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Ocean trawling emits as much CO2 as global aviation, study finds

Ocean trawling, condemned by green groups for destroying marine habitats and depleting fish populations, generates a similar volume of carbon emissions to the global aviation industry, driven by activity in Chinese national waters, according to a paper published in the journal Nature.

Bottom trawling, a fishing method that involves dragging heavy nets across the seafloor, is responsible for between 0.6 and 1.5 gigatonnes of carbon emissions a year, compared with the aviation industry’s emissions of close to 1Gt, the researchers found. The global aviation industry produces about 2 per cent of all human-induced carbon emissions.

The majority of this pollution occurs in less than 4 per cent of the ocean, in the sovereign fishing waters of coastal nations known as “exclusive economic zones”, said the paper.

Trawling by boats in Chinese waters generates by far the greatest volume of emissions, or about 770m metric tonnes of CO2, while trawlers in the economic zones of Russia, then Italy followed by the UK caused the next most pollution, the researchers said. The UK emits roughly 370m metric tonnes of carbon a year.

“The ocean floor is the world’s largest carbon storehouse. If we’re to succeed in stopping global warming, we must leave the carbon-rich seabed undisturbed,” said Trisha Atwood of Utah State University, a co-author of the paper. 

Oceans cover roughly 70 per cent of the earth’s surface and play a critical role in absorbing heat and CO2. The study said governments could achieve the triple benefits of carbon savings, increased marine biodiversity and greater fishing yields by protecting larger areas of the ocean.

It also endorsed an international initiative — which is likely to be a feature of this year’s China-hosted biodiversity conference, known as COP15 — to safeguard at least 30 per cent of the world’s oceans by 2030. 

Sediment on the seafloor acts as a long term carbon “sink” where emissions are stored, but trawling nets disrupt the seabed and release the gas back into the water. That means the ocean has less capacity to absorb CO2 from the atmosphere, and some of the newly released carbon is likely to escape into the air, said Enric Sala, the study’s lead author. 

Using satellite data based on fishing activity between 2016 and 2019, the researchers said roughly 1.3 per cent of the ocean was trawled each year, releasing up to 1.5Gt of CO2, a finding they said was “a preliminary best estimate”. 

Even if the same area of seabed were trawled repeatedly, at least 40 per cent of that amount would keep being released from the top metre of sediment annually for about 400 years, the paper said. 

Since most trawling occurs within the economic zones, governments could halt the practice, said Sala. “We need to make sure that the trawling effort vanishes.” 

The UK’s Marine Conservation Society, a charity, estimated in January that trawling occurred in 98 per cent of the nation’s offshore marine protected areas, and that only 2 per cent of the English seabed was legally protected from trawling. The UK government’s Marine Management Organisation is consulting on whether to ban trawling in four locations.

Sala said a study into the economics of trawling was under way, including the impact of ending the practice on jobs and food security. 

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In addition to delivering carbon benefits, the paper said the protection of certain areas of the ocean would help restore marine biodiversity and increase fishing yields. 

Yields could increase by 5.9m tonnes if 28 per cent of the ocean was protected, if all the fishing activity displaced from newly protected areas were redistributed evenly across non-protected regions, the paper said.

“When overfishing and other damaging activities cease, marine life bounces back,” said Reniel Cabral, a co-author.

Many of the priority areas for conserving marine biodiversity were also found to be within EEZs, though priority areas will change as the world warms, and Arctic waters, in particular, will become more important for conservation by 2050, the researchers said. 

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