Scottish seabird conservation action plan: vulnerability report

Threats Leading To Mortality

Bycatch

Seabirds can accidentally be caught and killed as bycatch in various types of fishing gear. Seabird bycatch is known to occur when birds target bait on long-line hooks; when birds swim into fixed nets when diving below the sea surface or when birds foraging for fish are caught in midwater trawls (Northridge et al., 2020). Less is known about seabird bycatch in other gear types and targeted monitoring is required to establish if seabird bycatch is an issue.

Long-Line Fisheries:

Seabirds captured in longline fisheries tend to be surface-feeders. They can be caught when attempting to take bait or when targeting caught fish as they are being hauled to the surface. Fulmar is the main species recorded as being bycaught in long-lines, along with gannet, and to a lesser extent Manx shearwater and some gull species (Dunn and Steel, 2001; Anderson et al., 2011; Northridge et al., 2020; Kingston et al., 2023). Long-lining tends to be concentrated along the edge of the continental shelf to the west and north of Scotland, with some coming further onto the shelf, including around the Shetland Isles.

Set Net Fisheries:

Set nets can be a hazard to birds that feed in the water column. This includes species that undertake plunge or pursuit diving such as auks (especially guillemots) but also cormorants, shags and gannets (Bærum et al., 2019, Northridge et al., 2020). Some surface-feeding species such as fulmar and gulls have also been reported as bycatch in set nets (Zydelis et al., 2013, Fangel et al., 2015, ICES, 2022). There is limited information on where set nets are used and there are a range of different types of netting in inshore waters, depending on the intended target species. Impacts are expected to be influenced by various factors including mesh size and soak time.

Midwater Trawl Fisheries:

It is mainly birds that feed in the water column, such as auks, that are at risk of occasional entanglement from midwater trawls (McCarthy et al., 2011). The majority of birds caught by UK vessels are guillemots, with fewer cormorants and razorbills (Northridge et al., 2020). Some gannets have been recorded as bycatch in the Scottish herring and mackerel fleets (Pierce et al., 2002). Surface-feeders may also be caught, for example gulls have been recorded in purse seine nets in Norway (Bradbury et al., 2017, Christensen-Dalsgaard et al., 2022). Pelagic fisheries are widely distributed across the continental shelf and beyond, with target species forming part of international stocks.

Aquaculture:

To a much lesser extent, some seabird species can also become accidentally entangled in nets, ropes and lines associated with aquaculture. Low numbers of seabirds, particularly gannets and occasionally gulls, skuas, cormorants and shags can be caught in finfish aquaculture top nets or side nets. Auks can be caught in the sub-sea nets of the cages (NatureScot, 2020). Fish farms are localised in distribution and limited mainly to the Northern Isles and west of Scotland.

Collision, Displacement & Barrier Effects

The next few decades are likely to see big changes to our climate. Moving to renewable energy, including offshore wind, will play a major role in cutting the emissions contributing to climate change.

However, seabird species have the potential to be impacted by marine renewable development, through both direct mortality and subtle non-lethal effects, which impact individual condition and demographic rates (e.g. survival and reproduction). Seabirds may be affected by three key effects: collision, displacement and barriers to movement, with vulnerability to these varying between species and across seasons. It is also dependent on the development characteristics and location.

Offshore windfarms may present a collision risk to commuting, feeding or migrating seabirds (Furness et al., 2013). Collision risk is assessed by combining the flight height of a given species and the ability of that species to avoid turbines. The flight height of some seabirds puts them at risk of death or injury from colliding with offshore wind farm structures, particularly large gulls, kittiwake, gannet, cormorant and shag (Furness et al., 2013, Johnston et al., 2014, Mendel et al., 2014, Johnston & Cook 2016). Some species appear to avoid flying through windfarms i.e. exhibit macro-avoidance. Evidence suggests there are species-specific responses to turbines and that to avoid collision, most birds adjust their flight paths at some distance from the turbines, rather than making last-second adjustments (Cook et al., 2018). However, some species such as shags and cormorants can be attracted to offshore developments by using structures to roost (Dierschke et al., 2016)

Some species are particularly sensitive to disturbance at sea, either from artificial structures such as wind turbines but also additional vessel traffic during wind farm construction (Jarrett et al., 2018, Fliessbach et al., 2019). This disturbance can lead to their displacement from important habitats (Searle et al., 2014, 2018, Warwick-Evans et al., 2017, Peschko et al., 2021, 2024, Lamb et al., 2024). Such displacements may incur greater energetic costs, which may ultimately affect survival or breeding success. In addition to simply causing birds to fly around a development and fly further than they otherwise would, the presence of offshore wind farms might have a greater impact by causing a barrier to movement, preventing birds from successfully migrating or accessing breeding colonies. Offshore wind developments may cause other impacts to seabirds such as changing prey populations and habitats (Grecian et al., 2010, Farr et al., 2021).

Exposure of seabirds to these pressures is likely to increase as the number of offshore wind farms increases and expands into waters in the North and West Scotland (Sectoral Marine Plan for Offshore Wind Energy, 2020^[4]).

Diving seabirds such as auks and shag have the potential to be killed or injured by collision with tidal stream turbines below the surface (Furness et al., 2012). Exposure to threats or impacts from tidal or wave energy generation is localised, confined to a limited number of locations predominantly in NW Scotland and the Northern Isles. Currently, there is one operational tidal stream project in Scottish waters. Several more projects have now secured funding and are anticipated to progress to construction/ operation.

Pollution

Marine litter:

Some seabirds that feed on detritus and plankton at the sea surface can accidentally ingest small floating plastic particles (O’Hanlon et al., 2017). Ingesting these plastics is potentially harmful and can cause damage to internal organs (Lavers et al., 2019; Rivers-Auty et al., 2023) resulting in scar tissue formation (Plasticosis -see Disease). Petrels’ are most at risk (Wang et al., 2021; Rivers-Auty et al., 2023). Unlike in other species of seabird, a specific anatomical feature of petrels’ - a narrowing between the proventriculus and gizzard - prevents pellets containing indigestible material in the gizzard being regurgitated. This is also true for fulmar and means that ingested plastic remains and accumulates in the gizzard reducing the amount of food that they can digest, leading to reduced body condition and potentially starvation.

Seabirds can also become entangled in marine litter such as in Abandoned, Lost or otherwise Discarded Fishing Gear (ALDFG), in recreational nylon fishing line, occasionally in other types of plastic litter at sea, and in plastic waste incorporated into seabird nest material, leading to injury or death (Wilcox, 2015, O’Hanlon et al. 2017, Lavers et al., 2019).

Marine litter is discarded through a wide range of human activities conducted both at sea and on land. Exposure to threats from litter is considered to be widespread and is assumed to occur throughout Scottish waters.

Oil pollution:

Oil pollution in the marine environment that has caused seabird deaths largely stems from chronic and widespread release of fuel oil, impacting seabirds that typically spend time on the sea surface such as auks (Mitchell et al. 2004). Released oil will contaminate the surface of the water, water column and seafloor as well as having an immediate impact on seabirds. Fouling with oil alters feather microstructure, which results in loss of water-proofing; thermal insulation; buoyancy and can impair flight (Leighton, 1993, Jenssen, 1994). Ingestion of oil from preening can lead to organ failure. Birds contaminated at sea may therefore die from drowning, hypothermia, starvation, or dehydration (Helm et al., 2014).

Accidental spillage of large volumes of crude oil during extraction or transportation can also cause mass mortalities and profound effects on populations, in particular if occurring during the breeding season near colonies. The effects are relatively short-term and localised.

Intentional Taking & Destruction of Adults/ Eggs

The intentional taking and destruction of seabirds (either adults or eggs) is illegal under the Wildlife and Countryside Act (1981). Certain activities are, however, permitted under licence. In a limited number of circumstances, for example where seabirds pose a threat to aviation safety or are a risk to public health and safety, licenses can be issued for lethal control. Some taking of gull eggs for human consumption is also licensed at limited sites.

Each year, up to 2000 young gannets (guga), have been harvested from the island of Sula Sgeir for the purpose of human consumption, as permitted by The Wildlife and Countryside Act 1981. The harvest was voluntarily suspended between 2022 and 2024 due to the outbreak of HPAI.