UK Cetacean Conservation Strategy: technical report

Section 2 – Summary of key pressures and threats acting on cetaceans in UK waters

8. This section provides a qualitative summary of updated evidence of the main pressures and threats acting on all species and species groups listed in the Strategy. It combines information sources used to inform the original quantitative assessment, with additional information and evidence in relation to the species and species groups (Table 1) that have been added since the original vulnerability assessment was carried out (Section 3).

9. The main pressures and threats described in this section are derived from the original detailed quantitative vulnerability assessment that accompanies this Strategy.

10. For some species and activities, there is limited data available, so evidence is referenced where possible and, in some instances, refer to historic records. The literature review has been carried out considering all time periods and includes data from published sources.

Removal of non-target species (i.e. bycatch or entanglement)

11. Bycatch or entanglement in fishing gear is considered one of the most significant threats to the conservation and welfare faced by cetaceans in UK waters, although the pressure varies by gear type, species and region. Bycatch monitoring programmes to date (Northridge et al., 2016; 2018; 2019; Kingston et al., 2021) have largely focused on monitoring set nets, pelagic trawls, and longlines with limited monitoring in other fisheries e.g. drift nets and creeling/pots.

12. Bycatch in bottom set gillnets in the Southwest Approaches and North Sea is a particular concern for harbour porpoise (Northridge et al., 2016; 2018; 2019; Kingston et al., 2021), although monitoring programmes have also recorded bycatch of other species including Atlantic-white sided dolphin (demersal gillnets; Reeves et al., 1999; Morizur et al., 1999; Couperus, 1997, Northridge et al., 2016), and white-beaked dolphin (Reeves et al., 1999). Records for other bycaught species with unknown gear types include: common dolphin (Tregenza et al., 1997; Evans et al., 2021), bottlenose dolphin (Deaville & Jepson, 2011; Northridge et al., 2016), Risso’s dolphin (Northridge et al., 2013), long-finned pilot whale (Northridge, 1991; Reyes, 1991; Leeney et al., 2008; Northridge et al., 2017), striped dolphin (Leeney et al., 2008), sperm whale (Leeney et al., 2008), fin whale (Leeney et al., 2008) and beaked whales (Reeves et al., 2013; Leeney et al., 2008).

13. Bycatch in trawl gear has also been reported for Atlantic white-sided dolphin (Ross, 2003), white-beaked dolphin (particularly in the North Sea; Morizur et al., 1999; Deaville & Jepson, 2011; Northridge et al., 2016), and long-finned pilot whale (Northridge, 1991; Reyes, 1991; Leeney et al., 2008; Northridge et al., 2017).

14. Evidence from UK strandings schemes and anecdotal records from fishers suggest bycatch events in creels and pots are very rare for most species. This is particularly the case for offshore species due to limited spatial overlap between their usual habitat and the distribution of this gear type (e.g. Atlantic white-sided dolphin, long-finned pilot whale). The exception to this is minke whale and humpback whale entanglement in creels and pots in Scotland, where records suggest that minke whale deaths due to entanglement in creel lines represent the single most frequently documented cause of anthropogenic mortality for the species in Scottish and UK waters (Northridge et al, 2010; Pierce et al., 2004; Deaville and Jepson, 2011; MacLennan et al., 2021; Leaper et al., 2022). Similarly, estimated rates of humpback whale entanglement in creel lines raise welfare concerns, with potential population impacts (MacLennan et al., 2020; Ryan et al., 2016).

Acoustic disturbance

15. Sound introduced in the marine environment by anthropogenic activities has the potential to result in masking^[1] or disturbance, hearing impairment and fatal or serious injury to cetaceans (Nowacek et al., 2007; Weilgart, 2007; Southall et al., 2019). Whilst the vulnerability to a particular activity may be low, when multiple operations and/or different activities occur at the same time or over an extended period, the cumulative impact is likely to be greater. However, evidence on the population-level impacts of cumulative noise is lacking as the impact of disturbance on reproduction and survival of many species in UK waters is not well understood. Cumulative impacts and vessel noise are recognised as two key sources of acoustic disturbance pressure for cetaceans in the marine environment.

Seismic / geophysical surveys, pile driving, unexploded ordnance (UXOs)

16. Impulsive noise from these sources are of particular concern for cetaceans due to the potential for rapid onset of noise and high sound levels. It is well documented that cetaceans can be affected by impulsive marine noise (Gordon et al., 2003; Southall et al., 2019) and studies have noted behavioural responses in the form of displacement of cetaceans to pile driving (Graham et al., 2017; Brandt et al., 2018; Benhemma-Le Gall et al., 2021; Fernandez-Betelu et al., 2021 Graham et al., 2023) and seismic/geophysical surveys (Stone & Tasker, 2006; Stone, 2015; Thompson et al., 2013; Kavanagh et al., 2019; JNCC, 2020; Stone et al., 2017; Rasmussen et al., 2016; Castellote et al., 2012; Dunlop et al., 2017; Noad & Dunlop, 2024; Barkaszi & Kelly, 2024). Studies have shown that seismic surveys and pile driving may lead to short-term displacement, but animals may return to the area (Thompson et al., 2013; Graham et al., 2019). The return of displaced animals post-construction of offshore windfarms can be relatively quick (Vallejo et al., 2017) although habitat loss due to repeated disturbance can induce stress and impact upon foraging and breeding success at the population scale. There is also the possibility for loss of hearing, both temporary (Temporary Threshold Shift – TTS) or permanent (Permanent Threshold Shift – PTS) from seismic surveys and pile-driving. To reduce the risk of this occurring, mitigation is commonly required for activities producing loud impulsive noise. Mitigation usually consists of monitoring a specified impact zone to ensure marine mammals are clear from the area and use of an coustic deterrence device (ADD), or soft-start procedures, to deter animals from the area before commencing. Whilst rare in the UK, noise abatement techniques (methods to reduce the sound output into the marine environment) can also be employed.

17. The clearance of UXOs (Unexploded Ordnance) has the potential to cause permanent hearing loss, physical injury and in extreme cases, death (von Benda-Beckmann et al., 2015). In 2011, clearance of UXOs from a practice range was thought to have triggered a mass stranding event of long finned pilot whale (Brownlow et al., 2015). High order explosions of UXOs have the potential to cause injury to cetaceans up to several kilometres (Robinson et al., 2022). ‘Low order’ methods of UXO clearance, such as deflagration (burning the explosive material without detonation), result in a substantial reduction in sound levels (Robinson et al., 2020, Lepper et al., 2024). The UK has agreed a position that all UXO clearance should be carried out using low order methods (Marine environment: unexploded ordnance clearance joint interim position statement - GOV.UK (www.gov.uk)).

Operational offshore wind farms

18. While low frequency noise from currently operational offshore wind farms is not thought to be a major concern for marine mammals (Madsen et al., 2006; Marmo et al.,2013), cumulative effects from larger and more numerous wind farms may be considerable (Tougaard et al., 2020).

Operational oil and gas platforms

19. Whilst the impact of drilling noise is potentially high (Boyd, 2008), there have been no studies that have investigated negative impacts due to noise from operational oil and gas platforms.

Acoustic Deterrent Devices (ADDs)

20. Acoustic Deterrent Devices (ADDs), which have historically been used to mitigate against seal interactions at aquaculture sites, can result in impacts on movement patterns, local density and may cause habitat exclusion in multiple cetacean species (Gӧtz & Janik, 2013; ICES, 2015, McGarry et al., 2017; Coram et al., 2014; Findlay et al., 2018; Boisseau et al., 2021; Findlay et al., 2021, Findley et al., 2024). ADDs are also used in some countries as mitigation prior to offshore pile-driving construction (Thompson et al., 2020), for UXO clearance and to deter animals from collision with tidal turbines (Graham et al., 2023). Where used, disturbance impacts from ADD noise are expected to be highest in coastal species, though the proliferation of offshore wind into deeper waters could overlap with the distributions of some offshore species.

Fish finders, depth sounders and other commercial sonar equipment

21. Fish finders and depth sounders occur on almost all vessels. However, the noise produced from current devices, although audible, is at frequencies that are not considered a significant concern (DeRuiter & Lurton, 2011; Deng et al., 2014) in the context of causing impacts to cetaceans. However, more recent studies have indicated echosounders may affect behaviour (Quick et al., 2016; Cholewiak et al., 2017). Some loud commercial sonars (e.g. those used in geophysical surveys) may produce impulsive sounds in frequencies audible to cetaceans, though these are often highly directional. Similar risks may also exist for audible antifouling devices, but these are unidirectional.

Pingers

22. Pingers are used as a management tool to reduce cetacean bycatch in fisheries. Their effectiveness in reducing harbour porpoise bycatch is well documented (e.g. Larsen et al., 2013, Larsen and Eigaard 2014, Northridge et al., 2011; Kindt-Larsen et al., 2019; Brennecke et al., 2022). However, extensive pinger use could lead to habitat exclusion (Kyhn et al., 2015). Pingers are currently required by law for use on vessels ≥12m in length operating in specific UK waters using bottom set gill nets or entangling nets: in area IV (North Sea) pingers are required where the total net length is ≤ 400 m and mesh size is 220m or more; in areas VIId, e, f, g, h and j (within the Celtic Sea) pingers are required on any bottom set gillnet or entangling net.

Dredging and mining

23. Noise emitted during dredging and mining operations is unlikely to cause damage to cetacean auditory systems, but masking and behavioural changes, including disturbance, are possible (Tillin et al., 2011; Todd et al., 2015, Pirotta et al., 2013; Thompson et al., 2023). However, our understanding of these impacts, particularly of deep-sea mining, on cetaceans is extremely poor, given the relatively recent development of the industry.

Vessel noise

24. Behavioural reactions have been observed in response to continuous noise from vessels (Richardson, 1995; Skov et al., 2014; Wisniewska et al., 2019; Erbe et al., 2019; Pirotta et al., 2012; Castellote et al., 2012) and reduced density around shipping lanes in the UK has been shown for several species (e.g. Heinänen and Skov, 2015, Anderwald et al., 2013). Noise from shipping vessels is thought to affect baleen whales in particular due to their peak hearing at low frequencies (Götz et al., 2009). However, smaller vessels and recreational craft may produce higher frequency noise than shipping which could lead to area avoidance behaviour in species such as harbour porpoise (IAMMWG et al. 2015). Vulnerability may be higher locally at certain times of the year and in specific locations (for example, southern North Sea). Reductions in vessel speed may help reduce the noise impact (Findley et al., 2023).

Sonar

25. Vulnerability to sonar is likely to be highest in deep diving species such as beaked whales (Cox et al., 2006), but cases of acute and chronic forms of gas embolism have been associated with common dolphin (Jepson et al., 2003; Deaville & Jepson, 2011; Jepson et al., 2013) and Risso’s dolphin (Jepson, 2003; Jepson, 2004; Jepson et al., 2005; Deaville and Jepson, 2011). Sonars are linked to behavioural responses such as disturbance, which are energetically costly but whether these lead to long-term population level impacts is poorly understood (Harris et al. 2017). Responses vary between and within individuals and populations (Harris et al. 2017).

Physical disturbance

26. Cetaceans may also be disturbed by the physical presence of vessels and other anthropogenic objects / activities in their habitat. There are many studies that indicate behavioural impacts (e.g. short-term disruption and reduction in foraging activities, and impacts on communication) from tourist vessel interaction with more targeted species such as inshore bottlenose dolphin, common dolphin and minke whale (Hastie et al., 2003; Neumann and Orams, 2006; Bejder et al., 2006; Stockin et al., 2008; Meissner et al., 2015, New et al., 2013; Pirotta et al., 2014; La Manna et al., 2016; Heiler et al., 2016; Christiansen et al. 2013; Christiansen and Lusseau, 2015; Lohrengel et al 2018; Vergara-Pena, 2020). There may also be some local disturbance impacts for non-target species as a consequence of activities targeting other species. Regional impacts on species not targeted at a UK scale may also be possible (e.g. white-beaked dolphin in Lyme Bay and Northumberland).

27. Vulnerability of cetaceans to disturbance from scientific studies is limited in UK waters, with certain activities such as photo-id surveys requiring licensing from Statutory Nature Conservation Bodies (SNCBs). Vulnerability may be greater for species highly targeted for research (coastal bottlenose dolphin or minke whale in the Moray Firth and west of Scotland and coastal bottlenose in Cardigan Bay), but disturbance from research vessels remains a small component of all vessel traffic.

Change to habitat and prey

28. Evidence of changes in access to habitat due to barriers to movement (e.g. from placement of structures on the seabed) of cetacean species is very limited (ICES, 2015). In the UK, there is potential for higher exposure to this pressure on the west coast of Scotland due to the interaction between the complex coastline and marine infrastructure / activities leading to ‘barriers’. ICES (2015) recognised that barrier effects may be locally significant but at the UK scale, there is negligible overlap of the pressure and the species.

29. There is very little evidence of effects on most cetacean species from changes to or loss of supporting habitat – either seabed or water column - but the pressure may be locally significant. Loss of habitat occurs with, for example marine development structures, but is usually small scale and considered on case-by-case basis during HRA/EIA. Local water column habitat changes, for example temperature changes from cooling water discharges from power stations, are unlikely to have population level effects due to their localised nature.

30. Changes in prey availability, due to competition with other marine predators, fishing or climate change may have an impact on species (e.g. harbour porpoise, common dolphin) (Santos and Pierce, 2003; Santos et al., 2004; MacLeod et al., 2007a; MacLeod et al., 2007b; Thompson et al., 2007). Starvation is a recorded cause of death in stranded individuals with reports of starvation in harbour porpoise, common dolphin, killer whale and minke whale, striped dolphin, humpback whale, fin whale and sperm whale (Deaville and Jepson, 2011). Opportunistic foragers or species with varied diets e.g. common dolphin (Young & Crockfoot, 1994; Santos et al., 2004), Atlantic white-sided dolphin (Hernandez-Milian et al., 2016), white-beaked dolphins (Fall and Skern-Mauritzen, 2014) and striped dolphin (Spitz et al., 2006) may be less vulnerable to changes in prey availability. Other species or populations may be more susceptible to a decline in prey biomass and this is particularly the case for those species that are more highly dependent on certain food sources (Lassalle et al., 2012, Bearzi et al., 2006). However, information on the implications of changes in prey availability is generally lacking.

Physical injuries/mortality

31. For most species it is likely that vessel strikes as a cause of death is unusual (Deaville and Jepson 2011; Deaville, 2016; ICES, 2015). However, monitoring of stranded animals shows that vessel strikes have been identified as a cause of death in harbour porpoise, short-beaked common dolphin, fin whale, minke whale, Risso’s dolphin, beaked whales and sperm whale (; Deaville et al., 2018). For fin whale and beaked whales, confirmed cases of ship strike represent over 20% of stranded animals in the UK between 1991 and 2017 (Deaville et al., 2018).

32. There are no documented cases of collisions resulting in injury between cetaceans and tidal turbines. Observations of close passes to the moving rotors of some devices have been published; however, degrees of avoidance ranging from a few metres up to 100 metres have been noted suggesting overall collision risk may be diminished by behavioural adaptions (Gillespie et al., 2021; Palmer et al., 2021). To date little effort has been placed on estimating the likelihood of injury or death as a result of collision with tidal energy devices. One study suggested worst cases scenario impact would result in “damage” however the magnitude of damage and therefore likelihood of death is too uncertain to apply these results to any risk framework at this stage (Carlson et al., 2012; 2014).

Marine pollution

Oil pollution

33. There are no published records of spills in UK waters impacting on cetacean mortality or distribution. It has also been suggested that cetaceans may be able to shed oil easily or avoid swimming in oil slicks (Parsons et al., 2010). However, elsewhere there is evidence of lethal and sub-lethal impacts to numerous cetacean species from spill events. The Exxon Valdez oil spill (Alaska) resulted in increased mortality rates (33% and 41%) for two populations of killer whales (Matkin et al., 2008). There have been unusual mortality events of bottlenose dolphins reported between 2010 and 2014 (Graham et al., 2017), potential population impacts for striped dolphin (Marques et al., 2023) and potential relocation of sperm whales (Ackleh et al., 2012) after the Deepwater Horizon oil spill. Numerous studies have also reported long-term sub-lethal impacts on bottlenose dolphin health following the Deepwater Horizon oil spill including lung disease, impaired stress functions and low reproductive success (Schwacke et al. 2014; Lane et al. 2015; Smith et al. 2022).

Chemical pollution

34. The impact of contaminants on cetaceans is well documented (Jepson et al., 1999, 2005; 2016; Hall et al., 2006; ICES, 2015) and vulnerability assessments ranked chemical pollution as medium or high for all species assessed. Known impacts include reproductive impairment of harbour porpoise, bottlenose dolphins, killer whale and beaked whales (Murphy et al., 2015; Jepson et al, 2016; Desforges et al., 2018; Williams et al., 2021; Feyrer et al., 2024), increased mortality due to infectious diseases and reduced foetal survival in common dolphins (Gosnell et al., 2024; Murphy et al., 2012; 2018; 2021), increase in skin lesions in bottlenose dolphin (Stylos et al., 2022), population declines and liver damage in striped dolphin (Jepson et al., 2016; Borrell et al., 2014) and potential immunosuppression in multiple cetacean species (Desforges et al. 2016; 2018). Available evidence suggested that the UK cetacean species with highest vulnerabilities are harbour porpoise (Williams et al., 2023), common dolphin (Jepson et al., 2013; Williams et al., 2023), white-beaked dolphin (Van De Vijver et al., 2003; Megson et al., 2022; Williams et al., 2023), coastal bottlenose dolphin (Jepson et al., 2016), Risso’s dolphin (Megson et al., 2022; Minoia et al., 2023), killer whale (Jepson et al., 2016; Williams et al., 2023), Atlantic white-sided dolphin (Williams et al., 2023) and minke whale (Kleivance and Skaare, 1998; Rotander, et al., 2012a; Rotander, et al., 2012b). However, for many species included in this Strategy such as Atlantic white-sided dolphins, despite evidence of the occurrence of contaminants in tissue samples (McKenzie et al., 1997; Tuerk et al., 2009), there is minimal species-specific information on the impact of these pollutants on health and vital rates (Pinzone et al., 2022).

Plastic pollution (ingestion)

35. Plastic ingestion is monitored through post-mortem examination of stranded animals. Its presence has been in noted several species including harbour porpoises, long-finned pilot whale, Cuvier’s beaked whale and white-beaked dolphin but has only been identified as the acute cause of death in one Cuvier’s beaked whale stranding (Deaville et al., 2018). Little evidence of direct plastic ingestion has been recorded for most other species in UK waters (Deaville et al., 2018).

Entanglement in marine litter and ghost nets

36. As stated earlier, in cases of entanglement in stranded animals, unless observed entangled in active gear, it cannot be determined if the material is from active fishing gear or abandoned, lost or otherwise discarded fishing gear. There is evidence and data of cases of entanglement, and injuries consistent with an animal having been entangled, in the UK stranding schemes of species including: humpback whales, sperm whales and killer whales.

Eutrophication

37. Nutrient enrichment in areas of agriculture, aquaculture or sewage may pose a threat to individuals (Simeone et al., 2015), but the consequence of exposure to the population is unlikely to be a concern. Offshore distribution of some species will limit their exposure.

Climate change

38. Evidence suggests that the predominant impacts of climate change on marine mammals are geographic range shifts, reduction in suitable habitats, food web alterations and increased prevalence of disease in marine mammal populations (Williamson et al., 2021; Coombs et al., 2019; Hammond et al., 2002; 2013; 2021; Santos and Pierce, 2003; Bull et al., 2021; Kebke et al., 2022; Cohen et al., 2018; Tracy et al., 2019; Sanderson and Alexander, 2020). However, with the diverse range of anthropogenic pressures presently acting on marine mammals in UK waters (e.g. offshore infrastructure development, commercial fisheries, tourism) and the difficulty in distinguishing causation from correlation with many observed changes in marine mammal populations, it remains challenging to separate climate change induced pressures from wider cumulative pressures (Martin et al., 2023).

39. In addition to the long-term cumulative impacts discussed above, there may also be more acute impacts of shifting climactic conditions. Violent storms may influence stranding events (Hamilton 2018), the frequency and intensity of which may increase with climate change. Harmful algal blooms (HABs) have also been implicated in the deaths of several cetacean species around the world, for both baleen whales (e.g. Häussermann et al. 2017) and toothed whales such as bottlenose dolphins (Fire et al. 2011; 2015), harbour porpoises (Star et al. 2017), common dolphins (De La Riva et al. 2009) and pilot whales (Bengtson Nash et al. 2017). Though HAB events are currently rare in the UK, predictions indicate the distribution of toxin-producing plankton species will shift northwards this century, with potential implications for the frequency of blooms in UK waters, particularly the central and northern North Sea (Townshill et al. 2018).