Bottom trawling, a fishing method that entails towing a net along the ocean floor to capture target species, does not release as much carbon as air travel, according to a response paper that sought to debunk research published in 2021 and picked up by global media outlets, including The Guardian.
The response paper, “Quantifying the carbon benefits of ending bottom trawling,” released in May 2023, claims that Sala et al. – the authors of the original research – overestimated trawling’s carbon output by two to three orders of magnitude, or 100 to 1,000 times more than they should have, in the model they used, contending that the discrepancies are due to incorrect fundamental assumptions of the carbon cycle and flawed validation.
“Because there is such an uncertainty, potentially quite a large number, we need to study this effect of trawling on the carbon in the seabed in much more detail to understand how these effects happen, when they happen, and where so that we can properly quantify it,” Jan Geert Hiddink, a researcher at Bangor University and the lead author of the response paper, said.
Trawling creates sediment plumes that take days to disperse, and trawling tracks – caused by trawl nets that can dig about one inch into the sediment – can persist on the surface for months to years. Sala’s model assumes trawling and its tracks expose carbon previously buried in less-disturbed, deeper ocean floor sediment layers.
According to Sala’s model, around 70 percent of buried carbon, or labile carbon, in the sediment can be remineralized or broken down when resuspended due to trawling.
Hiddink said when organic carbon sinks to the seabed, bacteria and invertebrates naturally mineralize the element. Any buried, organic carbon is most likely inedible to bacteria and, therefore, unlikely to remineralize at a high rate, even when reexposed to oxygenated, active layers of the ocean.
“Carbon that is buried deeper in the sediment would not be particularly reactive to oxygen, even if you’re mixing it into the water column, because if that were the case, it wouldn’t be buried in the first place,” Hiddink said. “It doesn’t mean [Sala’s original research] can’t be important, but it does mean that the model makes no sense.”
Hiddink said knowledge on the effects of trawling remains extremely limited, making conclusions like Sala’s difficult to fully prove.
“I tell researchers to take away from this that doing these massive extrapolations based on little knowledge is not necessarily very useful,” Hiddink said.
Another study published in February 2022 by Epstein et al. found that 61 percent of trawled areas had no significant difference in carbon levels compared to other areas of the ocean, 29 percent reported less organic carbon stored in trawled areas, and 10 percent reported higher amounts of carbon stored after trawling. Sala’s model, by contrast, would have predicted consistently less carbon in trawled areas compared to untrawled areas.
With the full impacts of bottom trawling on carbon release still uncertain, University of Washington’s Sustainable Fisheries Project criticized how the inflated carbon estimates were used to promote the expansion of marine protected areas (MPAs).
“The amount of carbon dioxide released from trawling is highly uncertain, and it’s possible that only a small fraction gets to the atmosphere. Making an equivalence between air travel and trawling is dangerous as it takes our attention away from real solutions that work: reducing fossil fuel emissions,” University of Washington Biogeochemist Gordon Holtgrieve in a recent article about Sala’s research.
Sala supports creating more MPAs and selling carbon credits generated by his calculated reduction in bottom trawling’s footprint to fund the development of more MPAs.
However, Hiddink said this approach could potentially increase carbon emissions by artificially lowering perceived emissions from trawling to offset actual emissions generated elsewhere. Incorrect assumptions regarding carbon emissions from bottom trawling might result in stricter industry regulations, larger protected zones, and reduced fishing areas, leading to potential conflicts over fishing rights or decreased fishing activities during a period of rising demand for seafood, Hiddink said.
“The key message to policymakers is that they shouldn't be taking those numbers by Sala at face value … they are very likely to be massively overestimated,” Hiddink said. “Therefore, they shouldn't be making policy based on those kinds of numbers.”
Hiddink acknowledged Sala’s paper had spurred much more interest and funding for future research into the carbon storage potential of the oceans’ seabeds.
“To be able to make these kinds of logical predictions, you need to understand the mechanisms through which these inputs happen that would allow you to address where, when, and by what fishing gear you would expect large carbon dioxide release,” Hiddink said. “That’s what we want to study now in much more detail, and it’s actually pretty hard. It requires collaboration between biogeochemists, fishery scientists, and marine ecologists.”
Photo courtesy of Anney_Lier/Shutterstock
