Trawling the bottom of the ocean is kicking up tons of carbon dioxide

Your shrimp cocktail comes with a side of carbon dioxide, according to scientists who have for the first time quantified greenhouse emissions caused by a destructive fishing technique known as bottom trawling.

Bottom-trawling ships deploy huge weighted nets — up to half a mile (0.8 kilometers) in length — that scour the ocean floor to scoop up shrimp, crab, cod, halibut and other fish. Scientists and environmentalists have long opposed bottom trawling for the damage it inflicts on seabed ecosystems such as coral reefs, and for killing sea turtles, sharks and other non-targeted marine species inadvertently caught in the drag nets.

There’s also a climate cost to be paid, according to a paper published Thursday in the journal Frontiers in Marine Science. Researchers calculated that bottom trawling’s disturbance of CO2 sequestered in seabed sediments results in as much as 370 million metric tons of the greenhouse gas being released into the atmosphere each year. That’s more than twice the CO2 emitted by the global fishing industry’s burning of fossil fuels.

The authors of the new paper also estimated that any released CO2 that remains in the ocean is acidifying the surrounding waters, which can dissolve the shells of crabs, mussels, sea urchins and other seafood people depend on.

“These are enclosed areas, particularly like the Mediterranean, where we can see that CO2 can create localized acidification that could be quite substantial,” said Trisha Atwood, the study’s lead author and an associate professor of watershed sciences at Utah State University. She noted that further research is needed to quantify the local impact of acidification as the researchers’ modeling looked at the ocean on a global scale.

The Frontiers study isn’t the first to connect trawling and CO2; a 2021 paper in the journal Nature, which analyzed measurements of CO2 in trawled areas, established for the first time that disturbed sediment had released the planet-warming gas into the ocean. The new research, based on that data, used computer models to show that 55% to 60% of carbon dioxide released from trawled sediment makes its way into the atmosphere from depths of at least 1,640 feet (500 meters), while the rest remains in the ocean.

And Ocean Rebellion activist protests against bottom trawling during a demonstration ahead of the COP26 summit in Glasgow in 2021.
| REUTERS

Though it can take decades for dissolved carbon dioxide in seabed sediments to be released into the atmosphere, depending on depth, the scientists’ models showed that CO2 from trawling entered the air relatively quickly between 1996 and 2020.

“It only takes about nine years for it to make it completely out of the ocean and into the atmosphere, and there’s enough CO2 that is being emitted by global trawling each year that people need to pay attention to it,” Atwood said. “Some of the CO2 is very, very old and might have been laid down in the ocean 10,000 years ago.”

Anastasia Romanou, a NASA scientist who studies the ocean carbon cycle and is a co-author on the new paper, said the speed at which CO2 from trawling reaches the atmosphere means that curbing the practice would have an almost immediate climate benefit. “Any mitigation effort will be very effective and we’ll be able to see the results,” said Romanou, a contributing author to the United Nations Intergovernmental Panel on Climate Change.

The findings are also still underestimating CO2 emissions from bottom trawling, Atwood said, as data wasn’t available for certain under-monitored trawling hotspots, such as Southeast Asia. The scientists identified the Baltic Sea, the East China Sea, the Greenland Sea and the North Sea as most impacted by bottom trawling.

The new research is likely to face pushback from some corners of the scientific world. After publication of the 2021 paper, a group of scientists wrote a response that questioned the methodology used and contended that the researchers had overestimated the amount of CO2 released from seabed sediment due to trawling.

The lead author of the 2021 paper, marine scientist Enric Sala, said in a statement that the new research validated his colleagues’ initial findings. “Many people dismissed the importance of the findings of the 2021 study, saying that the carbon in the water is inconvenient but that atmospheric emissions are what count,” said Sala, also a co-author of the new paper and executive director of the National Geographic Society’s Pristine Seas initiative. “This [new] report is essential in that it shows that roughly half of the emissions from bottom trawling do pollute the atmosphere and the other half increases the acidity of the ocean.”

Romanou said that while the estimated release of atmospheric CO2 from trawling would be a significant percentage of emissions from fishing, it’s a small figure in the context of overall global emissions. Still, bottom trawling’s footprint is growing, she noted, and policymakers are not yet considering its impacts on ocean acidity and CO2 emissions.

“As humanity expands trawling activities and intensifies them, we expect to see larger regions being affected,” Romanou said.