The Trouble with Trawling: the Massive Carbon Footprint of Bottom Trawling

Featured Image Caption: A shrimp trawler at sea. Deep sea trawlers harvest tens of millions of tons of sea food per year, in the process disturbing sea floor ecosystems and releasing carbon into the ocean and atmosphere. (Image Source: “Shrimp Trawling” by NOAA FishWatch, Public domain, via Wikimedia Commons.)

Source Article: Atwood, T.B., Romanou, A., DeVries, T., Lerner, P.E, Mayorga, J.S., Bradley, D., Cabral, R.B., Schmidt, G.A., Sala, E. (2024). Atmospheric CO₂ emissions and ocean acidification from bottom-trawling. Frontiers in Marine Science 10(18). https://doi.org/10.3389/fmars.2023.1125137. 

The most productive and least sustainable way to fish 

Bottom trawling has long been the bane of environmentalists. A trawler drags a net along the ocean floor, or benthic zone, to catch bottom-dwelling shrimp, squid and popular fish such as cod and haddock. It is an efficient method from the perspective of the fishing industry, and it contributes more to global fishery production than any other fishing method.

There are, however, considerable downsides. Benthic trawling displaces not just the target species, but everything else in its path, causing massive destruction to coral reefs, fish nurseries and other deep-sea environments. It leads to a very high rate of bycatch, reducing fish stocks. It increases ocean turbidity, or cloudiness, blocking light from reaching the sea floor and reducing kelp photosynthesis. For these reasons, trawling has been restricted in protected areas throughout the world. Pacific Island nations led by Palau even pursued a global ban on the practice in 2006, without success. 

Undersea emissions: the (other) dark side of bottom trawling 

Less attention has been paid to the impact of trawling on the global carbon cycle. The ocean floor is the world’s single largest carbon stock, containing unmineralized organic carbon that can remain buried in marine sediments for millennia. As trawlers scrape their way across the benthic zone they disturb this buried carbon, releasing it into the ocean as undersea emissions. This exposed carbon is decomposed by microbes, releasing carbon dioxide that contributes to ocean acidification and, if it leaves the ocean, atmospheric climate change.  

Previous research has estimated that 1.3% of the global oceans are trawled every year, disturbing millions of tons of carbon. Despite the scale of the problem and the urgency of addressing climate change, the destination of these undersea emissions had not yet been quantified. What portion is staying in the water and acidifying the oceans, and what portion is escaping and heating the planet? 

Researchers here sought to quantify these carbon flows and assess the impact of aqueous emissions on both climate change and ocean acidification. The group used previously published data on global trawling-induced sediment movement from 1996-2020, combined with two different ocean circulation models. These models simulate the complicated movement of CO₂ within the ocean and between the ocean and atmosphere. In addition to analyzing the impact of recent emissions, researchers modelled future scenarios up to the year 2100, using different assumptions about the future extent of deep-sea trawling.  (This study did not consider the emissions of the trawling ships themselves, which are also considerable.)

From the ocean to the atmosphere

The results showed that trawling emissions left the oceans relatively rapidly: 55-60% of emitted CO₂entered the atmosphere within a 9-year period. Interestingly, this proportion did not change when the model was extended to the end of the end of the century, and it was also consistent under different models and different trawling emissions scenarios. In absolute terms, this means that 8.5-9.2 Pg CO₂ made its way from the oceans to the atmosphere during the 25 years that were analyzed. This equals 0.34-.37 Pg CO₂ / year; to put this in perspective, this is roughly equal to the annual emissions produced by the entire economy of Argentina! It is also nearly twice as high as the emissions produced by every fishing ship on earth.

Meanwhile, the remaining 40-45% of trawled emissions remained dissolved in the ocean, contributing to ocean acidification. On a global scale the drop in pH is insignificant compared to that caused by fossil fuel emissions. On a local level, however, the story is different: intensively trawled, semi-isolated seas can hold concentrated levels of carbon and acidify more than surrounding parts of the ocean. In the East and South China seas, for example, the acidification caused by trawling over this period was roughly equal to the impacts of fossil fuel use in the region. Ocean acidification is linked to coral bleaching and the collapse of reef ecosystems. Decreased oceanic pH can also dissolve the calcium carbonate shells of crustaceans and snails, which threatens not just these species but the fish that rely on them as a food source.  

The primary global hotspot for trawling-induced emissions is the East China Sea, which is unsurprising considering that the Chinese trawling fleet is by far the largest in the world. Northern Europe, particularly the North and Baltic seas, were also major hotspots. Places adjacent to these hotspots, like the South China and Norway Seas, received elevated CO₂ levels due to the horizontal movement, or advection, of inorganic carbon across the ocean. The authors emphasize the limitations of this geographic data, which relied on ships’ automatic identification system, or AIS – not all ships are covered within this system, leading to underestimates in regions such as Southeast Asia.  

Food for thought 

The problems with bottom trawling are legion: it destroys marine ecosystems, reduces photosynthesis, reduces fish stocks through high bycatch rates, acidifies the ocean and contributes massively to climate change. What can we do to reduce these harmful impacts? Surprisingly, the Marine Stewardship Council (MSC) certification can be given to trawled fish, as long as they meet certain conservation criteria. Consumers can instead avoid consuming heavily trawled species (shrimp, cod and haddock, for example) altogether if the source is not known. Given the size of the industry, however, there are limits to what individual consumption decisions can accomplish alone.

Thankfully, other fishing methods exist that can replace trawling entirely. Given the urgency of tackling climate change, it might be time to renew the call for a global ban first proposed by Palau almost twenty years ago.  

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