The practice of bottom trawling is extensive in parts of the Baltic Sea, in particular in the southern parts and in Kattegat. However, this way of fishing does not only affect the fish stocks but also the life on the seafloor and the biogeochemical processes in the sediments and the water, explains marine ecologist Sofia Wikström at Stockholm University Baltic Sea Centre.

Sofia Wikström recently presented the latest research on environmental effects of bottom trawling in the Baltic Sea at the HELCOM Stakeholder conference, held in Helsinki. The conference is one component of the extensive work on the Baltic Sea Action Plan update, scheduled for next year.

– The largest effects of the bottom trawling are caused by the heavy trawl doors that are dragged on the seafloor, says Sofia Wikström.

– These doors dig deep into the sediment and translocate sediment, forming piles a few decimeters high.

In soft sediments the tracks can be seen for several years after the trawling event. Even the net that is stretched between the doors disturbs the uppermost centimeters of the sediment, extending over larger areas. Studies from around the world have shown that the disturbance of the seafloor affects the composition of the animal community living there.

– Many animals are killed by a passing trawl and repeated trawling in an area leads to decrease or loss of species that are sensitive to trawling, typically large, long-lived and non-mobile species, says Sofia Wikström.

Dragging of the the trawl doors at the seafloor leads to the suspension of sediments, which can be seen as orange in this sonar image from the research project lead by Clare Bradshaw at Stockholm University.

Resuspension of sediments causes turbid water

Apart from this direct effect the trawl doors also re-suspend large amounts of sediment. An ongoing research project at Stockholm University lead by professor Clare Bradshaw has documented the resuspension from trawling on Baltic soft bottoms and shown that the fine sediment particles stay for days in the water and can disperse over a kilometer away from the trawl track.

– Suspended sediment is known to be damaging to many marine species, says Clare Bradshaw.

The mixing of the seabed by the trawl may also disrupt biogeochemical processes in the sediment, potentially releasing stored nutrients and hazardous substances and speeding up the breakdown of organic matter. The researchers are currently working on quantifying the amount of substances released in the Baltic Sea this way and the contribution of trawling to overall carbon and nutrient cycles.

– This may be particularly important in the Baltic Sea, where eutrophication is a problem and excess nutrients are stored in seabed sediments, Clare Bradshaw explains.

Sediment som virvlas upp i vattnet av bottentrålning skadar flera bottenarter, där havspennor är ett exempel. Foto: Göransson, PAG

Effects call for restrictions in MPAs

Currently there is no specific HELCOM action from the BSAP or Ministerial Declarations that addresses measures that would reduce seabed loss and disturbance, although seafloor integrity is one of the eleven descriptors of Good Environmental Status under the Marine Strategy Framework Directive.

With the BSAP scheduled to be updated in 2021, the researchers believe the effects of bottom trawling are important for HELCOM to take into account when developing an indicator of cumulative impacts on benthic biotopes.

– The effects of bottom trawling on sediment resuspension and biogeochemical processes, not only on fauna composition, is important to include. We are happy to provide scientific input, says Sofia Wikström.

The effects are also important to consider in the work with the network of HELCOM Marine Protected Areas, MPAs. The researchers are currently modelling the spread of suspended sediment into the cod spawning area in the Bornholm Basin, which will allow evaluation of the current closed area.

– There might be a need for a buffer zone in that area. Overall, we need more restrictions on trawling in the protected areas and their vicinity, says Sofia Wikström.

Text: Lisa Bergqvist