Bottom trawling uses massive steel plates to scrape almost all living organisms off the seabed. Courtesy WWF.
According to a recent report by a group of scientists, the world’s appetite for fish is helping release as much CO2 from the seabed as the entire aviation industry aviation is dumping into the atmosphere. And the biggest culprit is bottom trawling. By Jeremy Torr.
Barcelona, April 2021. A report published in Nature, looking at the ramifications of commercial fishing and its sustainability, has calculated that some thousand million tonnes – one gigatonne – of carbon emissions is released from the oceans on average each year, because of bottom trawlers dragging their nets across the sea floor.
“Bottom trawling is a destructive practice” - Trisha Atwood, Utah State University. Courtesy USU.
Researchers primarily looked at bottom trawling, a fishing method that involves dragging heavy nets across the ocean floor to catch bottom-dwelling fish like shrimp, whiting, cod and other low-level feeders.
“The ocean contains unique biodiversity, provides valuable food resources and is a major sink for anthropogenic carbon,” notes the report. “But the destructive practice of trawling for bottom feeders disturbs the seabed, releasing some of the carbon it contains, which is then broken down by microbes and turned into carbon dioxide,” said assistant professor Trisha Atwood of Utah State University, one of the authors.
Research suggests bottom trawling nets penetrate the seafloor on average 2.4 cm with each pass. Every time that happens, the sediment is disturbed, releasing an estimated 0.52-1.47 billion metric tons of CO2 every year. In comparion global air travel releases 0.9 billion tons of CO2 into the atmosphere each year.”
This newly released CO2 then either stays in the sea where it contributes to acidification and coral reef degradation, or worse, it means the seawater cannot absorb as much CO2 from the atmosphere as it would normally. Juan Mayorga, one of the report’s researchers, compares bottom trawling to terrestrial farmers using bulldozers to harvest foodstocks from the soil.
One issue with bottom trawling is that it is completely indiscriminate. Millions of fish get thrown back dead. Courtesy Ecozine.
“Imagine farmers used bulldozers to scrape up … the soils of terrestrial environments such as savannas or marshlands (to get at the food),” he says. “As any farmer knows, this would induce erosion and damage the quality of the soil itself, stripping it of its valuable nutrients and releasing carbon as well.” But, he says, this is exactly what is happening through bottom trawling. It is totally unsustainable.
Even more bizarrely, bottom trawling is an easy fix. The report estimated that banning industrial fishing in under 4% of the oceans would eliminate 90% of the risk of carbon disturbance due to bottom trawling.
Worse still, it is not even a profitable industry sector. Those trawlers that do bring home a profit usually do so only because of generous government subsidies. Almost 450 million tonnes of bottom-trawled marine organisms, worth up to US$600 billion, have been dumped back in the ocean over the past 65 years because it was not what the trawlers were looking for.
“Had they landed that catch, they would have made US$560 billion,” noted researcher Maria Deng Palomares. “But they were just dumped overboard.” Palomares notes the worst aspect is that, in general, bottom trawlers are so expensive to operate that: “the only way to keep them afloat is by giving them government subsidies. In other words, it’s a wasteful and inefficient practice” she said.
“We can realise benefits quickly if countries work together.” - Enric Sala.
Consequently, the report’s authors have recommended that marine protected areas (MPAs) should be increasingly designated as “an effective tool for restoring biodiversity, expanding seafood supplies and storing climate-heating carbon.”
Lead author of the report, Enric Sala is pushing for the extension of MPAs, as the best way to stop the scoured of bottom trawling.
“It’s clear that humanity and the economy will benefit from a healthier ocean. And we can realise benefits quickly if countries work together to protect at least 30% of the ocean by 2030,” said Sala.
As Jessica Desmond, oceans campaigner at Greenpeace Aotearoa notes, bottom trawling is a real double whammy for the ocean - and the world.
“For two decades, environmentalists have been urging governments to protect ocean wildlife from the bottom trawling,” she says. “This new science shows that there’s another side to that coin – releasing one gigaton of carbon (every year) that has been stored away in the ocean.”