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Trawls and climate change – Küstenforschung

Bottom trawls promote global warming and impact marine ecosystems

Screenshot hereon.de

Fishing nets cause significant CO2 emissions because they stir up and release carbon bound in the seabed. It is also clear that they drastically affect ocean ecosystems. A new study from the Helmholtz-Zentrum Hereon models the influence of bottom trawls on sedimentary carbon in the North Sea. It also shows that restrictions on fishing in certain areas would benefit life in the ocean and on land.

What would happen if bottom trawls were banned in all currently designated marine protected areas? In the North Sea, trawls release CO2 emissions of the order of one million tons into the atmosphere annually by stirring up organically bound carbon. “This is a conservative estimate compared to other studies and corresponds to the emissions from the diesel engines of the fishing fleet itself,” says coastal researcher and lead author Dr. Lucas Porz from the Institute of Coastal Systems – Analysis and Modeling at Hereon. Current marine protected areas have little positive impact on carbon storage. But by designating ‘carbon protection zones’, both sedimentary carbon and habitats can be effectively protected.

Porz and his team first reviewed all available data and research on fishing activity in the North Sea and its impact on sediments and bottom-dwelling animals such as shrimp and mussels. These effects are incorporated into computer models that simulate the distribution of animals, sediments and organic carbon and their interaction with ocean currents. By comparing the simulations with and without bottom trawls, the researchers were able to estimate the impact. To investigate the effects of possible management measures, they redistributed fishing activity in the model from potential closure zones to surrounding areas.

“Bottom trawling in the North Sea has been intensively practiced for more than 100 years and it is therefore difficult to say how it is already changing the ecosystem,” Porz adds. For example, whether positive ecosystem effects can also occur through bottom trawling, such as increased availability of nutrients in the water column, is currently being investigated at Hereon. However, it is known that approximately one fifth of seabed dwellers do not survive contact with a bottom trawl. According to his research, without bottom trawls there would be approximately 14 percent more animals on the seabed of the North Sea. In addition, habitats are changed by fishing gear. Muddy bottoms are more affected than sandy bottoms because fishing gear penetrates deeper into muddy bottoms and takes longer to recover. The composition of the communities living on the seabed is also changing. (Source: Hereon press release)

Read the full Hereon press release:

==> Trawls and climate change

Porz, L., Zhang, W., Christiansen, N., Kossack, J., Daewel, U., & Schrum, C. (2024): Quantifying and mitigating the impact of bottom trawling on sedimentary organic carbon stocks in the North Sea. Biogeosciences, 21, 2547–2570, doi:10.5194/bg-21-2547-2024

Abstract:

The depletion of sedimentary organic carbon stocks through the use of bottom contact fishing gear and the potential climate impacts due to remineralization of the organic carbon to CO2 have been hotly debated recently. A problem that has remained unresolved concerns the fate of organic carbon released into the water column following disturbance by fishing gear. To solve this problem, a 3D coupled numerical ocean sediment macrobenthos model is used in this study to quantify the impact of bottom trawls on organic carbon and macrobenthos stocks in North Sea sediments. Using available information on vessel activity, gear components and sediment type, we generate daily time series of trawling impacts and simulate six years of trawling activity in the model, as well as four management scenarios in which trawling effort is redistributed from areas inside to areas outside the closure zones for the trawling. North Sea sediments present 552.2 ± 192.4kt less organic carbon and 13.6±2.6% less macrobenthos biomass in the towed simulations than in the non-towed simulations towards the end of each year. The organic carbon loss is equal to the aqueous emission of 2.0±0.7Mt CO2 Each year, about half of it is likely to accumulate in the atmosphere on a multi-decade time scale. The consequences were greater in years with higher trawl pressure and vice versa. The results showed high spatial variability, with large losses of organic carbon due to trawling in some areas, while organic carbon content increased in nearby non-trawled areas due to transport and redeposition. The area most affected was the heavily fished and carbon-rich Skagerrak. Simulated trawl closures in planned offshore wind farms (OWFs) and outside core fishing grounds (CFGs) had negligible effects on net sedimentary organic carbon, while closures in Marine Protected Areas (MPAs) had a moderate positive effect. The largest positive impact occurred with the closure of trawling in carbon protection zones (CPZs), which were defined as areas where organic carbon is both abundant and labile and therefore most vulnerable to disturbance. In that scenario, the net effects of trawling on organic carbon and macrobenthos biomass were reduced by 29% and 54%, respectively. These results show that carbon protection and habitat protection can be combined without requiring a reduction in net fishing effort.