Ocean warming harms seafood supplies and situation worsens | Science

North Sea cod have been affected by rising temperatures and overfishing.

PA Images / Alamy Stock Photo

By Erik Stokstad

Marine fish around the world are already feeling the effects of climate change – and some are shaking, according to the first major analysis of recent trends. Rising sea temperatures have reduced the productivity of some fisheries by 15 to 35 percent in eight decades, although in other places fish are booming because warming waters are better for them. The net effect is that the world's oceans can no longer produce so many sustainable seafood products, a situation that is likely to worsen as global warming accelerates in the oceans.

Research suggests that well-managed fisheries are more resilient to rising temperatures, says Rainer Froese, marine ecologist at the GEOMAR Helmholtz Ocean Research Center in Kiel, Germany, who did not participate in the work. "We need 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 reflect the temperature of the water in which they swim. When the water gets too hot, the enzymes they use for digestion and other functions are less effective, which hinders growth and reproduction. In addition, the hot water contains less oxygen, another stressor.

Despite these well-known problems, no one has ever examined the impact of climate change on fisheries around the world. Chris Free, a fisheries scientist, is immersed in the subject of his thesis at Rutgers University in New Brunswick, New Jersey. He has created a computer model of how fish populations react to temperature, drawing on a vast database of scientific stock assessments representing about one third of the world's fish caught. Free, now a postdoctoral fellow at the University of California, Santa Barbara, examined trends in the response of these stocks to changes in sea-surface temperature.

Managing a fish stock, in simple terms, is tantamount to taking money out of a bank account that pays interest. Each year, fishing boats can catch a certain amount without depleting the stock – this part is called maximum sustainable yield. A more productive fishery – where the water temperature is optimal and where food is abundant, for example – looks like a bank account with a higher interest rate, which means that more fish can be caught in a sustainable.

So what is climate change doing for sustainable fisheries? Out of 235 actions, Free and his colleagues found some winners. Nine stocks had become on average 4% more productive. These stocks are found in places where rising temperatures have made the waters too cold more suitable for fish, such as north and south of the equator. In Canada, for example, off the coast of Newfoundland and Labrador, the maximum sustainable yield has increased by 14% since 1930. And the fishery could be even better there. According to the new study, the productivity of Greenland halibut will increase by 51% at each degree of warming. It's like having a big boost for the interest rate of your savings account.

This good local news is offset by 19 stocks elsewhere, which are on average 8% less productive than before. Many of them are found in northern Europe and Japan, and they will likely continue to degrade as their environment continues to heat up. The boats that hunt Atlantic cod at sea in Ireland are facing a particularly bleak future: the maximum sustainable yield of this stock will decrease by 54% at each additional degree of warming, announced today. the team at the Science.

By combining winners and losers, the overall maximum sustainable yield of 235 stocks is now 4% lower than in 1930. This represents about 1.4 million tonnes less fish than could already be fished sustainably. "At first glance, it looks like a small number," says Free, "but it's a big problem for the lives of the people who depend on it."

The figure is probably an underestimate because there is little data from the tropics. Tropical fish already live in warm waters. As a result, they were probably more affected by rising temperatures than temperate fish. "The fish over there are already back to the wall when it comes to temperature," says Froese. "We expect the tropics to be the hardest hit."

The findings constitute "an important step forward," written in an accompanying commentary Eva Plaganyi from the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Canberra. Science. The study, she adds, provides a "solid foundation" for predicting the impact of rising temperatures on some stocks in specific locations.

The general decline is likely to increase, as previously predicted. Since 1930, average sea surface temperatures have increased by about 0.5 ° C. By the end of this century, warming will probably occur three times longer and marine heat waves will become more frequent. Although temperatures will become more favorable for fish in higher latitude waters, "these benefits can not last forever," says Free. "There is probably a turning point."

Fisheries managers can help the situation. The analysis suggests that stocks are harder hit by rising temperatures if they have been heavily overexploited. This is surprising, says Froese, because fishing tends to selectively eliminate larger fish and heavily fished stocks become smaller and mature faster. These smaller fish, which use oxygen more efficiently, could, in theory, be better able to cope with warmer, less oxygen-rich water. But the new study suggests that these stocks were less resistant to temperature increases.

One of the reasons could be that overfishing has wiped out genes to cope with warmer temperatures, says Froese. Whatever the mechanism chosen, fisheries experts know that reducing overfishing leads to larger and more sustainable catches. "Reducing overfishing, he says, is obvious."

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