
In about a quarter of the regions studied, fish had expanded their range. Off the Atlantic coast of the United States, sustainable catches of black sea bass increased by 6 percent over the study period.
Another quarter of the regions saw no significant changes in fish populations, like the northwest Atlantic Ocean, where Atlantic herring are abundant.
But half the regions did not fare as well. The northeast Atlantic Ocean — home to the Atlantic cod, the mainstay of fish and chips — saw a 34 percent decline in sustainable catches.
Over all, more populations of fish declined than increased over the eight decades in the study.
The researchers focused on sustainable catches, using a measure developed by the United Nations that quantifies the amount of food that can be repeatedly harvested from a base population of fish. “Fisheries are like a bank account, and we’re trying to live off the interest,” Dr. Pinsky said.
Several previous studies have predicted that climate change would lead to fewer ocean fish in the future, but the new research looked at historical data to determine that the declines had already begun.
“This is going to be one of those groundbreaking studies that gets cited over and over again,” said Trevor Branch, an associate professor at the University of Washington’s School of Aquatic and Fishery Sciences, who was not involved in the study. “Most of what I’ve seen before in terms of climate-change impacts have been speculative, in terms of, ‘We think this is what’s going to happen in the future.’ This one’s different.”
The researchers used a data set of 235 fish populations located in 38 ecological regions around the globe. The detailed data told them not only where the fish were but also how they reacted to environmental effects like changing water temperatures.
The team compared that data to records that showed how ocean temperatures had changed over time, broken down by the various regions. These regional analyses were important, because some parts of the ocean have warmed faster than others.
“We then connected those to which populations responded positively, negatively, and which didn’t respond at all,” Dr. Pinsky said.
The data revealed some other trends. Fish populations in the colder parts of their ranges tended to fare better than those located in warmer areas — for those fish, the extra heat was too much. This was especially troubling to the researchers, because the data they used was less detailed in the tropics. Fish losses in those regions may have been higher than in the regions the study focused on, Dr. Pinsky said.
Warm areas fared even worse when they were overfished. The researchers suggested that overfishing made fish even more vulnerable to temperature changes by hurting their ability to reproduce and damaging the ecosystem.
Guarding against overfishing and improving the overall management of fisheries can help, the researchers said. But ultimately, they said, the solution lies in slowing or halting climate change.
A separate study, published Wednesday in the journal Science Advances, found that limiting warming to 2.7 degrees Fahrenheit, or 1.5 degrees Celsius, above preindustrial levels — a goal of the Paris climate agreement — could result in billions of dollars in extra revenue for fisheries globally. Much of that would be in the developing world, where many people rely on fish for protein.
“We hope that this highlights the importance of accounting for the fact that climate change is driving shifts in productivity,” Dr. Free said of his research. “Fishery managers need to come up with new innovative ways of accounting for those shifts. That includes reducing catch limits in warm negative years, but it can also include increasing catch limits in cooler positive years. Having regulations that are adaptive to climate change is going to be really important for maximizing food potential.”
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