Annual carbon emissions from bottom trawling—a popular fishing method used to capture seafood at the bottom of the ocean—is equivalent to around 40% of annual transportation emissions in the U.S., a new study published Thursday found, reaffirming previous research on the harms of bottom trawling.
Bottom trawling is a fishing method used to catch large quantities of fish at once, which involves dragging nets towed by boats along the ocean floor to catch fish and other marine animals.
When the nets scrape the ocean floor, they disturb and cut up the coal housed in the marine sediment, releasing carbon dioxide into the water that eventually finds its way into the atmosphere.
Between 55% and 60% of the carbon dioxide made underwater by bottom trawling will be released into the atmosphere within the following nine years, the Thursday study published in Frontiers in Marine Science found.
Around 370 million metric tons of carbon emissions were released into the atmosphere every year between 1996 and 2020 due to bottom trawling, equivalent to around 40% of all U.S. transportation emissions in 2022 (1,023 million metric tons) according to the U.S. Energy Information Administration.
Annual bottom trawling emissions are more than double the annual fuel emissions from the global fishing fleet(179 million metric tons a year), almost double the annual emissions produced by volcanos (200 million metric tons) and 55% of all U.S. yearly agriculture emissions (671.5 million metric tons).
The researchers identified bottom trawling carbon dioxide emissions as the highest in the East China Sea, the North Sea, the Baltic Sea and the Greenland Sea.
A previous study published in 2021 found bottom trawling releases as much carbon dioxide as the entire aviation industry, and there’s less carbon in trawled areas of the ocean than untrawled areas.
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The practice’s controversy has led to the National Oceanic and Atmospheric Administration banning it from some parts of the seafloor off the U.S. West Coast, New Zealand restricting it from around 20% of its territorial sea and the U.K. banning it from at least one body of water.
“The good news is that reducing bottom trawling carbon emissions will deliver immediate benefits,” Enric Sala, study author and executive director of ocean preservation company Pristine Seas, said in a statement. “The bad news is, delaying action ensures that emissions from trawling will continue seeping into the atmosphere a decade from now.”
Some researchers don’t believe bottom trawling emits as much carbon as previously reported. Researchers who wrote a response paper to the 2021 study believe the study overestimated the carbon emissions of bottom trawling by between 100 and 1,000 times due to the researchers’ flawed assumption of the carbon cycle. Around 70% of buried carbon can be broken down by bottom trawling and release carbon dioxide, according to the 2021 study. However, buried carbon isn’t reactive to oxygen, and therefore most of it cannot be broken down, the response paper says. “It doesn’t mean [the 2021 study] can’t be important, but it does mean that the model makes no sense,” lead author of the response paper and researcher at Bangor University in Wales told SeafoodSource News. A separate paper released in 2022 challenged claims that there was less carbon in trawled areas compared to untrawled areas. It reviewed 49 papers and found 61% saw no difference, 29% reported less carbon in untrawled areas and 10% reported higher carbon levels in trawled areas of the ocean.
19 million tons. That’s how much seafood is caught by bottom trawling annually, making bottom trawling responsible for over a quarter of all wild-caught seafood captured each year, according to a study published in the Proceedings of the National Academy of Sciences of the United States of America.
Besides releasing carbon dioxide into the atmosphere, bottom trawling has other dangerous effects. Between 40% to 45% of carbon released by bottom trawling remains in the ocean, which increases ocean acidification and leads to harmful damage on aquatic plants and animals, according to Thursday’s paper. The practice can also lead to a change in nutrient levels, the destruction of habitats and plant roots and reduce photosynthesis in ocean-dwelling organisms, according to the U.S. Geographical Survey, an arm of the Department of the Interior. It’s also believed bottom trawling is wasteful: 50% of all unwanted and discarded fish by the fishing industry is caught through bottom trawling. This is equivalent to around 437 million tons of discarded fish, or a loss of $560 billion, the University of California, Santa Barbara reports.
Looking for Climate Solutions? Protect More Ocean, Researchers Find. (New York Times)