Bottom fishing can release CO₂ from the seabed, but the consequences for the climate are not straightforward.
New research, published in the journal Journal of Applied Ecology, shows that disturbing the seabed sets in motion multiple processes with contrasting effects. The overall climate impact of bottom fishing is the result of these processes and depends strongly on where fishing takes place and on whether carbon is released that would otherwise have remained stored in the seabed for long periods.
The seabed is an important store of carbon. When it is disturbed by bottom fishing, organic material is released into the water column. There, it is broken down more quickly due to exposure to oxygen, which can lead to additional CO₂ formation.
At the same time, the opposite can occur within the seabed itself. Disturbance can remove benthic organisms that play a key role in breaking down carbon, slowing these processes. In some sediments, bottom fishing can therefore reduce the breakdown of carbon.
"What we show is that the climate effects of seabed disturbance are more complex than just the formation of CO₂," says fisheries biologist Justin Tiano of Wageningen Marine Research. "Multiple processes occur simultaneously, some of which partially counteract each other."
Short and long-term
According to the researchers, it is important to distinguish between short- and long-term effects. The carbon that is rapidly released following disturbance is often carbon that would, even without disturbance, already have broken down relatively quickly.
"If you only look at the immediate emissions following disturbance, you miss a key part of the story," says Justin Tiano. "What ultimately matters for the climate is whether you lose carbon that would otherwise have remained stored in the seabed over the long term."
Climate impact varies strongly by location
In some areas, bottom fishing can disrupt the long-term storage of carbon, making it more important for the climate impact. In other areas, it involves carbon that is broken down and converted into CO₂ relatively quickly, and that would also have broken down relatively quickly even without bottom fishing. Areas where carbon is stored for long periods are therefore more important from a climate perspective. If this long-term storage is disrupted, the impact can be greater than the additional emissions in the short term.
This means it is not sufficient to focus only on where the most CO₂ is released. Instead, it is important to consider where carbon is retained over longer timescales, so that these areas can be better protected.
In the southern part of the North Sea, many sandy sediments are naturally dynamic and have low potential for long-term carbon storage. This means that additional disturbance there may have a relatively limited long-term climate impact. In contrast, in the northern part of the North Sea, where finer sediments accumulate and carbon storage is higher, disturbance can have more significant long-term consequences.
Policy requires a targeted approach
The climate impact of bottom fishing varies by location. The same fishing activity can have different effects in different areas.
The researchers therefore call for a targeted approach, in which the role of the seabed in carbon storage is assessed on an area-by-area basis.
Not a substitute for emission reduction
Protecting carbon stored in the seabed can contribute to climate policy, but it is not a substitute for reducing fossil fuel emissions. The greatest climate gains will come from reducing CO₂ emissions from other sources.
