A new study by the Helmholtz-Zentrum Hereon models the influence of bottom trawls on sedimentary carbon in the North Sea.

The researchers state that fishing trawls cause significant CO2 emissions as they stir up the carbon bound in the seabed and release it again. They also believe it is clear that they drastically affect ecosystems in the ocean. 

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 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. 

Methodical approach

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.

Consequences and recommendations for action

The most effective way to reduce additional emissions would be for fisheries to avoid clearly identified carbon-rich muddy grounds. There are also alternative fishing methods and gear that have a significantly lower impact on the seabed and the animals that live there.

“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.

The news release can be read here and the full journal paper is here.