Bottom trawling releases around 340 million tonnes of carbon dioxide into the atmosphere each year, according to the first study to estimate these emissions. That is nearly 1 per cent of all global CO2 emissions, a major contribution that has been overlooked until now.
Trawling involves dragging weighted nets across the seafloor to catch bottom-dwelling fish, crustaceans and shellfish. This practice is widely used around the world, but it is controversial because the fishing gear damages seafloor environments such as cold water reefs, where some corals may be thousands of years old.
“Bottom trawling is an extremely destructive form of fishing as the nets and weights dragged along the bottom destroy marine habitats that can take many years to re-establish and recover,” says Mika Peck at the University of Sussex, UK, who wasn’t involved in the research.
It also stirs up sediments, releasing the oxygen that microbes need to break down organic matter into carbon dioxide. Those sediments might otherwise continue to build up for many millennia, with the organic matter in them preserved by low-oxygen conditions – meaning the carbon is effectively locked away.
In 2021, Trisha Atwood at Utah State University in Logan and her colleagues combined studies looking at how much CO2 may be released during trawling with data on the extent of trawling worldwide from an organisation called Global Fishing Watch. The team concluded that massive amounts were released into the seawater.
But the big unanswered question was how much of the CO2 released from sediments ends up in the atmosphere.
“Lots of countries and different agencies started asking us about that research,” says Atwood. “But they basically said, if it just stays in the ocean, we don’t really care.”
So the team has combined forces with researchers who have developed computer models of ocean circulation. According to those models, around 55 per cent of the CO2 released into water by trawling will end up in the atmosphere after nine years.
“I was surprised that about more than half comes out,” says Atwood. “And that it comes out quite rapidly.”
According to the Global Carbon Budget, total CO2 emissions from human activities rose to 40.9 billion tonnes in 2023. So if the team’s estimate is correct, trawling is responsible for around 0.8 per cent of global emissions, compared with 2.8 per cent for aviation and shipping.
Conservationists say the findings strengthen the case for reducing trawling. “Many marine habitats are trawled more than once a year, resuspending sediments and liberating carbon to the atmosphere,” says Peck. “A ban of destructive fishing practices is key to the future of healthy marine ecosystems and those that depend on them.”
“Measures to reduce the carbon impact of bottom-towed fishing gear are urgently needed, though it must be done as part of a just transition,” says Gareth Cunningham at the Marine Conservation Society, which has been calling for a ban on trawling in so-called marine protected areas around the UK. “There isn’t a one-size-fits-all model, and solutions will vary from one location to another.”
But not all researchers are convinced by the numbers. “I’m very sceptical about their estimates,” says Jan Geert Hiddink at Bangor University in the UK.
Hiddink thinks much of the carbon that reaches the seafloor is in hard-to-break-down forms, such as in bones, meaning it isn’t released even when sediments are disturbed. Atwood’s team may be overestimating the quantity released by up to 1000 times, he argues.
Atwood says the estimate is based on actual measurements. “We took studies that measure the amount of CO2 that was coming off of the seabed in areas that are trawled,” she says.
There have been very few of these studies, she says, so there is a great deal of uncertainty, but the amount of CO2 released could be higher than these studies suggest as well as lower.
Governments need to start counting the CO2 emissions from trawling, says Atwood. “That will allow them to determine whether or not they should regulate those emissions,” she says.
What is clear is that the extent of trawling is greater than the study assumes, because the Global Fishing Watch trawling data is based on boats that emit automated signals to satellites, and many trawlers don’t carry these systems.
“We know that we’re underestimating the global extent of trawling and probably its intensity,” says Atwood.
There is also an opportunity for the trawling industry to sell carbon credits in exchange for reducing emissions, she says. “If you were to give it a price on today’s voluntary market, it’s a 100 million dollar market.”