- New research suggests that bottom trawling stirs up large amounts of carbon from the seabed and releases 55-60% of this carbon into the atmosphere within nine years.
- This amount of carbon is nearly double the annual emissions from the combustion of fuel by the entire global fishing fleet of about 4 million vessels, the study suggests.
- The authors say that the remaining 40-45% of the carbon would remain dissolved in the water, contributing to ocean acidification.
- However, this study has drawn criticism for potentially overestimating the amount of seabed carbon that trawling releases.
Nearly a quarter of the world’s wild-caught seafood is scooped up by bottom trawlers, fishing vessels that drag heavy nets over the seafloor. These boats fish the world over and support numerous global seafood supply chains. Yet critics have dogged them as environmentally unfriendly. Conservation experts say they can damage sensitive marine ecosystems, like deep-sea coral, and scoop up mobs of non-target species as bycatch. Research also suggests trawlers can disturb carbon deposits in seabed sediment, undermining the ocean’s ability to act as a carbon sink.
But how much carbon does trawling actually release? And what happens to that carbon? These questions have generated significant debate.
A new study published Jan. 18 in Frontiers in Marine Science attempts to answer at least one unresolved question: How much carbon does trawling put into the atmosphere?
The research draws on a 2021 study published in Nature that calculated that trawling stirs up and releases about 1 gigaton, or 1 billion metric tons, of carbon each year — nearly equivalent to the annual emissions from the global aviation industry. The new study takes things one step further. It recalculates the amount of released carbon by adjusting the models from the Nature paper. Then it quantifies how much of this carbon escapes from the sea into the atmosphere, adding to the mass amounts of human-produced carbon dioxide already there that are causing global warming.
It found that 55-60% of this carbon will make it into the atmosphere in about nine years. This is nearly double the annual emissions from the combustion of fuel by the entire global fishing fleet of about 4 million vessels, according to the authors. The remaining 40-45% of the stirred-up carbon would remain dissolved in seawater, the study says. There it would contribute to ocean acidification, which notably compromises the ability of many organisms at the bottom of the food chain to form shells and is already at record levels around the world.
“Under current climate policy, things like carbon markets … really work off of atmospheric emissions,” lead author Trisha Atwood, an aquatic ecologist at Utah State University and National Geographic’s Pristine Seas program, told Mongabay. “If that CO2 had just remained in the water, then from a policy standpoint, people didn’t really care … and we couldn’t answer whether or not they should care because we didn’t have that number.”
But that all changed when scientists from NASA Goddard Institute for Space Studies got in touch with Atwood, who co-authored the Nature study, and her colleagues.
“They said, ‘We have models to do this,’” Atwood said. “And these models have been used for the IPCC reports; they’ve been used for the global carbon budget. What these models basically do is develop an understanding of how the ocean and the atmosphere interact together, and how they and where they exchange carbon or CO2.”
The researchers found that carbon emissions from bottom trawling, which can produce large sediment plumes that are visible from space, were particularly high for some parts of the world, including the East China, Baltic, North, and Greenland seas. They also suggest that trawling activity, and the ensuing release of carbon, may be elevated in Southeast Asia, the Bay of Bengal, the Arabian Sea, parts of Europe, and the Gulf of Mexico, although there are insufficient data on trawling activity for these parts of the world to be sure.
This lack of data could be because many vessels don’t use satellite tracking systems, despite many national and international laws requiring them to do so. According to another study published earlier this month, 75% of industrial fishing activity isn’t tracked on public systems. This suggests the Frontiers paper’s calculations may be on the conservative side.
“We definitely think that we are underestimating the expanse of global bottom trawling because our way of tracking bottom trawlers for this paper uses a very particular type of signal that not every boat has — it’s called an AIS signal,” Atwood said.
William Austin, chair of the United Nations Ocean Decade Programme for Blue Carbon, who wasn’t involved in this research, said the study highlights the “need to better understand and constrain the potentially significant atmospheric emissions” from bottom trawling.
“There is no doubt that both ecosystems and sediment carbon stores are vulnerable to these pressures,” Austin, who has studied carbon storage in marine sediment and published research on its disturbance by trawlers in the U.K., told Mongabay in an emailed statement, “and the good news is that we can manage these activities to deliver better outcomes for sustainable fisheries, biodiversity gain, and probably our climate system.”
However, both the 2021 Nature study and the new Frontiers study have drawn some criticism. After the publication of the Nature study, a team of scientists published several rebuttals to it in the same journal, including one suggesting that trawling produced significantly less carbon than the Nature study said it did.
Atwood said she and her colleagues stand by their conclusions. “Some of their assumptions are fundamentally incorrect about what we did,” she said about the rebuttal. “And that they provide no quantitative support for what they are saying.”
However, Ray Hilborn, a fisheries scientist at the University of Washington’s School of Aquatic and Fishery Sciences, who co-authored one of the rebuttals, said he believes the new study is flawed based on its reliance on the previous Nature study. He said the Nature study wrongly assumed that carbon on the seafloor exists in a homogeneous layer, when in fact there’s a deeply buried layer of carbon as well as a top layer of carbon that’s “naturally stirred up” by organisms like clams and polychaete worms. Moreover, Hilborn said, trawling only tends to impact that top layer.
“It’s certainly misleading,” Hilborn told Mongabay. “Their overall estimate of how much carbon gets suspended has the same problem the earlier Sala [Nature] paper did.
“There is still a lot of uncertainty about how much sediment disturbance trawling generates,” he added. “It’s not nailed down.”
Banner image caption: A trawler off the coast of the Netherlands. Image by Paul van de Velde via Flickr (CC BY 2.0).
Elizabeth Claire Alberts is a senior staff writer for Mongabay’s Ocean Desk. Follow her on Mastodon, @[email protected], Blue Sky, @elizabethalberts.bsky.social, and Twitter @ECAlberts.
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Citations:
Atwood, T. B., Romanou, A., DeVries, T., Lerner, P. E., Mayorga, J. S., Bradley, D., … Sala, E. (2024). Atmospheric CO2 emissions and ocean acidification from bottom-trawling. Frontiers in Marine Science, 10. doi:10.3389/fmars.2023.1125137
Black, K. E., Smeaton, C., Turrell, W. R., & Austin, W. E. (2022). Assessing the potential vulnerability of sedimentary carbon stores to bottom trawling disturbance within the UK EEZ. Frontiers in Marine Science, 9. doi:10.3389/fmars.2022.892892
Collins, J., Kleisner, K., Fujita, R., & Boenish, R. (2023). Atmospheric carbon emissions from benthic trawling depend on water depth and ocean circulation. EarthArXiv Preprint. doi:10.31223/x5xd2p
Hilborn, R., & Kaiser, M. J. (2022). A path forward for analysing the impacts of marine protected areas. Nature, 607(7917), E1-E2. doi:10.1038/s41586-022-04775-1
Hiddink, J. G., Van de Velde, S. J., McConnaughey, R. A., De Borger, E., Tiano, J., Kaiser, M. J., … Sciberras, M. (2023). Quantifying the carbon benefits of ending bottom trawling. Nature, 617(7960), E1-E2. doi:10.1038/s41586-023-06014-7
Paolo, F., Kroodsma, D., Raynor, J., Hochberg, T., Davis, P., Cleary, J., … Halpin, P. (2024). Satellite mapping reveals extensive industrial activity at sea. Nature, 625(7993), 85-91. doi:10.1038/s41586-023-06825-8
Sala, E., Mayorga, J., Bradley, D., Cabral, R. B., Atwood, T. B., Auber, A., … Lubchenco, J. (2021). Protecting the global ocean for biodiversity, food and climate. Nature, 592, 397-402. doi:10.1038/s41586-021-03371-z
