More than a quarter of the wild seafood the world eats comes from the seabed. Shrimp, skate, sole, cod and other creatures – mostly flat animals – that roam the bottom of the sea are scooped up in large nets. These nets, called bottom trawls, net millions of tons of fish worth billions of dollars each year. But they also damage coral, sponges, starfish, worms and other sand dwellers as the nets scrape the seabed. Environmentalists sometimes compare the practice to strip mining or clear-cutting of forests.
According to a new study in the journal Frontiers in Marine Science, bottom trawling can be even worse than many people thought. Dragging nets through the sand – which occurs over some 5 million square kilometers, a little more than 1 percent of the ocean floor—is not just a threat to marine life. The study found that generating carbon-rich sediment on the ocean floor releases about 370 million metric tons of planet-warming carbon dioxide each year, about the same as operating 100 coal-fired power plants.
“I was pretty surprised,” said Trisha Atwood, a watershed scientist at Utah State University and the paper’s lead author. The findings, Atwood added, suggest that limiting bottom trawling could have “almost immediate benefits” for the climate.
The paper follows a study by some of the same scientists published in the journal Earth in 2021 – one that has also attracted a lot of media attention criticism from other researchers who thought its results were far off. In 2021, Atwood’s team found that bottom trawls unlock more carbon from the ocean floor than all the world’s airplanes emit each year. But they couldn’t say how much of that carbon ended up in the atmosphere warming the Earth and how much of it stayed in the water.
So this is what they wanted to do in the latest study. The team used fishing vessel data to map regions where trawlers have disturbed the seabed – such as the North Sea off the coast of Europe – and applied ocean circulation models to estimate how much carbon dioxide flows from the sea into the air. They found that more than half of the carbon released by trawling enters the atmosphere—and it does so relatively quickly, in less than a decade.
“The key finding here is that these emissions are not negligible,” said Juan Mayorgas, a marine data scientist at the National Geographic Society and co-author of the paper. “They are not small. They cannot be ignored.”
The world’s oceans are spongy in their ability to absorb carbon, soaked a quarter up of all the carbon dioxide that humans spew into the air. In fact, a lot more carbon is stored in the ocean than in all the soil and plants on earth. But until recently, little attention was paid to how much the oceans emit. “We know the oceans are not a closed system,” Mayorgas said. “At the same time the ocean absorbs CO2, it puts it out.”
Most climate goals and policies do not take into account emissions from sea-based activities such as trawling. Atwood and Mayorgas said their study could help change that. “Now,” Mayorgas said, “countries can put all the information on the table and say, ‘Here’s how many jobs trawling produces, here’s how much food it produces, here’s how much carbon it emits.’
But there is one big caveat: Not everyone agrees with their research. The 2021 paper — which provided data for the new study — drew significant backlash from scientists who called the results “wildly overrated.”
“I am very skeptical of their estimates,” Jan Geert Hiddink, a marine biologist at Bangor University in the Netherlands, said in an email. The team’s emissions estimates are off by “several orders of magnitude,” he said, and “will likely lead to incorrect management actions.”
Hiddink, who co-authored a comment in Earth criticize the 2021 paper, arguing that carbon stored in the ocean floor is much less likely to be converted into carbon dioxide than Atwood’s team assumes in their models. He said that trawling in some places – such as shallow coastal areas that have muddy sediment and contain more carbon than deeper, sandier areas – is likely to inject some carbon dioxide into the water and atmosphere, but that more detailed research is needed to understand exactly how much is released. Hiddink suggested that some of the carbon dioxide that Atwood’s team claims is released by rustling the ocean floor is actually released naturally by microbes breaking down decaying fish skeletons and other organic matter.
“There’s no way that the kind of numbers they’re talking about are at all realistic,” said Ray Hilborn, a fisheries scientist at the University of Washington. (Hilborn was criticized for financial support from the fishing industry for his research. In responseHilborn said he has been open about funding sources, noting that he has also received support from environmental groups such as The Nature Conservancy and Environmental Defense Fund.)
Atwood said Hiddink’s criticism is “entirely theoretical” and does not line up as closely with empirical studies as her team’s models. Enric Sala, a researcher at the National Geographic Society and lead author of the 2021 paper, also pushed back against Hiddink’s points, saying in a prepared statement that they “lack quantitative support.”
Atwood and her colleagues admit, however, that it is not entirely clear how easily the sediment scooped up by trawl lines releases carbon dioxide. Studies on that issue are “extremely limited,” the authors wrote. She said the latest paper is valuable in finding out how much carbon dioxide rises into the air after trawlers release it into the water.
“All of us agree,” Atwood said, “that this is an area in which we need more research.”
