When Bryce Stewart dove after the jagged, steel-weighted nets of a rumbling scallop dredger to the bottom of the Irish Sea 22 years ago, he witnessed destruction he would never have could see from a boat.

“Half-crabs. Crushed sea urchins. Starfish missing 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 across the ocean floor to catch fish and seafood – is one of the most harmful ways to feed the world. It destroys ecosystems and sweeps away unwanted marine life which is thrown overboard. From 1950 to 2014, bottom trawlers discarded $560 billion in bycatch, more than the value of all longline catches during the same period.

Today, scientists fear another environmental catastrophe bubbling beneath the surface: climate change.

According to a study published in the journal Nature in 2021, trawlers that churn 1.3% of the world’s seabed generate more carbon dioxide than the emissions of the entire aviation industry. Even if only some of this makes it to the surface, the practice prevents the seas from absorbing as much CO2 from the atmosphere and inhibits the growth of plant life.

Ocean trawling is like digging a garden over and over again, Stewart said, comparing the nets to a plow. “If you’re constantly disturbing the seabed, you basically have to keep restarting that carbon storage process.”

European consumers can check seafood labels to find out how they were caught

Marine protected areas are extensively trawled

The history of trawling is long and controversial.

In 1376, King Edward III of England received a complaint that fishermen were using an iron and netting implement which “sinks so heavily and barely over the ground that it destroys the flowers of the land below”.

According to an excerpt from the petition published in a book by marine scholar Callum Roberts, the plaintiffs considered trawling to be causing the “destruction of fisheries” and doing “great damage [to] the commons of the kingdom.” King’s officials agreed to ban the practice near the coast, although they did not pass a law to enforce it.

In the centuries since – and particularly on the high seas since World War II – bottom trawling has exploded. Globally, it has become one of the most common industrial fishing methods, responsible for around a quarter of wild caught fish.

Across West Africa, authorities have seized fish from foreign bottom trawlers

In Europe, for example, an estimated 59% of marine protected areas allow trawling, according to a study published last year by the New Economics Foundation think tank. More trawling took place inside protected areas than outside, according to the study.

The UK government has declared around 40% of the marine protected areas of England’s seas. But it only granted 2% legal protections against trawling and dredging, according to a report by the Marine Conservation Society, a UK charity that fights for healthier oceans. From 2015 to 2018, only one of the country’s offshore protected areas escaped bottom trawling and dredging, according to the report.

“The first port of call is to make sure the protected areas we have are actually doing the job and protecting the seabed,” said Frith Dunkley, ecologist and co-author of the report. “Generation by generation we have forgotten what the seabed looked like because it has been altered so much over time by fishing.”

Bottom trawling exacerbates climate change

These changes are worsening extreme weather conditions across the world.

As the nets rake the sea floor, they lift soils that store carbon. The Nature study found that this releases nearly 1.5 gigatonnes of CO2 into the water, some of which can end up in the atmosphere, where it traps sunlight and warms the planet.

But dissolving carbon also has a cost. The oceans, which absorb 25% of global CO2 pollution, absorb less as the concentration increases. Scientists are now trying to determine how much this contributes to climate change.

The uncertainties are huge, but so is the scale of the problem, said Juan Mayorga, a marine scientist at the University of California, Santa Barbara, who co-authored the study. “The seabed is the biggest carbon reservoir on the planet,” Mayorga said, “and we are disturbing it.”

Healthy oceans are vital for the tourism and fishing industries, and are also a powerful carbon sink

What scientists do know is that bottom trawling relies on burning large amounts of fossil fuels. The energy required to propel vessels when passing heavy nets over rough terrain is greater than that of most fishing vessels. As a result, fish caught by bottom trawling have a carbon footprint almost three times higher than otherwise, according to a study published in the journal Environmental Research Letters in 2017.

Reducing fuel subsidies would mean that bottom trawling would disappear, Mayorga said: “These industries in many parts of the world only survive on heavy government fuel subsidies.”

‘Scallop discs’ to stop net dragging

Some marine scientists have called for an outright ban on bottom trawling. Others have suggested that a slower phase-out would be sufficient, 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 hard to call for a sweeping ban, said Nicolas Fourier, campaign manager for conservation group Oceana. “We call for the phasing out of this type of fishing gear,” he said. “This is not compatible with any of our climate or biodiversity commitments.”

Environmental groups call on policymakers to regulate the fishing industry

Consumers can play a role by eating less fish and checking labels to see if the fish they buy has been bottom-trawled. In the short term, the fishing industry could help by switching to lighter nets that do not touch the ground.

There are also tentative research breakthroughs. Stewart, the scientist who started diving after scallop dredges two decades ago, came across a new way to catch the mollusk last year. His research team placed LED lights underwater and discovered that they draw scallops, which have 200 tiny eyes and are attracted to light. The team is currently testing the practice to see how it might work at scale.

Because fishermen can set traps instead of dragging heavy nets along the seabed, Stewart said, these “scallop clubs” have a much lower impact.

Edited by: Tamsin Walker