Trawls and Climate change
Bottom trawls promote global warming and affect marine ecosystems
Fishing trawls cause significant CO2 emissions as they stir up the carbon bound in the seabed and release it again. It is also clear that they drastically affect ecosystems in the ocean. A new study by 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 trawling were banned in all currently designated marine protected areas? In the North Sea, trawls cause CO2 emissions in the order of one million tons to be released into the atmosphere every year 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 could be effectively protected.
Porz and his team first reviewed all available data and studies on fishing activity in the North Sea and its impact on sediments and bottom-dwelling animals such as shrimps and mussels. These effects were 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 trawling, the researchers were able to estimate the impact. In order to investigate the effects of possible management measures, they redistributed the fishing activity in the model from potential closure zones to surrounding areas.
„Bottom trawling in the North Sea has been practiced intensively for more than 100 years and it is therefore difficult to say how exactly the ecosystem is already being changed by it,“ adds Porz. Whether, for example, there could also be positive ecosystem effects from bottom trawling, such as increased availability of nutrients in the water column, is currently being investigated at Hereon. However, it is known that around a fifth of seabed dwellers do not survive contact with a bottom trawl. According to his study, there would be around 14 percent more animals on the seabed of the North Sea without bottom trawling. In addition, the habitats are altered by the fishing gear. Muddy beds are more affected than sandy beds, as the fishing gear penetrates deeper into muddy beds and they take longer to recover. The composition of the communities living on the seabed also changes. (Source: Hereon Press Release)
Read the complete Hereon Press Release:
==> Trawls and Climate change
Porz, L., Zhang, W., Christiansen, N., Kossack, J., Daewel, U., & Schrum, C. (2024): Quantification and mitigation of bottom-trawling impacts 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 by the use of bottom-contacting fishing gear and the potential climate impacts resulting from remineralization of the organic carbon to CO2 have recently been heavily debated. An issue that has remained unaddressed thus far regards the fate of organic carbon resuspended into the water column following disturbance by fishing gear. To resolve this, a 3D-coupled numerical ocean sediment macrobenthos model is used in this study to quantify the impacts of bottom trawling 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 6 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 of trawling closure zones. North Sea sediments contained 552.2±192.4 kt less organic carbon and 13.6±2.6 % less macrobenthos biomass in the trawled simulations than in the untrawled simulations by the end of each year. The organic carbon loss is equivalent to aqueous emissions of 2.0±0.7 Mt CO2 each year, roughly half of which is likely to accumulate in the atmosphere on multi-decadal timescales. The impacts were elevated in years with higher levels of trawling pressure and vice versa. Results showed high spatial variability, with a high loss of organic carbon due to trawling in some areas, while organic carbon content increased in nearby untrawled areas following transport and redeposition. The area most strongly impacted was the heavily trawled and carbon-rich Skagerrak. Simulated trawling closures in planned offshore wind farms (OWFs) and outside of core fishing grounds (CFGs) had negligible effects on net sedimentary organic carbon, while closures in marine protected areas (MPAs) had a moderately positive impact. The largest positive impact arose for trawling closures in carbon protection zones (CPZs), which were defined as areas where organic carbon is both plentiful and labile and thereby most vulnerable to disturbance. In that scenario, the net impacts of trawling on organic carbon and macrobenthos biomass were reduced by 29 % and 54 %, respectively. These results demonstrate that carbon protection and habitat protection can be combined without requiring a reduction in net fishing effort.
