Bottom trawling is a fishing method that uses heavy nets to catch animals that live on and in the seafloor like shrimp and flatfish. This technique has been widely criticised by scientific community because it is considered to be particularly damaging to marine ecosystems. Bottom trawling notoriously has the highest rates of bycatch among all fishing techniques, and the mechanical contact between the nets and the seafloor can alter seafloor habitats. It is estimated that when seafloor dwellers like starfish, shells and worms come into contact with a bottom trawling net, about one fifth of them don’t survive.

Lately, researchers have pointed out that aside from the ecological damage, bottom trawlers may also have unforeseen consequences for the climate because the sediment stirred up by trawlers can be rich in organic matter that can turn to CO2 after being exposed to oxygen in the water. The magnitude of this effect has proved very difficult to estimate due to the complexity of processes involved and because reliable data are scarce. We set out to solve this problem by analysing the available field data thoroughly and by using computer simulations to model the impacts of trawlers on sedimentary carbon on large scales.

We chose the North Sea as our study area. Not only is it one of the most heavily monitored marine regions globally, but also one of the most intensively trawled. Thanks to data made available by GlobalFishingWatch and the International Council for the Exploration of the Sea (ICES), we know where fishing vessels are active, how large they are, in which speed they are trawling and what kind of gear they are using. Some areas of the North Sea are trawled very frequently, others are avoided completely. We also had access to plenty of measurement data (more than 2000 sampling points) including sediment type and organic carbon content on the seafloor. Typically, natural muddy sediment contains up to a few percents of organic matter by weight, whereas sandy sediment tends to contain less than one percent.

How can we meaningfully analyse this data to find the effects of trawling on organic carbon? Simply comparing carbon contents and fishing activity did not give us much insight; Since North Sea fishermen generally favour muddier, carbon rich areas, a direct causality between fishing pressure and carbon contents cannot be inferred from a correlation between the two.

Instead, we compared the intensity of bottom trawling to the ratio of carbon to mud in the sediment, mud being defined as the fine-grained sediment fraction. The reason for looking at this ratio is that organic matter tends to stick primarily to smaller sediment particles like silt and clay rather than coarser ones like sand or gravel. Our rational was that when trawling gear resuspends sediment, most of mineral particles are likely to re-settle nearby, whereas part of the organic matter would be dissolved. In other words, we expect trawling to change the carbon content of the sediment more than it does to the grain size composition, thus the ratio between the two would change. Indeed, we found exactly this trend in the data, but only in samples taken at intensely trawled areas. In less intensely trawled areas, the was no clear effect.

In order to better understand these processes mechanistically, we applied a numerical model. The model simulates the interaction between macrobenthos (animals living at the seafloor) and organic carbon (Fig.1). The main source of labile (degradable) organic carbon at the seafloor is microalgae. Each year, following the growth periods of these algae in spring and summer, their dead bodies sink to the seafloor as organic matter. Seafloor-living animals interact with this form of organic carbon in two ways: (1) They consume it as food and (2) they bury it in the seafloor in a process called bioturbation. Bioturbation occurs when animals burrow and move around in the sediment, some even pack their faecal pellets underground. The result of these activities is that fresh, degradable and bioavailable carbon is moved from the oxygen-rich sediment surface into deeper layers where oxygen is mostly absent. Generally, carbon is preserved more effectively in the absence of oxygen.

As more carbon and sediment is added year by year, the organic particles are buried deeper and deeper. If left undisturbed, they can be stored there for thousands of years, perhaps longer. Thus, marine sediment constitutes one of the few continuous, long-term carbon sinks in the Earth system. This sets it apart from the shorter-lived “green carbon” ecosystems, such as forests, which lose their function as a carbon sink when trees die and decompose.

In our model, bottom trawls interfere with this process of carbon sequestration in three ways: firstly, they resuspend surface sediment into the water column, which has the effect of exposing deeper layers to oxygenated waters. Secondly, they kill some of the benthic animals, and dead animals do not consume nor bioturbate carbon. Thirdly, the gear components penetrating the sediment act to mix it up, similar to the action of a plough in a field.

What did the model show us? In short: Bottom trawls caused a loss of organic carbon on the order of a few hundred thousand tons from the sediment each year in the North Sea. A surprising result is that the main drivers for this loss include not only resuspension of carbon causing its exposure to oxygen but also the decrease in bioturbation (Fig. 1). The reduced consumption and gear-induced mixing could only offset this loss to a small extent. This means that habitat protection and carbon protection could go hand in hand.

The corresponding underwater CO2 emissions are estimated at about 1 million tons per year in the North Sea assuming that 60% of remineralized carbon is eventually turned to atmospheric CO2. This is about as much as the fishing vessels emit due to fuel combustion. It is also much less than previous estimates made using simpler models. Muddy areas are particularly susceptible to this carbon loss. This is notable since historically, marine protection primarily favoured hard substrate like sand reefs, which are not very effective at trapping sedimentary carbon. Our research shows that if policymakers do decide to prioritize certain marine areas for protection for the sake of carbon benefits, muddy depocenters in shelf seas including marginal seas that actively accumulate carbon would the most effective choice.