Publication - Publication

Offshore wind energy - draft sectoral marine plan: habitat regulations appraisal

Published: 18 Dec 2019
Marine Scotland Directorate
Part of:
Energy, Marine and fisheries

The habitats regulations appraisal is completed in accordance with the Habitats Regulations that implement the EC Habitats and Birds Directives in UK waters and has been completed for the sectoral marine plan for offshore wind.

Offshore wind energy - draft sectoral marine plan: habitat regulations appraisal
Appendix H: Mammals Literature Review

Appendix H: Mammals Literature Review

H.1 Marine Mammal and Otter Baseline Characterisation

H.1.1 Scope of Review

This appendix presents a general review of the marine mammal populations found in Scottish waters.  It is designed to provide a screening context for the HRA of the Plan Options in Scottish Waters and includes information on the background ecology, population, distribution, abundance and foraging behaviour of the following marine mammal species:

  • Grey seal (Halichoerus grypus);
  • Common seal (Phoca vitulina);
  • Bottlenose dolphin (Tursiops truncatus); and
  • Harbour porpoise (Phocoena phocoena).

The European otter (Lutra lutra) has also been included in the scope of this review as it is recorded living on and near the coast around Scotland and travelling/foraging within the marine environment.

It should be emphasised that this is a broad-scale characterisation only.  It seeks to draw upon available information to understand the value of the area in general terms.  It does not specifically focus on the interest features of the European/Ramsar sites but provides a context for the assessment in recognition of the importance of mammals in this area.

H.1.2 Major Information Sources

Due to the scale of the proposed development and work being proposed in Scottish waters and the subsequent need for a coherent and comprehensive ecological management system for these waters, there is currently a significant level of research effort to improve the evidence base on the abundance and distribution of marine mammals in Scottish waters.

Detailed reviews of marine mammal distributions and ecology in UK waters have been carried out by the Sea Mammal Research Unit (SMRU), University of St. Andrews, as a contribution to the UK Department of Energy and Climate Change (DECC) Offshore Energy Strategic Environmental Assessment (SEA) (DECC, 2009; DECC, 2016) and previous oil and gas SEAs (Hammond et al. 2004 and Hammond et al. 2005).

Information on the status of seals around the UK coast is also reported annually by the SMRU-advised Special Committee on Seals (SCOS) (SCOS, 2017; SCOS, 2018).  Studies on the movements of seals fitted with satellite tags have been undertaken by;; Cunningham et al. 2009; Cronin et al., 2013; McConnell et al. 1999; Tollitand Thompson et al. 1996 to investigate their distributions in UK waters.  Research on seal site use, population structure, fidelity and philopatry (i.e. behaviour of remaining in, or returning to, an individual's birthplace) was reviewed by SCOS (2011) with specific research undertaken by Allen et al. (1996), Goodman (1998), Pomeroy et al. (2000), Pomeroy et al. (1994); Härkönen and Harding (2001); Cordes et al, (2011) and SMRU (2011). At sea movement data and terrestrial colony count data for seals has been used to produce high resolution broad-scale maps of distribution by the SMRU (Jones et al., 2015; Russel et al., 2017)

The ‘Small Cetacean Abundance in the European Atlantic and North Sea’ programmes (SCANS, SCANS-II and SCANS-III) undertook widespread ship based and aerial surveys of cetaceans in the North Sea and adjacent waters in the summers of 1994, 2005 and 2016 (SCANS, 1995; SCANS-II, 2008; SCANS-III, 2017).  The programme provides the most comprehensive wide-scale survey data on cetacean abundance, distribution and density available in northwest European waters.  Comprehensive information on cetacean distribution in northwest European waters is also presented in Reid et al. (2003).  This report provides a compilation of cetacean sighting records from a variety of systematic surveys and opportunistic sightings amounting to over 2,500 days of observation carried out since 1973.  Survey data for cetaceans recorded along the shelf edge (and offshore waters beyond this) of the UK, Ireland, France and Spain is provided by the Cetacean Offshore Distribution and Abundance (CODA) survey (CODA, 2009).

Understanding of cetacean population structure was reviewed in the ASCOBANS/HELCOM small cetacean population structure workshop (Evans and Teilmann, 2009) and more recently by the Inter-Agency Marine Mammal Working Group (IAMMWG, 2015).

The WDCS (Whale and Dolphin Conservation Society) additionally undertook a review identifying critical habitat for cetaceans to help highlight potential Marine Protected Areas (MPA’s) for cetacean species (Clark et al. 2010).

The results of a number of research studies to determine the distribution, abundance and population structure of bottlenose dolphins around the Scottish coast is presented in Thompson et al. (2011).  The collaborative project commissioned by Scottish Natural Heritage (SNH) was carried out by the University of St Andrews’ Sea Mammal Research Unit (SMRU), the Scottish Association for Marine Science (SAMS), the Hebridean Whale and Dolphin Trust (HWDT) and the University of Aberdeen’s Lighthouse Field Station (AULFS), with additional support from a wide range of different individuals and bodies.  The results of this collaborative research have also been presented in Cheney et al (2013) and provide the first comprehensive assessment of abundance of bottlenose dolphins in the inshore waters of Scotland. More recently, Nykänen (2019) investigated the fine‐scale population structure and connectivity of bottlenose dolphins in European waters (including UK populations).

Studies and surveys at a more local scale on the bottlenose dolphins have been undertaken by a range of organisations including the University of Aberdeen/SMRU (Hastie et al. 2003, Bailey 2006, SMRU, 2007), Talisman Energy (UK) Limited (2006), WDCS, 2009) and the Hebridean Whale and Dolphin Trust (Mandleberg, 2006). The potential for long distance movements of the bottlenose dolphins around UK waters has been investigated by a collaboration of organisations including Cetacean Research and Rescue Unit (CRRU), AULFS, HWDT and Irish Whale and Dolphin Group (IWDG), which has resulted in the first evidence for long-term re-sightings between the Moray Firth, Inner Hebrides and across international borders to the Republic of Ireland (Robinson et al, 2012).

Heinänen and Skov, (2015) used spatial modelling and the results of detailed analyses of 18 years of survey data in the Joint Cetacean Protocol (JCP) undertaken to inform the identification of discrete and persistent areas of relatively high harbour porpoise desnity in the UK marine area within the UK Exclusive Economic Zone (EEZ).

Satellite tagging of harbour porpoises showing evidence of animals moving from northern Denmark to the western part of the North Sea and Shetland has been undertaken by the National Environmental Research Institute (NERI), Denmark (Teilmann et al. 2008; Sveegaard et al. 2011).  Surveys on harbour porpoise abundance and distribution at a regional scale and have been undertaken by a range of authors including Bailey and Thompson (2009), Goodwin and Speedie (2008), Robinson et al. (2007) and Marubini et al. (2009).

Detailed research on otter ecology in coastal and marine environments has been undertaken around Shetland by members of the Institute of Terrestrial Ecology and University of Groningen (Kruuk et al. 1991; Kruuk and Moorhouse, 1991; Kruuk et al.. 1993; Kruuk, 1995; Kruuk et al. 1998 and Nolet et al. 1993).  In addition, reviews of otter ecology have been produced as part of the Life in UK Rivers project, a project which aimed to develop methods for conserving the wildlife and habitats of rivers within the Natura 2000 network of protected European sites (Chanin, 2003) and by Mason and Macdonald (2008).

A number of other surveys and studies on marine mammals and otters have also been included where appropriate.

H.2 Pinnipeds

H.2.1 Grey Seal Halichoerus grypus

13.1.1 Ecology

The grey seal is the larger of the two seal species found in British waters, with males reaching a length of 2.45m and weigh over 300kg.  In Scotland, grey seals predominantly inhabit remote islands and coastlines, giving birth to their pups from September to early November.  About 38% of the world population of grey seals is found in Britain and over 88% of British grey seals breed in Scotland (Tables H1 and H2), the majority in the Hebrides and in Orkney.  There are also breeding colonies in Shetland, on the north and east coasts of mainland Britain and in SW England and Wales (SCOS, 2018).  The Monach Islands off the Outer Hebrides holds the largest single breeding colony in the UK, (contributing over 20% of annual UK pup production) with Faray and Holm of Faray (uninhabited islands in the northern part of Orkney) supporting the second-largest breeding colony in the UK, contributing around 9% of annual UK pup production.

Grey seals are generalist feeders, foraging mainly on the sea bed at depths of up to 100m although they are probably capable of feeding at all the depths found across the UK continental shelf. They take a wide variety of prey including sandeels, gadoids (cod, whiting, haddock, ling), and flatfish (plaice, sole, flounder, dab). Amongst these, sandeels are typically the predominant prey species. Diet varies seasonally and from region to region (SCOS, 2018). Tracking of individual seals has shown that they can feed up to several hundred kilometres offshore, with foraging trips lasting between 1-30 days although most foraging probably occurs within 100km of a haulout site (SCOS, 2018).  Research into the diving behaviour of grey seals around the Orkney coast found that they appear to use the whole water column with a mean dive duration of 3.82 minutes recorded (SMRU, 2011).

13.1.2 Population structure, philopatry and site fidelity

A range of studies suggest that site fidelity and philopatry is common in grey seals.  Allen et al. (1995) examined the genetic variability and differentiation of grey seals in two widely spaced British breeding colonies.  Samples were collected from adults and pups on the island of North Rona, off the north-west coast of Scotland, and on the Isle of May, situated at the mouth of the Firth of Forth on the east coast.  Highly significant differences in allele frequencies between these two sites indicated considerable genetic differentiation.  The authors suggested that although grey seals are known to range over very large areas outside the breeding season, site fidelity of adults and philopatry of pups for these breeding colonies must be sufficiently common to have effects, through genetic drift, at the sub-population level.

Within Europe there are two apparently reproductively isolated populations, one that breeds in the Baltic, usually pupping on sea ice in the spring, and one that breeds outside the Baltic, usually pupping on land in Autumn and early winter.  These populations appear to have been reproductively isolated at least since the Last Glacial Maximum (between 26,500 and 19,000-20,000 years ago).

Little movement of breeding animals between sites in the Inner Hebrides, Outer Hebrides, Orkney and North Sea has been indicated by a spatially-explicit model that has been used to estimate the British grey seal population from geographically structured pup production estimates (SCOS, 2011).  This finding is further supported by recent results from other grey seal population models that indicate an absence of large scale redistribution of breeding females between regions (Graves et al. 2009) again implying a high degree of philopatry (SCOS, 2011).  The lack of large scale redistribution is also supported by Pomeroy et al. (2000).  The study showed that grey seals born and marked by tagging and cohort brands at colonies on North Rona, Outer Hebrides since 1960 and at the Isle of May, Firth of Forth since 1981 have shown philopatry (particularly for females) and that females born at N. Rona pupped closer to their natal sites than would be expected by chance providing evidence of fine scale natal site fidelity.  Pomeroy et al (1994) also found evidence of philopatry with 93% of marked females in 1985 returning to N. Rona to breed in at least one season up to 1989 with 27% present in all five years.

13.1.3 Population trends

Pup production estimates are typically used to compare the relative sizes of grey seal populations (SCOS, 2017). The total number of pups born in 2014 at all annually surveyed UK colonies was estimated to be 65,000.  Regional estimates were 4,541 in the Inner Hebrides, 15,732 in the Outer Hebrides, 23,849 in Orkney, and 6,426 in the Firth of Forth.  A further 4,193 pups were estimated to have been born at other scattered colonies.  Overall, there has been a continual increase in pup production since regular surveys began in the 1960s. Between 1992 – 2009 there were indications that population growth was levelling off, for example the number of pups born in the Hebrides and Orkney have remained approximately constant since 1992 (SCOS, 2010). However, the most recent surveys in 2014 and 2016 shows an increase in pup production.  Grey seal pup production in 2016 can be seen in Table H1 and the major breeding colonies are highlighted in Image H1.

Table H1: Grey seal pup production estimates for the main colonies surveyed in 2014

Region Location 2016 Pup Production Pup production in 2014 Average Annual Change in Pup Production From 2014-2016
Scotland Inner Hebrides 4,541 4,054 +5.8%
Outer Hebrides 15,732 14,316 +4.8%
Orkney 23,849 23,758 +0.2%
Firth of Forth  6,426 5,860 +4.7%
All other Scottish colonies incl Shetland & mainland 4,193 3,875 +4.0%
Total  54,741 51,863 +2.7%
England and Wales Donna Nook +East Anglia 5,919 5,029 +8.5%
Farne Islands 2,238 1,600 +18.3%
SW England (last surveyed 1994)* 350 250 -
Wales * 1,650 1650 -
Total 10,157 8,527 +9.1%
Northern Ireland*   100 100 -
Total (UK)   64,998 60,490 +3.7%

*Estimated production for colonies that are rarely monitored

(Source: SCOS, 2017)

Image H1: Grey Seal Breeding Colonies in the UK

Image H1: Grey Seal Breeding Colonies in the UK

(Source: Russel et al., 2017)

13.1.4 Grey seal movement at sea

Grey seals can undertake wide ranging seasonal movements over several thousand kilometres (McConnell et al. 1999; Jones et al., 2015; Russel et al., 2017). However, while grey seals may range widely between haul out sites, tracking has shown that most foraging probably occurs within 100 km of a haul-out site (SCOS, 2018). For example, Cronin et al. (2013) found that the mean distance travelled by tagged seals during foraging trips was 50 km from haul out sites in south west Ireland. McConnell et al. (1999) found that most tagged grey seals stayed relatively close to haul out sites in the North Sea (mean distance of 39.8 km travelled on a foraging trip) with an average of 43% of the grey seals’ time spent within 10 km of a haul out site. This is reflected in seal usage maps which shows the highest densities of seal movements to occur around the seal colonies with the largest populations (such as Orkney, North Rona, the Monach Isles, the Farne Islands, and Donna Nook) (Image H2).

Image H2: Mean grey seal at-sea usage

Image H2: Mean grey seal at-sea usage

(Source: Russel et al. 2017)

H.2.2 Common Seal Phoca vitulina

13.1.5 Ecology

The common seal (also known as harbour seal) is the smaller of the two native UK seals measuring up to approximately 1.85m in length and typically weigh 80-100kg.  Britain is home to approximately 30% of the population of the European sub-species of common seal (having declined from approximately 40% in 2002).  Scotland holds approximately 79% of the UK common seal population (SCOS, 2018).

The population along the east coast of England (mainly in The Wash) was reduced by 52% following the 1988 phocine distemper virus (PDV) epidemic. A second epidemic in 2002 resulted in a decline of 22% in The Wash, but had limited impact elsewhere in Britain. Counts in the Wash and eastern England did not demonstrate any immediate recovery from the 2002 epidemic and continued to decline until 2006. The counts increased rapidly from 2006 to 2012 but have remained relatively constant since. In contrast, the adjacent European colonies in the Wadden Sea experienced continuous rapid growth after the epidemic, but again, the counts over the last 5 years suggest that the rate of increase has slowed dramatically. Major declines have now been documented in several harbour seal populations around Scotland, with declines since 2001 of 76% in Orkney, 30% in Shetland between 2000 and 2009, and 92% between 2002 and 2013 in the Firth of Tay. However the pattern of declines is not universal. The Moray Firth count apparently declined by 50% before 2005, remained reasonably stable for 4 years, then increased by 40% in 2010 and has fluctuated since, showing no significant trend since 2000. The 8 Outer Hebrides apparently declined by 35% between 1996 and 2008 but the 2011 count was >50% higher than the 2008 count. The recorded declines are not thought to have been linked to the 2002 PDV epidemic that seems to have had little effect on harbour seals in Scotland (SCOS, 2018).

Common seals are widespread around the west coast of Scotland and throughout the Hebrides and Northern Isles (SCOS, 2011).  On the east coast, their distribution is more restricted with concentrations in the major estuaries of the Thames, The Wash, Firth of Tay and the Moray Firth.  In Scotland, common seals are typically found in a wide variety of coastal habitats and come ashore in sheltered waters, including on sandbanks, in estuaries and along rocky areas (particularly on the west and north coasts).  Adult common seals can remain very faithful to particular haul-out areas, moving around the same group of favoured locations on a seasonal basis and over a number of years (SNH, 2006).  Tollit et al. (1998) found that the diet of common seals in north eastern Scotland primarily consisted of sandeel with octopus, gadoids and clupeids also consumed.

Research into the diving behaviour of common seals around the Orkney coast found that they appear to utilise the entire water column with dives lasting a mean duration of 4.14 minutes (SMRU, 2011).

13.1.6 Population structure, philopatry and site fidelity

Genetic analysis on common seals in European waters has found evidence of significant genetic differentiation between different areas.  Goodman (1998) identified six distinct population units: Ireland-Scotland, English east coast, Wadden Sea, western Scandinavia (Norway-Kattegat-Skagerrak-west Baltic), east Baltic and Iceland. Within the Ireland-Scotland population there is probably occasional movement of animals between regions, but there is no evidence from satellite telemetry of any long-range movements (for example, between the east and west coasts of Scotland) comparable to those observed in grey seals.  Similarly, studies of the movements of branded seals in the Kattegat/Skagerrak indicate that there is only limited movement within the western Scandinavia population.  However, satellite telemetry has revealed some interchange between the Wadden Sea and the English east coast populations outside the breeding season (SCOS, 2010).  This evidence and the fact that the phocine distemper virus spread rapidly throughout European common seal populations indicates that movement of individuals  between locations does occur, but reproduction does not occur in the regions they visit (SCOS, 2010).

In the Moray Firth, repeat sightings of adult branded seals have shown that at least some individuals remain faithful to a single haul out site, or small group of sites both during the breeding season and in successive years (H Corpe and PM Thompson unpublished data, cited by Corpe, 1996).  However, recent evidence from Cordes et al (2011) have shown that when looking at long term trends (decadal), breeding females may be changing their pupping sites in the Moray Firth to new sites.  It has been suggested that this is due to changes in site characteristics, rather than changes in foraging behaviour.  On the west coast of Sweden, Härkönen and Harding (2001) observed that all of the branded adult females returned to within 7 km of their natal site.  Males, in contrast, showed an increased rate of dispersal from their natal site from the age of two, yet appeared to remain within approximately 50 km of their natal site.

13.1.7 Population trends

Overall, the common seal population has increased from 25,600 (rounded to the nearest 100) in the 2007-09 period to 31,400 animals during the 2013-2016 period, but remain slightly below the 1996-97 level of 32,800 (SCOS, 2017). However, as reported in SCOS 2008 to 2016, patterns of changes in abundance have not been universal; although declines have been observed in several regions around Scotland some populations appear to be either stable or increasing. The common seal population is currently showing a decline in many populations along its range in the UK in comparison to population counts in the mid-1990s.  The Orkney common seal population counts in 2016 were >30% lower than the same areas in 2013 (SCOS, 2017). The Orkney Harbour seal population has continued to decline since the late 1990’s and has been falling at an average rate >11% p.a. since 2001 (SCOS, 2011).

Counts in the Outer Hebrides in 2008 were 35% lower than the peak count in 1996.  Counts in the Strathclyde region in 2009 were 15% higher than 2007 and like the counts in the mid-1990s.  Surveys of the east coast populations in 2010 showed contrasting trends. The Firth of Tay SAC population has declined at an average rate of 20% p.a. since 2002 with the 2016 count 90% lower than the peak counts between 2000 and 2002 (SCOS, 2017). There is an increase in the population at the Moray Firth.  The total counts in 2016 were 26% higher than the previous two years, although still significantly lower than the 1996-97 counts (SCOS, 2017). The combined count for the English east coast population (Donna Nook to Scroby Sands) in 2010 was 5% higher than the 2009 count which was 21% higher than the 2008 count, meaning that population is nearing pre-epidemic levels in 2002. The west coast of Highland Region appears to be stable, while the north coast of Highland Region has declined by 35% since the 2005 survey and is approximately 60% lower than in 1997.  Interestingly, the nearest European population in the Wadden Sea has shown a rapid growth in populations which increased 12% between 2008-2009 and has grown at 13% p.a. since the 2002 PDV epidemic (SCOS, 2011).  The distribution of harbour seals 2016 in Great Britain and Ireland, by 10km squares can be seen in Image H3.  A summary of counts of harbour seals from 2011-2016 can be seen in Table H2.

Table H2. Counts of harbour seals by region (2011-2016)

Region Location Count Estimate
Scotland Southwest Scotland 1,200
West Scotland 15,184
Western Isles 2,739
North Coast and Orkney 1,349
Shetland 3,369
Moray Firth  940
East Scotland 368
England Northeast England 86
Southeast England 5,061
South England 23
Southwest England 0
Wales 5
Northwest England  10
Total Britain 30,334

Image H3: Haul-out count data for harbour seals between 1996 and 2015

Image H3: Haul-out count data for harbour seals between 1996 and 2015 

(Source: Russel et al., 2017)

13.1.8 Common seal movement at sea

Harbour seals are not believed to travel as far as grey seals, usually staying closer to haul out sites, typically within 40-60 km (for example Thompson et al., 1996; Cunningham et al., 2009; Tollit et al., 1998; SCOS, 2011). This is reflected in at-sea usage maps which show the highest densities of seal movements in close proximity to haul-out sites with foraging areas off much of the east and west coast of Scotland, with hotspots of activity north of St Andrews, in the Moray Firth, northeast of Orkney, the east of Shetland and the Hebrides (Image H4) (Russel et al., 2017). However, seals tagged in The Wash have been observed making longer trips of between 75 and 120 km offshore during foraging trips averaging ten days in duration (SCOS, 2004). All seals tagged in The Wash were highly consistent in their individual foraging habits, repeatedly travelling to the same areas. No seasonality in behaviour was apparent, though diet varies seasonally based on the availability of prey (Hall et al., 1998). All but one of the seals tagged remained faithful to the haul-out site at which they were recorded (SCOS, 2004).

Image H4: Mean harbour seal at-sea usage

Image H4: Mean harbour seal at-sea usage

(Source: Russel et al., 2017)

H.3 Cetaceans

H.3.1 Bottlenose Dolphin Tursiops truncatus

13.1.9 Ecology

The bottlenose dolphin has a worldwide distribution in tropical and temperate seas of both hemispheres in both shelf and coastal waters.  In coastal waters, bottlenose dolphins favour river estuaries, headlands and sandbanks, mainly where there is uneven bottom relief and/or strong tidal currents (DECC, 2009; DECC, 2016).  Bottlenose dolphins are a social dolphin, commonly forming groups of 2-25 individuals with larger schools occurring in offshore areas (Reid et al. 2003).  Bottlenose dolphins in Scotland have been found to feed primarily on gadoids such as cod, saithe and whiting as well as Atlantic salmon and cephalopods (Santos et al. 2001).

13.1.10 Population structure

Bottlenose dolphins in the North Atlantic appear to consist in two forms, coastal and offshore. The better known coastal form is locally common in the Irish Sea (particularly Cardigan Bay) and off north east Scotland (particularly the inner Moray Firth), and in smaller numbers in the Hebrides (west Scotland), and off south west England.  Little is known about the offshore form of bottlenose dolphins, including the relationship between the offshore and coastal forms (Clark et al. 2010).  More detailed studies in the North West Atlantic suggest that inshore and offshore (pelagic) populations are ecologically and genetically discrete (Hoelzel et al. 1998).

Nykänen et al (2019 also found a high level of genetic differentiation between coastal and pelagic populations. The study also found that genetic structure was defined at an unprecedented fine‐scale level for coastal dolphins, leading to identification of five distinct coastal populations inhabiting the following areas: Shannon estuary, west coast of Ireland, English Channel, coastal Galicia, east coast of Scotland and Wales/west Scotland. Demographic connectivity was very low among most populations with <10% migration rate, suggesting no demographic coupling among them. The study concluded that each local population should therefore be monitored separately.

Based on the current understanding of near-shore bottlenose dolphin population and community structure the IAMMWG (2015) identified six MUs in British waters:

  • Coastal West Scotland and the Hebrides (CWSH, to 12nm);
  • Coastal East Scotland (CES, to 12nm);
  • North Sea (NS, represented by ICES Area IV, excluding coastal east Scotland) It should be noted that very few bottlenose dolphin are seen in this area and, although there is no conclusive evidence, those seen are thought to belong to the Coastal Scottish group;
  • The Channel and South West England (CSW) (ICES Divisions VIId to h);
  • Irish Sea (IS) (ICES Division VIIa); and
  • Oceanic waters (OW) (ICES Divisions VIa-b, VIIb, c, k and j, excluding coastal west Scotland).

The location of these areas can be seen in Image H5.

Image H5: Bottlenose dolphin Management Units (MU)

Image H5: Bottlenose dolphin Management Units (MU)

(Source: IAMMWG, 2015)

13.1.11 Abundance and distribution of different populations

Photo-identification studies indicate that around 200-300 individual dolphins occur regularly in Scottish coastal waters.  The majority of sightings of bottlenose dolphins around the Scottish coastline are concentrated on the east and west coasts with numbers on the east coast being approximately five times higher (Thompson et al. 2011).  A population of 195 dolphins (95% highest posterior density interval-HPDI: 162-253) has been estimated for the east coast and 45 (95% HPDI: 33-66) dolphins for the west coast (Cheney et al. 2013).  Estimates an order of magnitude higher have been estimated for offshore bottlenose dolphins from wide-scale surveys such as SCANS-II, SCANS-III and CODA.

As discussed in Section 13.1.2, four distinct populations are currently considered to occur around Scotland, these are the North Sea, Outer Hebrides, Inner Hebrides and an offshore form.

The three parapatric communities of bottlenose dolphins in Scottish coastal waters, which vary considerably in population size have marked contrasts in their ranging patterns.  On the west coast there are two small and socially segregated communities of dolphins, one of which includes approximately 15 individuals that have only been recorded in the waters around the Sound of Barra whereas the other is double in size and ranges more widely throughout the Inner Hebrides and mainland coasts.  Due to the lack of movement between the two west coast communities, it has been recommended by Cheney et al (2013) that these communities should be considered as separate units for management purposes, pending further study.  On the east coast there is a population of nearly 200 interacting dolphins between the Moray Firth and Fife with individual differences in ranging behaviour and site fidelity.  Information on the abundance and distributions of these populations is shown in Table H3. A recent report by Robinson et al. (2012) provides evidence for the first time of bottlenose dolphins being re-sighted between the Moray Firth, Inner Hebrides and the Republic of Ireland, suggesting that a small number of individuals do have long range dispersal routes of up to 1,277km. This research provides evidence that communities are discrete from one another and should be managed as separate units, but that these wide ranging individuals may provide an important function in gene pool flow meaning North Atlantic bottlenose dolphin populations are not completely isolated.

Analyses of photo-identification data and genetic studies indicate that coastal bottlenose dolphins can make long-distance movements between the east and west coast of Scotland (Cheney et al. 2013), although this appears extremely rare with relatively few reports of bottlenose dolphins on the north coast of mainland Scotland or around Orkney and Shetland suggesting that the north coast and Northern Isles are not significant bottlenose dolphin habitat (Thompson et al. 2011).

Research has also indicated a level of population genetic structure among the resident populations of dolphins from East Scotland and West Scotland, and those in adjacent waters in Wales and Ireland.  However, there appears to be insufficient genetic divergence to suggest that these are demographically isolated populations.  At the same time, the analyses indicated that animals from these four areas do not form a single randomly mating population (Thompson et al. 2011).  It has however been identified that inshore and offshore populations are ecologically and genetically discrete (Hoelzel et al, 1998 and Thompson et al, 2011).

Table H3: Abundance and distribution of Scottish bottlenose dolphin populations

Area Population Background Information
East North Sea
(Eastern Scotland from Caithness to the borders with England)
The population of bottlenose dolphins on the north east coast of Scotland is estimated at around 193 animals (Thompson et al. 2011), with all animals considered highly mobile (Cheney  et al, 2013).  The core range of the north east Scottish population is the Moray Firth which has 2 communities; community 1, animals that are regularly seen outside the inner Moray Firth (38% of individuals seen 2006-2007); and community 2, animals which are rarely seen outside in inner Moray Firth (20% of individuals seen 2006-2007). The remaining individuals comprise calves and juveniles which are yet to be assigned to a community.  Bottlenose dolphin is listed in Annex II of the Habitats Directive, and the importance of this population, and the inner Moray Firth, is reflected in the designation of part of this area as a Special Area of Conservation (SAC).  In the 1980s, the population’s known range was focused in the inner Moray Firth but since the early 1990s, the population’s range has expanded and they now inhabit the coastal waters of eastern Scotland from north of the Moray Firth to the Firth of Forth.  Surveys along the southern coast of the Moray Firth from 2001-2005 encountered bottlenose dolphins along the majority of the coastline, primarily in waters <25m depth (Robinson et al. 2007).  Observations are also occasionally made in northern England (Thompson et al. 2011; Evans and Teilmann, 2009).  Occasional visual and acoustic surveys in offshore waters of the Moray Firth have encountered very few bottlenose dolphins (Hastie et al. 2003, Bailey 2006, Talisman Energy (UK) Limited, 2006, SCANS-II 2008).  The Whale and Dolphin Conservation Society conducted surveys of the outer Moray Firth in 2008 and recorded no bottlenose dolphin sightings, with the main cetaceans recorded consisting of harbour porpoise and minke whale (WDCS, 2009).  
West Outer Hebrides
(Island of Barra)
In total a maximum of 15 dolphins were identified from surveys in and around the Sound of Barra in 2007.  The dolphins appeared to have a very small range and they were encountered during every visit to the area (Thompson et al. 2011).
Inner Hebrides / mainland In the Inner Hebrides a maximum of 25 dolphins were identified in 2007.  In contrast to the dolphins using the Sound of Barra this community ranged widely with identifications from Kintyre to the north coast of the Isle of Skye and Gairloch (Thompson et al. 2011).  Systematic data from HWDT shows greater relative densities in certain parts of the Inner Hebrides.  During the monthly surveys carried out by HWDT in the summer months between 2003-2005, the southern coast of Islay (at the entrance to Loch Indaal), Mull, Tiree and Skye were found to be of particular relevance for bottlenose dolphin sightings (Mandleberg, 2006).
Offshore Offshore High densities of bottlenose dolphins have been recorded in the deep waters of the North Atlantic, often in association with long-finned pilot whales and Atlantic white-sided dolphins such as in the vicinity of Rockall Bank and over the Wyville Thompson Ridge and Ymir Ridge (Reid et al .2003). In European offshore waters, the CODA survey (in waters deeper than 200m) estimated bottlenose dolphin abundance in the summer of 2007 as 19,295 with an abundance of 5,700 in waters north of 53ºN, including offshore Scottish waters (CODA 2009).

H.3.2 Harbour Porpoise Phocoena phocoena

13.1.12 Ecology

Harbour porpoise distribution is restricted to temperate and sub-arctic (primarily 5 14ºC) seas of the Northern Hemisphere.  The harbour porpoise is the most commonly recorded cetacean in UK waters, primarily occurring on the continental shelf (Hammond et al, 2005; and DECC 2016).  In coastal waters, they are often encountered close to islands and headlands with strong tidal currents (Pierpoint, 2008; Marubini et al, 2009 and DECC, 2016).  Harbour porpoise often show large seasonal variations in their distribution (Read & Westgate, 1997 and Sveegard et al, 2011).  These seasonal changes may be linked to migrations/changes in the distribution of prey (Sveegaard et al, 2011).

Harbour porpoises around the coasts of Scotland forage on a range of species including sandeels (Ammodytidae), gadoids such as whiting (Merlangius merlangus) and clupeids (herring and sprats) (Santos et al. 2004; MacLeod et al. 2007).

13.1.13 Population structure

In the last three decades, photo-ID has become a standard tool in cetacean monitoring, being used for the estimation of various population parameters such as abundance, home range size and use, movements and for the better understanding of social structure.  It relies upon natural markings (such as nicks in the dorsal fin or tail flukes, pigmentation patterns or chevrons).  However for harbour porpoise it would be unrealistic to use photo-ID for distinguishing populations (Evans and Teilmann, 2009).  Instead satellite tagging and genetic studies have been used to investigate population structure.

Johnston et al. (2005) investigated the fine-scale distribution of harbour porpoises, using satellite telemetry and found that harbour porpoises can range over large areas (up to 11,289 km2) but generally concentrated their movements in small focal regions often proximate to islands, headlands, or restricted channels.

Satellite-tracking data has shown evidence of several animals moving from northern Denmark to the western part of the North Sea and Shetland.  The study tagged 63 harbour porpoises with satellite transmitters in Danish waters from 1997 to 2007 (Teilmann et al. 2008; Sveegaard et al. 2011).  Twenty-four harbour porpoises were tagged on the northern tip of Jylland (Skagen) at the border between Skagerrak and Kattegat and 39 harbour porpoises were tagged in the Inner Danish Waters (see Image H6).  The majority of the tagged harbour porpoises remained within several hundred kilometres of the area where they were first tagged.  Six animals (approximately 10% of the total number tagged) however undertook long distance travel into western parts of the North Sea.  These animals were recorded approximately 800-1000km away from the point they were first tagged, travelling to various locations offshore from the East Coast of the UK and the Shetland Isles (Image H7).

Image H6: Locations (1 Per Day) of the 63 Porpoises Tracked Between 1997 and 2007

Image H6: Locations (1 Per Day) of the 63 Porpoises Tracked Between 1997 and 2007

Locations from porpoises tagged in the IDW are red and locations from porpoises tagged in Skagen are blue

(Source: Teilmann et al. 2008)

Image H7: Tracks Made by the Six Tagged Harbour Porpoise Recorded Moving into the Western Part of the North Sea

Image H7: Tracks Made by the Six Tagged Harbour Porpoise Recorded Moving into the Western Part of the North Sea

(Source: Teilmann et al. 2008)

The identification of different stocks or subpopulations for harbour porpoise was undertaken by ASCOBANS Population Structure Workshop based on genetic studies and the combining of information from other approaches (e.g. telemetry). The Workshop identified 14 distinct stocks for the North Atlantic. The stocks relevant to UK waters are the North Eastern North Sea & Skagerrak (NENS), South Western North Sea & Eastern Channel (SWNS), Celtic Sea (plus South West Ireland, Irish Sea & Western Channel) (CES) and North West Ireland & West Scotland (NWIS) (Evans and Teilmann, 2009). More recently the IAMMWG identified three MUs appropriate for harbour porpoise in UK waters (IAMMWG, 2015):

  • North Sea (NS) (comprising ICES area IV, VIId and Division IIIa [Skagerrak and northern Kattegat]). Noting that the northern and western boundary with Division VIa is arbitrary and that there will be an interchange of animals here with the West Scotland MU;
  • West Scotland (WS) (comprising ICES area VIa and b). Noting that the boundary with the North Sea MU is arbitrary and that there will be an interchange of animals here and also with the Irish Sea and Celtic Sea MUs; and
  • Celtic and Irish Seas (CIS) (comprising ICES area VI and VII, except VIId).

13.1.14 Abundance and distribution

The harbour porpoise in the most abundant cetacean in UK waters (Clark et al. 2010; DECC, 2016). The estimated abundance of harbour porpoise in different areas of the UK from the SCANS-II survey is presented in Table H4.  While sighted throughout the year, peak numbers are generally recorded in summer months from June to October.

Table H4: Estimated abundance and density of harbour porpoise in UK waters

Area Abundance Density
Northern Isles and Moray Firth 10,254 0.274
Northern North Sea 23,766 0.177
Central North Sea (north) 47,131 0.294
Central North Sea (south) 88,143 0.562
Southern North Sea and Channel 40,972 0.331
Celtic Shelf 80,163 0.408
Irish Sea 15,230 0.335
Minches and western Scotland 12,076 0.394
Shelf waters west of Scotland and Ireland 10,002 0.067
Total (strata overlapping UK waters) 328,142 0.317

(Source: SCANS-II)

Density surface modelling of harbour porpoise from the SCANS project in 1994 suggested high densities of animals north of Scotland and in the western central and northern North Sea.  The highest density of porpoises in UK waters in the SCANS survey was recorded off the Northern Isles and the Inner Moray Firth at 0.783km animals/km2 and an abundance of 24,355 animals.  Repeat surveys for SCANS-II in 2005 showed considerable differences in distribution (DECC, 2009).  While areas of high density remained north of Scotland, density around Orkney and Shetland appeared considerably lower than in 1994 (see Image H8).  In addition a more southerly distribution in the North Sea was observed in 2005 with elevated densities in the southern North Sea and only moderately low densities for the northern North Sea and Moray Firth.  The reason for the change is unknown but changes in prey abundance and distribution have been suggested.  The Minches and western Scotland (Hebrides) were not surveyed in the original 1994 SCANS survey but high densities were recorded in SCANS-II. Various studies that have investigated harbour porpoise distribution at a more local level are summarised in Table H5.

Image H8: Modelled Density of Harbour Porpoise in 1994 SCANS and 2005 SCANS Surveys

Image H8: Modelled Density of Harbour Porpoise in 1994 SCANS and 2005 SCANS Surveys

(Source: DECC, 2009)

More recently collected data from the SCANS-III survey carried out in 2016 (SCANS-III, 2017) showed that the shift seen in harbour porpoise distribution in the North Sea from the northwest in 1994 to the south in 2005 was maintained in 2016, with highest densities found in the southwestern North Sea, and north and east of Denmark.

The series of abundance estimates for harbour porpoise shows no change and a stable trend in abundance over the 22 years covered by the SCANS surveys (SCANS-III, 2017).

Table H5: Regional studies into harbour porpoise abundance and distribution

Area Summary Data Source
Hebrides  Sightings collected by the Sea Watch Foundation during vessel surveys in West Scotland were critically analysed and used to construct a predictive habitat model for harbour porpoises in the Greater Minch.  Four ‘Hotspot’ areas with high relative abundance were identified in (1) the region between Ardnamurchan, Coll and the Small Isles, (2) southeast of Barra, (3) northeast of Skye to Gairloch, and (4) west of Pairc Peninsula (Isle of Lewis) to Shiant Islands. Marubini et al. 2009
Moray Firth  An examination of the animals using the southern coastal waters of the outer Moray Firth was carried out between May and October 2001 to 2005 inclusive.  The most commonly sighted cetaceans were harbour porpoises (comprising 60% of all encounters).  Robinson et al. 2007
West Coast During May-August, 2002-2004 a boat-based visual survey, employing effort-related line transect methodology was conducted for specified areas of the west coast of the UK.  Harbour porpoise density showed a significant decrease in the South West, with an increase for West Scotland over the study period, whilst the highest numbers for both Northern Ireland and the Firth of Clyde were recorded in 2003.  The porpoise population around the Firth of Clyde in July was estimated at 1645 (823LCI-3289UCI) individuals and off the West Scotland during August and September 3105 (2032LCI-4745UCI) individuals. Goodwin and Speedie, 2008.
Moray Firth Used broad-scale surveys in the Moray Firth SAC to model habitat preferences of different marine mammals.  The predictions from the model indicated highest relative densities occurring within the centre of the area from the outer SAC boundary towards the Inverness Firth and also off Helmsdale. Bailey and Thompson, 2009.

Heinänen and Skov, (2015)used spatial modelling and the results of detailed analyses of 18 years of survey data in the Joint Cetacean Protocol (JCP) undertaken to inform the identification of discrete and persistent areas of relatively high harbour porpoise density the UK marine area within the UK Exclusive Economic Zone (EEZ). The following persistent high-density areas were identified (Image H9):

  • Three coastal areas off west Wales (Pembrokeshire and Cardigan Bay), and north-west Wales (Anglesey, Lleyn Peninsula), and part of the Bristol Channel (Camarthen Bay)
  • Smaller areas north of Isle of Man (winter) and on the Northern Irish coast near Strangford Lough
  • Western Channel off Start Point, Cornwall (summer)
  • North-western edge of Dogger Bank (summer)
  • Inner Silver Pit
  • Offshore area east of Norfolk and east of outer Thames estuary (winter)
  • Smith Bank, Outer Moray Firth (summer)
  • Coastal areas off north-west Scotland, including the Minches and eastern parts of the Sea of Hebrides.

In addition, the following areas were also identified as persistent high-density areas, but due to less than three years of survey effort they were not included in the above list:

  • Parts of high density zone between western edge of Dogger Bank and Norfolk coast, including both the Inner and Outer Silver Pit areas
  • Offshore area north of Shetland
  • Edge of the Norwegian Trench
  • Shelf edge off south-west Cornwall.

Image H9: Map showing selected persistent high-density areas of harbour porpoise with survey effort from three or more years, as derived from the statistical manipulations used in the present report.

Image H9: Map showing selected persistent high-density areas of harbour porpoise with survey effort from three or more years, as derived from the statistical manipulations used in the present report.

(Source: Heinänen and Skov, 2015)

H.4 European Otter Lutra lutra

H.4.1 Ecology

The Eurasian or European otter (Lutra lutra) is native to the UK. It belongs to the Mustelid family that includes weasels, stoats, badgers, polecats, pine martins and mink.  The otter has a broad geographical distribution that extends west-to-east from Ireland to Japan and north-to-south between the Arctic Circle and Southeast Asia (Chanin, 2003).

In Scotland otters are widely distributed with strongholds in the Highlands, Hebrides and Shetland (Strachan, 2007).  Populations of otters in England and Wales are confined mainly to fresh water but coastal habitats in Scotland are also readily exploited.  Coastal holts are typically found within 100 m of the shore but can be further inland (Chanin, 2003; Kruuk, 1995). The diet of otters in coastal areas of Scotland is dominated by fish such as eelpout (Zoarces viviparous) and rocklings (Ciliata mustela), sea-scorpions (Taurulus bubalis), butterfish (Pholis gunnellus) as well as consuming some crabs (Kruuk et al. 1991; Chanin, 2003).

Otters have large ranges with average linear territories along rivers of 40-50km recorded for dog (male) otters and about half this distance for females (Strachan, 2007; Kruuk et al. 1993; Birkeland, 2003; Chanin, 2003; Mason and Macdonald, 2008).  The range of otters utilising coastal areas in less well understood.  In Shetland resident females have been recorded living in ranges of 5-14 km along the coast, males had larger ranges of up to 19 km.  Transient dog otters have been recorded moving 40km along the coast (Kruuk, 1995).

H.4.2 Use of Marine Environment by Otters

Otters make use of shallow, sheltered, inshore marine areas.  Feeding is usually within 100m of the shore (Kruuk et al. 1998).  Kruuk and Moorhouse (1991) found that on 500 otter dives, 84% occurred within 50m of the coast.  Otters also typically dive to less than the 10m depth contour with deeper water generally avoided (Scottish Executive, 2007). A detailed study in Shetland during 1983 to 1985 found that although otters were recorded diving as deep as 14m, the vast majority of dives occurred at shallower depths (54% of 3558 dives were in water less than 2m deep and 98% were in less than 8m) (Nolet et al. 1993; Kruuk (1995).  Evidence from accidental captures of otters in lobster pots has shown that they are capable of diving to depths of 15m (Twelves 1983).

The preference for foraging in shallow water is thought to be related to energetic costs (with deep dives more demanding and allowing less time for foraging along the bottom (Nolet et al. 1993). This energetic need for shallow water could possible explain the low densities observed along steep coasts (Kruuk et al. 1998).

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