Protection of marine habitats and biodiversity is a major challenge globally. In Europe, a high proportion of marine species and habitats show an unfavourable conservation status and the loss of marine biodiversity has not been halted, despite ambitious goals and legislation.
Fishing is one of the key pressures on the marine environment, both through resource extraction and through the damage done to the seabed. European waters are some of the most intensively bottom-trawled areas in the world; there are a number of regions where more than half of the seabed is trawled each year, in extreme cases up to 99 percent of the seabed, with some hotspots trawled more than ten times per year.
In addition to the documented effects on extracted species, there are serious concerns that this intensive trawling has negative effects on benthic ecosystems, both through direct effects on the seabed and by suspension of sediment and associated substances. This affects biological diversity, production of fish and biogeochemical processes in the sediment that regulate nutrients and carbon cycles. Therefore, reducing bottom trawling and its impacts are important measures for an ecosystem-based fisheries management, in accordance with the Common Fisheries Policy. It is also instrumental for protection and restoration of marine biodiversity in Europe and for achieving the targets for biodiversity and seabed integrity in the EU Marine Strategy Framework Directive. This needs to be acknowledged in the development and implementation of the EU Biodiversity Strategy for 2030.
Different parts of an otter trawl disturb the seabed in different ways; the ground gear rolls across the seabed leaving shallow parallel tracks (left) while the trawl doors dig deeper into the sediment, displacing piles of sediment up to approx. 50 cm high and 1-2 m wide (right). Photo: Clare Bradshaw
Direct effects on the seabed
Different types of trawling gear are used on different types of seabed and to catch different species, but have a number of features in common. As well as the net for collecting the catch, they all have components that keep the gear close to or on the seabed, protect the gear from being damaged by rough surfaces, keep the net open and often parts that force or herd organisms into the net.
These gear components interact with the seabed in different ways. For example, during otter trawling the gear is kept on the seabed, and the net opened horizontally, by two trawl doors, also called otter boards, which can weigh more than a ton each. These doors displace up to several decimetres of sediment, depending on their size and on seabed type. Weighted ground gear at the front of the net of otter trawls only penetrates a few centimetres into the sediment, but has a much larger spatial ‘footprint’ than the trawl doors.
This physical disturbance has large consequences for the habitats and species living on the seabed (see factbox).
Bottom trawling, by otter trawls in this illustration, has a range of effects on the seabed, such as physical displacement and disturbance of the seabed, suspension of sediment and potential release of buried substances. Illustration: Evelina Jonsson/Azote
Sediment is suspended, affecting water quality
Bottom trawling also suspends seabed sediment, both by the direct contact of the gear with the seabed and by the hydrodynamic turbulence around it. The effect is largest on silty or clayey seabeds. Bottom water turbidity (cloudiness) can be increased by several orders of magnitude immediately after a trawl has passed.
Both the turbidity itself and the increased sedimentation that occurs when the particles settle are often detrimental to marine organisms, particularly those that are not able to move away from the area. Suspended particles may clog fish gills and decrease visibility so that feeding and predation are impaired. Filter-feeding animals’ feeding apparatus may become clogged and the quality of the particles available as food in the water may decrease. The survival of eggs and larvae is decreased, for example by particles sticking to their surfaces, making them less buoyant. Plants and algae are affected by reduced light penetration and particles settling on their surfaces.
After a trawl has passed, the sediment plume can extend tens of metres above the bottom and remain in the water for days. During this time, water currents can transport the suspended sediment several kilometres away, where it may settle out, increasing the amount of sediment reaching the seabed. In areas of high trawling intensity, semi-permanent turbid bottom water may form.
Since turbidity caused by trawling is not restricted to the specific area trawled, this needs to be considered in conservation planning, particularly when aiming to protect seabed habitats or species likely to be sensitive to turbidity. This can either be achieved by increasing the size of protected areas, introducing buffer zones around the perimeter, or reducing trawling in the vicinity.
Composite image of many overlapping trawl door tracks in the Bornholm Basin, Baltic Sea, as seen by acoustic multibeam surveying. The colours illustrate the bottom topography. The image is approx. 750 m wide and each trawl door furrow is approx. 2 m wide.
Effects on biogeochemical processes
There is also a growing concern about whether seabed disturbance and suspension of sediment might affect important biogeochemical processes in the sediments. However, this is one of the most poorly studied and understood aspects of bottom trawling impacts, since effects depend on the frequency of trawling, type of sediment and faunal community, and short- and long-term effects may be different.
Physical disturbance of the seabed disrupts its natural 3-D structure, disrupting carbon and nitrogen cycling between the sediment and water. Sediment mixing and suspension may also stimulate the breakdown of organic matter, a process that can result in decreased oxygen levels in the water. When trawling removes surface sediments, surface-dwelling organisms, including the majority of the microbes involved in biogeochemical processes, are also removed. Alterations to the abundance and type of burrowing fauna is also important since these animals play a crucial role in biogeochemical cycling.
Scaling up these results to larger spatial and temporal scales is very difficult. However, some calculations have suggested that bottom trawling could affect the carbon storage capacity of sediments, release carbon dioxide and thus potentially contribute to overall carbon cycling, the effects of climate change and ocean acidification. Others have suggested trawling may contribute locally or regionally to nitrogen dynamics.
Lastly, sediment is also an archive for hazardous substances, but these can be suspended and released by bottom trawling, making them bioavailable to organisms. Suspended contaminated sediment seems to be more detrimental than suspended clean sediment, and a range of physiological stress responses in marine animals/species have been observed in lab and field studies.
Trawl doors may be several metres in diameter and weigh several tons. Photo: Clare Bradshaw
Extend protection to all types of habitats
Given the large impact of bottom trawling beyond fish stocks, there is a need to reduce bottom trawling impacts in Europe in order for fisheries to be compatible with achieving the objectives of the environmental legislation in the EU, i.e. the Water Framework Directive, the Marine Strategy Framework Directive and the Habitats Directive.
Some measures have already been taken in European countries to decrease negative effects of trawling on marine ecosystems, including trawling restrictions in sensitive habitats in some marine protected areas (MPAs) and initiatives to protect representative and undisturbed seabeds. Efforts have been made to alter mobile fishing gears to make them less destructive, and to replace mobile gears with passive gears, such as creels (traps) for crustaceans. Still, bottom trawling is impacting species and habitats of conservation concern.
Commercial trawling, including bottom trawling, still occurs inside more than 50 percent of European MPAs. Very few have a complete ban on bottom trawling and many MPAs lack appropriate monitoring to assess effects of trawling or trawling-induced sediment dispersal on benthic species. Trawl-free areas are in many cases small, which means that there is a risk for suspended sediment from surrounding trawled areas to affect the species and habitats inside. Thus, to fully protect sensitive benthic ecosystems, it is important to close larger areas than today from bottom trawling. For instance, a general ban on trawling in coastal areas, as already adopted in some member states, would protect coastal fish stocks, recruitment, spawning grounds and sensitive habitats and reduce conflicts with other fishing gears.
Slow-growing, stationary fragile species, such as the deep water coral Desmophyllum pertusum (left), are particularly sensitive to bottom trawling. Direct contact with the fishing gear results in fragmentation and burial (right). Photo: Robert Kautsky/Azote (left), Tomas Lundälv (right)
Along with the work to protect sensitive environments, there are strong arguments for establishing areas free of any human disturbance in all types of habitats that occur in a region, not only the most sensitive ones. Firstly, such areas represent the natural species composition and biogeochemical processes and can serve as ecological references to study the effects of bottom trawling and other disturbances on the marine environment and to be able to assess if, as some have suggested, there is reason to ban bottom trawling more generally. Lack of comparable, non-trawled areas has been identified as a major challenge for studies of long-term effects of trawling on benthic communities and seabed biogeochemistry. Secondly, protecting areas representative of different types of marine ecosystems is in line with the precautionary principle of ecosystem-based marine management, providing a refuge for benthic species that can help rebuild populations in impacted areas.