Cod in the North Sea have suffered from rising temperatures, as well as overfishing.

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Warming oceans are hurting seafood supply—and things are getting worse

Marine fish around the world are already feeling the effects of climate change—and some are reeling, according to the first large analysis of recent trends. Rising sea temperatures have reduced the productivity of some fisheries by 15% to 35% over 8 decades, although in other places fish are thriving because warming waters are becoming more suitable. The net effect is that the world’s oceans can’t yield as much sustainable seafood as before, a situation that is likely to worsen as global warming accelerates in the oceans.

A silver lining is that the research suggests well-managed fisheries are more resilient in the face of rising temperature, says Rainer Froese, a marine ecologist with the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, who was not involved in the work. “We have to stop overfishing to let the gene pool survive, so that [the fish] can adapt to climate change,” he says. “We have to give them a break.”

As cold-blooded animals, fish mirror the temperature of the water they swim in. When the water gets too warm, the enzymes they use for digestion and other functions are less efficient, impairing growth and reproduction. In addition, warm water contains less oxygen, a further stressor.

Despite these well-known problems, no one had looked at the impact that climate change has had so far on fisheries around the world. Chris Free, a fisheries scientist, dove into the topic for his dissertation at Rutgers University in New Brunswick, New Jersey. He created a computer model of the way fish populations respond to temperature, relying on a large database of scientific assessments, conducted between 1930 and 2010, of stocks that represent roughly a third of the fish caught around the world. Free, now a postdoc at the University of California, Santa Barbara, looked for patterns of how these stocks had responded to changes in sea surface temperature.

Managing a stock of fish, in simple terms, is like withdrawing cash from a bank account that earns interest. Each year, a certain amount can be caught by fishing boats without depleting the stock—that portion is known as the maximum sustainable yield. A more productive fishery—where water temperature is optimal and food plentiful, for example—is like a bank account with a higher interest rate, which means more fish can be sustainably caught.

So what has climate change done to sustainable fishing? Out of 235 stocks, Free and his colleagues found a few winners. Nine stocks had become on average 4% more productive. These stocks are in places where rising temperatures have made too-cold water more suitable for fish, such as far north and south of the equator. Off the coast of Newfoundland and Labrador in Canada, for example, the maximum sustainable yield has increased by 14% since 1930. And fishing there could get even better. According to the new research, the productivity of Greenland halibut will increase 51% with each degree Celsius of warming. That’s like getting a big, fat boost to the interest rate on your saving account.

This local good news is outweighed by 19 stocks elsewhere that are on average 8% less productive than before. Many of these are around northern Europe and Japan, and they will likely continue to decline as their environment continues to heat up. Boats chasing Atlantic cod in the Irish Sea face a particularly grim future: The maximum sustainable yield of this stock will shrink by 54% for each additional degree of warming, the team reports today in Science.

Combining winners and losers, the overall maximum sustainable yield of the 235 stocks is now 4% lower than in 1930. That’s about 1.4 million fewer tons of fish than could be sustainably caught previously. “At first glance, it feels like a small number,” Free says, “but it’s a big deal for the lives of people who depend on them.”

The figure is probably an underestimate because there are few data from the tropics. Fish in the tropics already live in warm water, so they have likely suffered more from recent temperature rises than have fish in the temperate zone. “Fish there are already with their back to the wall with respect to temperature,” Froese says. “We expect the tropics to be hardest hit.”

The findings are “an important advance,” Éva Plagányi of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Canberra writes in an accompanying commentary in Science. The study, she adds, offers “a solid foundation” for predicting how rising temperature will impact particular stocks in particular places.

The overall decline will most likely steepen, as forecasts have previously suggested. Since 1930, average sea surface temperatures have risen by about 0.5°C. By the end of this century, more than three times that amount of warming will likely happen, and marine heat waves will become more frequent. Although temperatures will become more favorable to fish in higher latitude waters, “those benefits can’t last forever,” Free says. “There probably is a tipping point.”

Fishery managers can help the situation. The analysis suggests stocks are harder hit by rising temperature if they have been heavily overfished. That is surprising, Froese says, because fishing tends to selectively remove larger fish and heavily fished stocks evolve to be smaller and mature faster. These smaller fish, which are more efficient at using oxygen, might, in theory, be better able to cope with warmer water that has less oxygen. But the new study suggests these stocks were less resilient to temperature increases.

One reason could be that excess fishing wiped out the genes for coping with warmer temperatures, Froese says. Whatever the mechanism, fisheries scientists know that curbing overfishing leads to larger and more sustainable harvests. “Reducing overfishing,” he says, “is a no-brainer.”