5.3. Pinnipeds

5.3.1. Harbour Seal

                        Ecology

  1. Harbour seal typically weigh between 80 kg to 100 kg and is the smaller of the two species of pinniped that breed in the UK (SCOS, 2022). Harbour seal become sexually mature at four to six years old for males and three to five years of age for females (Lowry, 2016). Harbour seal are long-lived animals with individuals estimated to live to between 20 and 30 years (SCOS, 2022).
  2. Harbour seals are central place foragers and come ashore in sheltered waters, often on sandbanks and in estuaries as well as rocky areas (SCOS, 2022). This species requires haul out sites on land for resting, moulting, and breeding, and disperse from these sites to forage at sea. In order to reduce time and energy searching for prey, animals are likely to travel directly to areas of previously or predictably high foraging success (Bailey et al., 2014). Harbour seals persist in discrete metapopulations and tend to stay within 50 km of the coast (Carter et al., 2022), although most foraging trips are over shorter ranges (Russell and McConnell, 2014). Harbour seal are generalist feeders and their diet varies both seasonally and from region to region (Hammond et al., 2005). The analysis of stable isotopic composition and concentration of mercury and selenium ions in blood of harbour seals from the North Sea demonstrated that harbour seals diet is comprised of 30% juvenile cod, 29% of plaice Pleuronectes platessa and 23% of monkfish Lophius piscatorius as well as European hake Merluccius merluccius and haddock (Damseaux et al., 2021).
  3. Harbour seal breed in small groups scattered along the coastline. Pups are born in June and July having moulted their white coats prior to birth. During lactation, females spend much of their time in the water with their pups and, although they will forage during this period, distances travelled at this time are more restricted than during other periods (Thompson, 1994). In recent years, a very limited number of breeding season surveys have been carried out on behalf of NatureScot in areas designated as SACs for harbour seals in Scottish waters (Stevens, 2023). Given that there are no harbour seal breeding surveys conducted in the East Scotland or Northeast England seal MUs, these are not considered further in this report.
  4. The annual moult of harbour seals in Scotland occurs in August and that is when the greatest and most consistent numbers of harbour seals are hauled out ashore (SCOS, 2022). As such, the main harbour seal population surveys are carried out when seals are moulting.

                        Distribution and occurrence

  1. Harbour seals are found around the coasts of the North Atlantic and North Pacific from the subtropics to the Arctic (SCOS, 2022). The largest population of harbour seals in Europe is in the Wadden Sea and currently, approximately 32% are found in the UK (SCOS, 2022). Harbour seals are widespread around the west coast of Scotland and throughout the Hebrides and Northern Isles. On the east coast, their distribution is more restricted with concentrations in the major estuaries of the Thames, The Wash, the Firths of Forth and Tay, and the Moray Firth.
  2. Major declines have now been documented in several harbour seal populations along the east coast of England and around Scotland (SCOS, 2022). The pattern of declines is not universal, e.g. the decline following 1998 phocine distemper virus outbreak in England affected mostly the Wash population but had limited impact elsewhere (SCOS, 2022). A sudden change in the East Scotland seal MU population trend was observed in 2002 and the nature of this change remains unknown (Stevens, 2023). As previously described for the Firth of Tay and Eden Estuary SAC in section 4.3.2, between 1990 and 2002, the majority of the East Scotland seal MU population was located in this SAC. At this time counts in the SAC remained stable, representing approximately 85% of the East Scotland seal MU count (Stevens, 2023). The population within the SAC then declined rapidly and monotonically from 2002 to 2021 to a count of 41 individuals, representing an approximate 95% decline. As such, now the SAC accounts for approximately 16% of the haul out counts in the East Scotland seal MU. Such rapid population decline appears to be restricted to the SAC, and there is now evidence that this decline may be slowing (Stevens, 2023). There has also been a redistribution of harbour seals within the East Scotland seal MU and additional groups of harbour seals are located in the Firth of Forth, Montrose Basin and around the Aberdeenshire coast ( Figure 5.18   Open ▸ ). Within the Northeast England seal MU, harbour seal haul outs were primarily recorded at Lindisfarne in north Northumberland and within the Tees Estuary (near Middlesbrough) ( Figure 5.18   Open ▸ ).

Figure 5.18:
August Harbour Seal Haul Out Counts (2021) based on Stevens (2023)

Figure 5.18: August Harbour Seal Haul Out Counts (2021) based on Stevens (2023)


  1. The telemetry data confirmed that harbour seal usage within the Array marine mammal study area is very limited. A total of 46 harbour seals were tagged in the East Scotland seal MU between 2001 and 2018 and no seals were tagged in the Northeast England seal MU (Stevens, 2023). A further four harbour seals that were tagged elsewhere entered the seal MUs of interest ( Figure 5.19   Open ▸ , please refer to volume 3, appendix 10.2, Annex B for more details). In total, there were 50 harbour seals tracked within the seal MUs of interest and of these, four had telemetry track data recorded within the Array marine mammal study area ( Figure 5.20   Open ▸ ) (Stevens, 2023). All four of these were tagged within the Firth of Tay and Eden Estuary SAC, thus showing connectivity between the SAC and the Array marine mammal study area ( Figure 5.20   Open ▸ ). None of these individuals showed connectivity with harbour seal SACs outside of the East Scotland seal MU (Stevens, 2023). The Firth of Tay and Eden Estuary SAC is located approximately 113.7 km from the Array marine mammal study area. Given that average foraging range of harbour seal is 50 km and that only 8% of all harbour seals tagged in the East Scotland seal MU showed connectivity with the Firth of Tay and Eden Estuary SAC, it is concluded that there is limited connectivity with the SAC.
  2. Harbour seal tracks were recorded in the central and southern part of the Array marine mammal study area ( Figure 5.20   Open ▸ ). No clear patterns in distribution of harbour seal across the Array marine mammals study area can be concluded based on DAS sightings as only a single individual was recorded on two occasions (two harbour seals in total; refer to volume 3, appendix 10.2, annex A for more detail).

Figure 5.19:
Telemetry Tracks of Harbour Seals that Entered the Seal MUs of Interest

Figure 5.19: Telemetry Tracks of Harbour Seals that Entered the Seal MUs of Interest


Figure 5.20:
Telemetry Tracks of Harbour Seals with Connectivity to the Array Marine Mammal Study Area

Figure 5.20: Telemetry Tracks of Harbour Seals with Connectivity to the Array Marine Mammal Study Area


                        Density/abundance

                        Published literature
  1. The main population surveys are carried out when harbour seals are moulting, during the first three weeks of August. The most recent August haul-out count for the whole of Scotland is for the count period 2016 to 2019 and 2021, where a total of 26,378 harbour seals were counted (Stevens, 2023). For England and Wales, in 2021 a further 3,659 harbour seals were counted and in Northern Ireland 818 were counted. This results in a total count of 30,855 harbour seals in the UK during the period 2016 to 2021 equating to an estimated population of approximately 42,854 harbour seals in the UK (excluding the Republic of Ireland) (Stevens, 2023).
  2. In the East Scotland seal MU, the population has been in decline since the 1996 to 1997 survey period where the highest counts of 764 individuals were recorded. In the 2016 to 2019 survey block, the haul out counts within the East Scotland seal MU had increased for the first time since the decline, to 343 (when compared to 224 in 2011 to 2015) but have since declined again in the most recent 2021 surveys to 262 (Stevens, 2023). The current scaled population estimate for the East Scotland seal MU is 364 harbour seals (Stevens, 2023). In the Northeast England seal MU, harbour seal haul out counts are low, with the most recent haul-out count of 89 harbour seals for the 2021 count period resulting in a population estimate of 124 harbour seals (Stevens, 2023). There were sudden declines in the population noted in 1988 and 2002 and phocine distemper virus is considered to be the cause of these declines (Stevens, 2023).
  3. Mean harbour seal at sea usage within the Array marine mammal study area is very low, with mean usage of 0.000016 animals per 5 x 5 m grid cell, equating to a density of 0.00000064 animals per km2 (Carter et al., 2022) ( Figure 5.21   Open ▸ ). The peak count of harbour seal within grid cells overlapping the Array marine mammal study area is 0.000303 harbour seals, which, assuming uniform density within a grid cell is a density of 0.00001212 animals per km2.
                        Surveys
  1. A single harbour seal was recorded on two occasions during DAS, in April 2021 and May 2022 (refer to volume 3, appendix 10.2, annex A for more detail) and as such, design or model-based density and abundance estimates for these species are not available.
  2. Seagreen boat-based surveys show that harbour seals were seen in low numbers during most months in 2010, with the only exceptions being October and November when no harbour seals were recorded (Sparling, 2012). However, it should be noted that the Seagreen boat-based surveys were conducted in the areas closer to the shorelines and Firths of Forth and Tay, where the harbour seal usage is higher ( Figure 3.2   Open ▸ , Figure 5.19   Open ▸ ). No harbour seals were recorded during the Seagreen boat-based surveys in 2017 (Seagreen Wind Energy Limited, 2018). Only three sightings of harbour seal were made over the 25 months of Berwick Bank aerial survey (SSE Renewables, 2022a).

Figure 5.21:
Mean Harbour Seal at Sea Usage Based on Carter et al. (2022)

Figure 5.21: Mean Harbour Seal at Sea Usage Based on Carter et al. (2022)


                        Summary of the densities
  1. Harbour seals normally feed within 50 km around their haul out sites and therefore their presence in the offshore waters is limited ( Table 5.5   Open ▸ ). Due to low numbers of harbour seal sightings during DAS, design or model-based density and abundance estimates for this species are not available. As previously described in paragraph 186, Carter et al. (2022) reports mean density of harbour seal across the Array marine mammal study area as 0.00000064 animals per km2.
  2. Following feedback received in the Ossian Array Scoping Opinion from MD-LOT and NatureScot (MD-LOT, 2023), harbour seal was scoped into the assessment and a thorough review of its distribution and ecology within the regional marine mammal study area was carried out in section 5.3.1. However, considering relatively short foraging ranges, low numbers of harbour seals detected during DAS, low at-sea harbour seal usage ( Figure 5.21   Open ▸ ) and a limited connectivity with the Array marine mammal study area based on the telemetry data ( Figure 5.19   Open ▸ , Figure 5.20   Open ▸ ), the potential for effects on harbour seal populations on the east coast of Scotland is very low. Therefore, in the Array marine mammal consultation note 1 (Ossian OWFL, 2023c) the Applicant invited the stakeholders to reconsider their position on the advice provided with regard to including harbour seals in the assessment of potential impacts as a result of the construction, operation and maintenance and decommissioning of the Array. The Applicant suggested to exclude this species from further analysis in the marine mammal Array EIA Report Chapter. As such, no harbour seal density is presented here as being taken forward to the assessment.

 

Table 5.5:
Comparison of Main Data Sources Densities for Harbour Seal

Table 5.5: Comparison of Main Data Sources Densities for Harbour Seal

 

                        Seasonality

  1. No clear patterns in seasonality of harbour seal can be concluded based on DAS and historic survey sightings ( Figure 3.2   Open ▸ ) due to low numbers of animals identified to species level (refer to volume 3, appendix 10.2, annex A for more detail).

5.3.2. Grey Seal

                        Ecology

  1. Grey seal is the larger of the two pinniped species which occur around the British Isles. Males weigh up to 300 kg and females weigh up to 200 kg. The average lifespan for this species is between 20 to 30 years, however, females tend to live longer than males. Females mature at between three and five years old and males around six years, although they are probably not socially mature until eight years old (Hall and Thompson, 2009).
  2. Grey seals breed, rest, moult and engage in social activity when they gather in colonies on land (known as haul outs). Haul out events occur also at sea on exposed sandbanks, but their frequency is low, and their duration is on average shorter than those events on land (Russell and Lonergan, 2012).
  3. Female grey seals tend to return to the same breeding site at which they were born in order to give birth. Preferred breeding locations in the UK include remote, uninhabited islands or coasts and in small numbers in caves (SCOS, 2022). These sites allow females with young pups to move inland away from busy beaches and storm surges. Seals may also breed on exposed, cliff-backed beaches but these locations limit the opportunity to avoid storm surges and it may result in higher levels of pup mortality (SCOS, 2022). In the UK, grey seals breed in the autumn, but there is a clockwise cline in the mean birth date around the UK (SCOS, 2022). The majority of pups in south-west Britain are born between August and October; in north and west Scotland pupping occurs mainly between September and late November; in east Scotland between August and December and in eastern England pupping occurs mainly between early November to mid-December. Grey seal give birth to a single, white-coated pup which is weaned over a period of 17 to 23 days (SCOS, 2022). Pups shed their white natal coat (lanugo) and develop their first adult coat, with moult occurring at the time of weaning after which pups remain on the breeding colony for up to two to three weeks before going to sea. Following this, the female comes into oestrus and mating occurs, after which adult females return to sea to forage and build up fat reserves.
  4. Along the Scottish coast, grey seals exhibit an offshore foraging behaviour (Damseaux et al., 2021). Wyles et al. (2022) studied the influence of geomorphological features of the seabed on at-sea behaviour of grey seal. The study found that features such as slopes, foot slopes and hollows attract grey seal individuals as these may host prey aggregations, and/or lead to increased prey capture success. Grey seal have a selective diet. A study on the diet of grey seals in Scottish waters found that 50% of prey items were plaice and sole Solea solea and 46% of prey items were sandeels (Damseaux et al., 2021). Hammond et al. (2005) also highlighted that grey seal diet comprises primarily sandeels, gadoids and flatfish, in that order of importance, but varying seasonally and from region to region. Gosch (2017) also reported that there are significant regional and temporal differences in the diet of grey seal. Seals in shallow waters show a preference for demersal and groundfish species such as cephalopods and flatfish, whilst seals foraging in deeper waters, over sandy substrates, will target pelagic and benthopelagic species such as blue whiting Micromesistius poutassou and sandeels (Gosch, 2017).
  5. Grey seals tend to forage in the open sea, returning to land regularly to haul out. Foraging trips can be wide-ranging, however, tracking studies have shown that most foraging is likely to occur within 100 km of a haul out site (SCOS, 2022). During breeding season grey seal tend to forage within 20 km from the breeding site (pers. comm. with NatureScot).

                        Distribution and occurrence

  1. Globally, grey seal concentrate in three regions: eastern Canada and the north-east USA, around the coast of the UK, especially in Scottish coastal waters, and the Baltic Sea. All populations are known to be increasing, however, numbers are still relatively low in the Baltic where the population was reduced by human exploitation and pollution (SCOS, 2022, Galatius et al., 2020).
  2. The nearest designated haul out sites for grey seals from the Array marine mammal study area are Kinghorn Rocks and Inchmickery and Cow and Calves (designated for harbour and grey seal) and Fast Castle, Inchkeith and Craigleith (seasonal grey seal haul out sites) ( Figure 3.6   Open ▸ ). The closest site, Fast Castle, is located approximately 113 km from the Array marine mammal study area.
  3. In the East Scotland seal MU, grey seal haul out sites are concentrated around sites in the Firth of Forth and the Tay and Eden estuaries as well as further north around Peterhead and Fraserburgh ( Figure 5.22   Open ▸ ). Within the Northeast England seal MU, the largest concentrations of haul outs can be found along the north Northumberland coast and within the Tees Estuary ( Figure 5.22   Open ▸ ).

Figure 5.22:
Most Recent August Grey Seal Haul Out Counts (2015, 2016, 2018, 2021). Note That Not All Sites Were Surveyed Each Year

Figure 5.22: Most Recent August Grey Seal Haul Out Counts (2015, 2016, 2018, 2021). Note That Not All Sites Were Surveyed Each Year


  1. A total of 105 grey seals of all age were tagged in the seal MUs of interest ( Figure 3.6   Open ▸ ) between 1990 and 2014, 74 in the East Scotland seal MU and 31 in the Northeast England seal MU (Stevens, 2023). Of these, 84 were adults ( Figure 5.23   Open ▸ ). A further 33 adult grey seals that were tagged elsewhere entered the seal MUs of interest (refer to the volume 3, appendix 10.2, annex B for more detail). In total, there were 101 adult grey seals tracked within the seal MUs of interest and of these, 18 were tracked within the Array marine mammal study area (Stevens, 2023). Fifteen of these individuals (five female and ten male) that showed use of the Array marine mammal study area also showed connectivity with at least one grey seal SAC ( Figure 5.24   Open ▸ ). Connectivity with only the Isle of May SAC was observed in two adult grey seals, with only the Berwickshire and North Northumberland Coast SAC in nine adult grey seals, and with both SACs in four adult grey seals (Stevens, 2023). Given that only approximately 2% of all grey seals tagged in the seal MUs of interest showed connectivity with the Isle of May SAC, it is concluded that there is limited connectivity with the SAC.
  2. Connectivity was also noted between the Humber Estuary SAC (one male of unknown age), the Faray and Holm of Faray SAC (one adult male) and the Monach Islands SAC (one adult male). Given that these last three SACs are located at further distances from the Array marine mammal study area, there are lower levels of expected connectivity and therefore these SACs will not be considered further.
  3. Adult grey seals tracks were recorded throughout the Array marine mammal study area, with a higher density of tracks in the south of the Array marine mammal study area ( Figure 5.23   Open ▸ , Figure 5.24   Open ▸ ). No clear patterns in distribution of grey seal across the Array marine mammals study area can be concluded based on DAS sightings (refer to volume 3, appendix 10.2, annex A for more detail).

Figure 5.23:
Telemetry Tracks of Adult Grey Seals Tagged in the Seal MUs of Interest

Figure 5.23: Telemetry Tracks of Adult Grey Seals Tagged in the Seal MUs of Interest


Figure 5.24:
Telemetry Tracks of Adult Grey Seals with Connectivity to the Array Marine Mammal Study Area and SACs

Figure 5.24: Telemetry Tracks of Adult Grey Seals with Connectivity to the Array Marine Mammal Study Area and SACs


  1. The movement data was also obtained from the telemetry tags on 45 pup and juveniles, with 37 individuals tagged in the seal MUs of interest ( Figure 5.25   Open ▸ ). It is important to note that pup and juvenile movements may not be representative of the typical movement patterns of adult grey seals, since recently weaned pups are known to disperse widely to haul out locations far from their birth colony location (Stevens, 2023). Of these 45 grey seal pups, 13 were tracked within the Array marine mammal study area, 11 of which were also tracked inside a grey seal SAC ( Figure 5.26   Open ▸ ). Connectivity with only the Isle of May SAC was observed in two grey seal pups, with only the Berwickshire and North Northumberland Coast SAC in five grey seal pups, and with both SACs in four grey seal pups. No connectivity was observed between these grey seal pups and any SAC outside the East Scotland and Northeast England seal MUs.
  2. During the Seagreen boat-based surveys grey seals were recorded in the highest numbers over sandy shallow banks such as Scalp Bank, Marr Bank, Wee Bankie and Berwick Bank, which are thought to be important areas for sandeels, a key prey item of grey seal (Sparling, 2012). During the Berwick Bank aerial surveys grey seals were recorded throughout the surveyed area ( Figure 3.2   Open ▸ ) with the mean encounter rate of 0.011 animals per km (95% CI = 0.014 to 0.007; unidentified seal sightings were precautionarily assigned to grey seal due to their prevalence in the region).

Figure 5.25:
Telemetry Tracks of Grey Seal Pups Tagged in the Seal MUs of Interest

Figure 5.25: Telemetry Tracks of Grey Seal Pups Tagged in the Seal MUs of Interest


Figure 5.26:
Telemetry Tracks of Grey Seal Pups with Connectivity to the Array Marine Mammal Study Area and SACs

Figure 5.26: Telemetry Tracks of Grey Seal Pups with Connectivity to the Array Marine Mammal Study Area and SACs


                        Density/abundance

                        Published literature
  1. Grey seals encountered during harbour seal August moult surveys are counted during SMRU surveys as these provide useful information on their summer distribution. However, at this time of year, grey seal numbers at haul outs can be more variable from day to day and therefore are not an accurate reflection of grey seal abundance in each region. The UK wide grey seal population is estimated using a population model that combines regional pup production estimates and August haul-out counts scaled to population estimates (Stevens, 2023).
  2. In 2019, total UK pup production was estimated at 67,850 (95% CI = approximately 60,500 to 75,100) based primarily on estimates from less frequently aerial surveyed colonies as well as ground count data (Stevens, 2023). Pup production in Scotland in 2019 was estimated at 54,050 individuals, equating to approximately 79.7% of all pups born in the UK. The overall trend in pup production in the East Scotland seal MU has been increasing in recent years, however, the distribution of pup production appears to be changing (Stevens, 2023). Prior to the 1990s, the Isle of May SAC was the dominant location for pup production but since 2012, pup production estimates at the Isle of May have been overtaken by the Fast Castle colony (Stevens, 2023). Pup production estimates at the Isle of May are now considered to be stable or potentially declining (SCOS, 2023). In the Northeast England seal MU, pup production comes entirely from the Farne Islands, where the pup production has been showing a rapid increasing trend (Stevens, 2023).
  3. Grey seal August counts are estimated to be stable in the East Scotland seal MU and increasing in the Northeast England seal MU ( Figure 5.27   Open ▸ ) (Stevens, 2023). The most recent August grey seal counts took place in 2021 in both East Scotland and Northeast England MU and resulted in a scaled August population estimates of 10,783 and 25,913 grey seals, respectively (SCOS, 2023).
  4. The UK total grey seal population size at the start of the 2022 breeding season was estimated to be 162,000 grey seals of which 129,100 (approximately 80%) were in Scotland (Stevens, 2023).

Figure 5.27:
August Haul Out Counts of Grey Seals Within Seal MUs of Interest

Figure 5.27: August Haul Out Counts of Grey Seals Within Seal MUs of Interest


  1. Mean grey seal at sea usage within the Array marine mammal study area is low, as the hotspots are located closer to the shore and in the vicinity of the Berwickshire and Northumberland Coast SAC, Firth of Forth, Tay and Eden Estuary and north of Aberdeen ( Figure 5.28   Open ▸ ). The average value of the mean at sea usage within the Array marine mammal study area was estimated at 4.49 animals per 5 x 5 km grid cell, equating to a density of 0.180 animals per km2 (Carter et al., 2022).
                        Surveys
  1. Grey seal were not observed during DAS in sufficient numbers for robust modelling to be undertaken, and as such their density can only be estimated via design-based methods. Peaks in grey seal presence occurred in winter. As such, the most precautionary grey seal densities calculated across the Array marine mammal study area were estimated for winter with 0.033 (95% CI = 0.014 to 0.054) animals per km2.
  2. Seasonal density estimates estimated from Berwick Bank aerial surveys highlighted that mean monthly densities of grey seal were highest during spring months. The absolute density was estimated as 0.276 animals per km2 with a peak mean density during spring months of 0.321 (SSE Renewables, 2022a). However, it should be noted that Berwick Bank aerial surveys took place at a distance closer to the shore ( Figure 3.2   Open ▸ ), where grey seal usage is higher compared to the Array area ( Figure 5.28   Open ▸ ). These densities are therefore higher than seasonal design-based estimates from DAS data with 0.034 animals per km2 (estimate based on data during non-breeding season (refer to Table 4.3   Open ▸ ).

Figure 5.28:
Mean Grey Seal at Sea Usage Based on Carter et al. (2022)

Figure 5.28: Mean Grey Seal at Sea Usage Based on Carter et al. (2022)


                        Summary of the densities
  1. The east coast of Scotland and northern England provide important breeding and haul out habitats for grey seal (SCOS, 2023). Comparison of key data sources for grey seal is shown in Table 5.6   Open ▸ .
  2. To provide the most precautionary figure, the design-based absolute density estimates from the DAS presented in Table 5.6   Open ▸ are based on the non-breading bio-season (September to December). However, the predicted estimate of mean density for the Array marine mammal study area from Carter et al. (2022) of 0.180 animals per km2 is higher than the absolute density of 0.034 animals per km2 based on DAS data ( Table 5.6   Open ▸ ). Given uncertainties associated with identification of seals to species level based on aerial survey data, the density of 0.180 animals per km2 based on Carter et al. (2022) will be taken forward to the assessment.

 

Table 5.6:
Comparison of Main Data Sources Densities for Grey Seal. Densities In Bold Will Be Taken Forward to the Assessment in the EIA (Carter et al. (2022) Density Will Be Taken Forward to the Assessment)

Table 5.6: Comparison of Main Data Sources Densities for Grey Seal. Densities In Bold Will Be Taken Forward to the Assessment in the EIA (Carter et al. (2022) Density Will Be Taken Forward to the Assessment)

 

                        Seasonality

  1. Grey seal sighting rates during Seagreen boat-based surveys were lowest over the autumn and winter (Seagreen Wind Energy Limited, 2018). During the Berwick Bank aerial survey grey seals were recorded every month, except one (SSE Renewables, 2022a). Based on DAS sightings, grey seals were most abundant in summer (June) (refer to volume 3, appendix 10.2, annex A for more detail).
  2. Higher encounter rates of grey seals at sea during summer are likely to be related to the capital breeding habit of grey seals and possibly indicative of a period of intense foraging where adult seals are at-sea gaining energy reserves prior to the breeding season. During autumn (August to December) grey seals aggregate to breed at traditional colonies between August and December and therefore the number of seals might be expected to be low as a large proportion of the population will be hauled out to breed.