Physical seabed disturbance can occur via abrasion (the scraping of the substrate), resuspension of the substrate (siltation), removal of the substrate, and deposition (smothering). The impacts associated with such disturbances include the biotic impacts linked to the physical action and include additional mortality through e.g. collisions with bottom-contacting mobile and set fishing activities. Other activities, such as aquaculture, tourism/recreation, coastal infrastructure, hydrodynamic dredging, shipping (anchoring), and cable burial may also contribute.
Effects on benthic habitats and associated biota
Mobile bottom contacting gears (MBCG), hereafter “bottom trawls," are defined as mobile gears that contact the seabed during deployment, including bottom otter trawls, bottom seines, dredges, and beam trawls. Physical disturbance of benthic habitats by bottom trawls (mostly in the > 12 m vessel category) is evaluated using vessel monitoring system (VMS) and logbook data and provides information on the extent of the pressure, its magnitude, and its potential impact on the seabed habitats and associated benthic communities. A proportion of vessels < 12 m in overall length fishing with bottom trawls is not represented in this advice because they are not monitored systematically with VMS. Fishing effort not monitored by VMS is considered to be negligible in most areas of the Celtic Seas, except the Irish Sea and nearshore waters west of Ireland (up to 15%). Consequently, bottom trawl pressure and impacts will be underestimated, especially in coastal waters.
Results show that this pressure varies geographically across the ecoregion. ICES estimates that bottom trawls used by commercial fisheries in the > 12 m vessel category have been deployed over approximately 328 000 km2 of the ecoregion in 2022. This corresponds to ca. 61% of the ecoregion's spatial extent shallower than 800 m (Figure 6). The total fishing effort of bottom trawls in the Celtic Seas ecoregion has been relatively stable since 2012.
Continued physical disturbance as a result of mobile fishing activities, shipping, and dredging can negatively affect the community structure of shallow subtidal, shelf, and deep‑water sediment habitats. Physical damage from bottom trawling and dredging also impact benthic habitats through displacement or overturning of boulders and cobbles, removing or damaging epifaunal species, disturbing sediments, and damaging fragile deep-sea communities including sponge aggregations, coral gardens, and deep-water coral reefs. Extensive areas of biogenic reefs may also have been impacted through direct contact e.g. Modiolus modiolus reefs in the Celtic Sea, S. spinulosa in the west of Scotland, maerl beds along the coasts of Ireland and Britain, and cold-water corals such as Lophelia sp. in deep‑water habitats on the western shelf of the Celtic Sea ecoregion.
Effects on vulnerable marine ecosystems
All bottom-contacting gears that make contact with VMEs are assumed to impact them. Faunal communities in deep-sea sedimentary habitats live in cold waters with a relatively low supply of food, slow life-histories, low population growth, slow recovery rates, and long lifespans. Even low fishing rates may have serious impacts under these conditions. Mobile gears such as trawls are known to have the greatest impact on VMEs. Declining condition (density, body size) of deep-sea sponge aggregations have been observed associated with the impacts of demersal fisheries. Extensive areas of biogenic reefs may also have been physically impacted, e.g., cold water corals such as Lophelia (e.g. Desmophyllum pertusum) in deep-water habitats on the western shelf of the Celtic Sea ecoregion.
Smothering due to resuspension of sediments can also result in physical loss and damage. The degree of permanence of any habitat changes will be dependent on the substrate type, the presence of sensitive or vulnerable communities, and the frequency of the activity.
Within the Celtic Seas ecoregion, the fishing footprint is extensive, covering almost all (88%) of the 400–800 m depth of the Celtic Seas ecoregion. Ninety-five percent of areas with known VME occurrence or likely occurrence have been found to have been fished between 2009 and 2011. Fishing in the 400–800 m depth contour accounts for 11% (UK), 15% (Ireland), and 5% (France) of each country's fished area. Otter trawls constitute the dominant gear grouping spatially within this depth range, closely followed by static gears. Within otter trawls, the largest footprint within this depth range belongs to benthic-directed fisheries. Within the static gears, the largest footprint belongs to gillnets, followed by longlines. Deep-sea bottom longlining has been shown to have a lower impact on VMEs, reducing bycatch of cold-water corals and limiting additional damage to benthic communities. However, as longlining mainly targets large colonies with complex morphologies (e.g. coral gardens), it may cause shifts in the faunal composition of VME habitats.
