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The destructive fishing practice destroys underwater ecosystems, kills marine life and kicks up soils needed to store carbon dioxide.
When Bryce Stewart dived after the toothed, steel-weighted nets of a scallop dredger rumbling over the bottom of the Irish Sea 22 years ago, he witnessed destruction he could never have seen from a boat.
"Half crabs. Smashed up sea urchins. Starfish missing some of their arms," said Stewart, a marine ecologist at the University of York. "There was literally a trail of dead and dying things on the seabed."
Bottom trawling — a powerful practice in which heavy nets are dragged along the floor of the ocean to catch fish and seafood — is one of the most harmful ways to feed the world. It destroys ecosystems and sweeps up unwanted marine life that gets thrown overboard. From 1950 to 2014, bottom trawlers discarded $560 billion of bycatch, which was more than the value of all catches from longline fishers over the same period.
Now scientists fear another environmental disaster bubbling under the surface: climate change.
Trawlers churning 1.3% of the global sea floor stir up more carbon dioxide than the emissions of the entire aviation industry, a study published in the journal Nature in 2021 found. Even if only some of that makes it to the surface, the practice stops seas from absorbing as much CO2 from the atmosphere and prevents plant life from growing.
Trawling oceans is like digging up a garden again and again, Stewart said, comparing the nets to a plow. "If you're constantly disturbing the seabed, then you have to basically keep restarting that process of storing carbon."
The history of trawling is long and controversial.
In 1376, King Edward III of England received a complaint that fishermen were using an instrument of iron and net that "runs so heavily and hardly over the ground that it destroys the flowers of the land below."
According to an extract of the petition published in a book by marine scientist Callum Roberts, the complainants considered trawling to cause the "destruction of the fisheries" and do "great damage [to] the commons of the realm." The king's officials agreed to ban the practice near coasts, though they did not pass a law to enforce it.
In the ensuing centuries — and particularly in the deep sea since World War II — bottom trawling has exploded. Globally, it has become one of the most common industrial methods of fishing, responsible for about a quarter of wild-caught fish.
In Europe, for instance, an estimated 59% of marine-protected areas permit trawling, according to a study published last year by the New Economics Foundation think tank. More trawling happened inside protected zones than outside them, the study found.
The UK government has declared about 40% of English seas marine-protected areas. But it has given just 2% legal protections against trawling and dredging, according to a report from the Marine Conservation Society, a UK charity fighting for healthier oceans. From 2015 to 2018, just one of the country's offshore protected zones escaped bottom trawling and dredging, the report found.
"The first port of call is to make sure the protected areas we have got are actually doing the job and protecting the seabed," said Frith Dunkley, an ecologist and co-author of the report. "Generation by generation, we've forgotten what the seabed used to be like because it's been so dramatically changed over time by fishing."
Those changes are worsening weather extremes across the world.
When nets rake the floor of the sea, they kick up soils that store carbon. The Nature study found that this releases nearly 1.5 gigatons of CO2 into the water, some of which may make it into the atmosphere, where it traps sunlight and heats the planet.
But the carbon that dissolves also carries a cost. Oceans, which take in 25% of the world's CO2 pollution, absorb less of it as the concentration rises. Scientists are now trying to work out how much this contributes to climate change.
The uncertainties are considerable, but so is the magnitude of the problem, said Juan Mayorga, a marine scientist at the University of California, Santa Barbara, a co-author of the study. "The sea floor is the largest carbon reservoir in the planet," Mayorga said, "and we're disturbing it."
What scientists do know is that bottom trawling relies on burning large quantities of fossil fuels. The energy needed to power vessels as they run heavy nets over rough terrain is greater than for most fishing boats. As a result, fish caught by bottom trawling have a carbon footprint almost three times greater than otherwise, according to a study published in the journal Environmental Research Letters in 2017.
Cutting fuel subsidies would mean that bottom trawling would fade out, Mayorga said: "These industries in many parts of the world only survive because of heavy government subsidies for fuel."
Some marine scientists have called for an outright ban on bottom trawling. Others have suggested a slower phase-out would be enough, starting first with a ban on trawling in marine-protected areas before expanding it to cover the rest of the ocean.
In Europe, bottom trawling is so common that it would be difficult to ask for a radical ban, said Nicolas Fourier, campaign director at conservation group Oceana. "We are asking to phase out this type of fishing gear," he said. "It's not compatible with any of our climate or biodiversity commitments."
Consumers can play a role by eating less fish and checking labels to see if the fish they buy was caught through bottom-trawling. In the short term, the fishing industry could help by switching to lighter nets and ones that don't touch the floor.
There are also tentative research breakthroughs. Stewart, the scientist who started diving after scallop dredgers two decades ago, stumbled across a new way of catching the mollusk last year. His research team placed LED lights underwater and found that they drew in scallops, which have 200 tiny eyes and are attracted to the light. The team is now trialing the practice to see how it could work at scale.
Because fishers can lay traps instead of dragging heavy nets along the seabed, Stewart said, these "scallop discos" have a much lower impact.
Edited by: Tamsin Walker