The harbour (or common) seal was recognised to comprise five subspecies (IUCN 2016). It is now generally accepted (Society of Marine Mammalogy) to comprise three (Westlake and Corry-Crowe 2002; Berta and Churchill 2012): the Atlantic (P. v. vitulina, Linnaeus 1758), Pacific (P. v. richardii, Gray 1864) and Ungava harbour seal (P. v. mellonae, Doutt 1942).

European regional assessment: Least Concern (LC)

This species is widespread and abundant. Population size well exceeds the threshold for the population size criterion of the IUCN Red List (i.e. less than 10,000 mature individuals in conjunction with appropriate decline rates and subpopulation qualifiers). The overall population trend for the area has not been quantified, but it is not believed to approach the threshold for the population decline criterion of the IUCN Red List (i.e. declining more than 30% in ten years or three generations). For these reasons, the Harbour Seal is evaluated as Least Concern at the European and EU 27 regional level. There is concern, however, for local populations. Indeed, they are classed as Vulnerable (Svalbard, Kalmarsund) and Critically Endangered (Greenland and Iceland) in parts of the European area. In both Svalbard and Kalmarsund, the classification is based on the limited genetic variation rather than population declines. The consideration genetic (and in some places phenotypic) differentiation across the European range (Anderson and Olsen 2010) makes such vulnerabilities of particular concern.

Harbour Seals are one of the most widespread of the pinnipeds inhabiting coastal areas of the Northern Hemisphere, from temperate to polar regions.

Within the European Red List boundary, they extend from Svalbard in the north to northern France in the south, and from Greenland in the west to Sweden (Kalmarsund) in the east. Within this spatial extent, there are some notable absences including the Faroe Islands; they are occasionally sighted in the Faroe Islands which historically hosted a subpopulation, as did the southern Baltic (as far east as Estonia; Härkönen and Isakson 2010). On a more local scale, there is little evidence for changes in distribution, even in areas which have exhibited declines (Banga et al. 2023).

The total population estimate for the area is around 130,000 individuals (Table 1; Banga et al. 2023, SCOS 2022, Galatius et al. 2022, Merkel et al. 2013, Nilssen et al. 2021, Nilssen and Bjørge 2019, Nilssen and Bjørge 2017a, 2017b, Hoekstein et al. 2022, Granquist 2021). The main (over 5,000 individuals) population centres are Scotland and the Wadden Sea (around 35,000 each), rest of Denmark and Sweden (c. 20,000 combined), Iceland (c. 15,000), Norway (10,000), England and Ireland (c. 5,000 each). Although overall the population in Europe has been recovering from historic hunting and bounty schemes, population trajectories vary even within metapopulations (Carroll et al. 2020) with increasing, stable and declining trends. Indeed, the following areas are showing evidence of depletion (since a baseline year of 1992) including Iceland, parts of Scotland (North Coast and Orkney, Shetland, Moray Firth, Eastern Scotland) and Northern Ireland (Thompson et al. 2019, Granquist 2021, Banga et al. 2023). Furthermore, although the current trend is unknown, abundance in Greenland is depleted compared to historic levels (1950s). There is also evidence of recent declines in Southeast England and the Dannish Kattegat (Banga et al. 2023, SCOS 22). Many of the current declines are due to unknown cause. The large Wadden Sea population appears to have reached carrying capacity around 2014 (Galatius et al. 2022).

Harbour Seals are a relatively small phocid with limited sexual dimorphism (Atlantic males/females: 153-156/140–146 cm and 75-104/67–83 kg length and mass respectively) (Härkönen and Heide-Jørgensen 1990, Lydersen and Kovacs 2010). The degree of which varies within their range. There is geographic variation in size and appearance. Harbour Seals can live for 25-30 years, reaching sexual maturity around four to five years old. 

Harbour Seals haul out and breed on a variety of habitats: rocky shores, sand banks, mud flats, rivers, and ice flows. In some haul-outs, they have become habituated, to a degree, to human presence. Foraging ranges vary markedly between regions (average extents from below 5 km to over 50 km according to Sharples et al. 2012)  and individuals with trip extents of  over 200 km and durations of  over three weeks (Lesage et al. 2004, Peterson et al. 2012). At-sea distribution is likely to vary by age class and sex (Lowry et al. 2001). Harbour Seals eat small-medium sized fish and cephalopods, with extensive geographic and seasonal variation. In freshwater, salmonoids are often the main prey. Even within limited geographic ranges they exploit a wide variety of prey (over 50 species in the UK; Wilson and Hammond 2019). They give birth to a single precocial pup which, having usually moulted in utero, can swim from birth. This allows harbour seals to take advantage of habitats for pupping that afford protection from land predators and, to some degree, human disturbance. The females continue to forage during the lactation period (Thompson et al. 1994, Bowen et al. 2001) which is around three to four weeks (Drescher 1979). The timing of pupping varies but in the Northeast Atlantic is generally between May and September (Temte 1994). Mating occurs at the end of the lactation period, with males defending at-sea territories. Moulting occurs after mating and timing varies with age and sex; juveniles moult first then adult females and finally adult males (Reder et al. 2003).  

Compared to the largely sympatric Grey Seal, they show relatively high fidelity to haul-out areas, though do occasionally move considerable distances to both haul out and breed. Dispersal appears to be highest in the pup/juvenile stage, though little is known about rates. For example, there can be significant genetic differentiation over relatively short distances even when no apparent obstacles exist (Carrol et al. 2021). However, the relationship between separation and distance is not straightforward with movements of adults between haul-outs which are over 150 km apart (Russell et al. 2016).

In the Northeast Atlantic, predators of Harbour Seal include Killer Whales (Orcinus orca), Polar Bears (Ursus maritimus), Grey Wolves (Canis lupus) and Grey Seals (Halichoerus grypus).

There are a multitude of potential pressures on Harbour Seals (reviewed in Blanchet et al. 2021). The relatively large number of apparently closed, and in many cases small, subpopulations make the species vulnerable to range losses (Andersen and Olsen 2010). One of the most concerning aspects of Harbour Seal populations is their sudden and rapid decline with no obvious cause in some populations. This may be indicative of their susceptibility to multiple stressors. Current key potential threats incorporate both human activities and natural processes (disease, competition, and predation) which to some extent are mediated by current or historic human activities. 

Harbour Seals generally live in coastal areas and thus have considerable overlap with human activities. Historically their main threat was deliberate killing through hunting, culling and bounty schemes. Hunting is now only allowed in Norway and Sweden, where it is now licenced, and the numbers allowed to be killed are informed by population monitoring as part of a management plan. As such, the hunting should not be a long-term threat to population viability. In 2019 hunting was banned in Iceland and in 2020, the provision for killing of seals in the UK to protect fisheries was removed. Other current human activities that could impact Harbour Seals include fisheries, agriculture, shipping, oil and gas exploration, recreational activities and, most recently, renewable energy developments. It is difficult to attribute cause and impact on a population level and to date, evidence for population level impacts has been restricted to hunting, fisheries bycatch, and pollutants.  Bycatch in fisheries occurs throughout the majority of the range with likely population level impacts in some areas (Moan and Bjørge 2021). Historically, there have been relatively high levels of some environmental pollutants, including polychlorinated biphenyl (PCBs), found in Harbour Seals (Sonne et al. 2020) which have had population impacts (e.g. Wadden Sea; Reijnders 1986). As well as directly impacting reproduction, pollutants can cause vitamin deficiency (Brouwer et al. 1989) and immune system suppression (Ross et al. 1995, Desforges et al. 2016).

With the exception of direct killing, for most human activities, the impact on Harbour Seal vital rates and thus populations are unclear. Availability of haul-out sites can be impacted by developments and commercial kelp production but there is no indication that haul-out sites are limited within this area. Similarly, disturbance on haul-out sites could potentially impact numbers using local sites, but Harbour Seals appear to have some resilience to disturbance at haul-out sites (Paterson et al. 2019). Disturbance during breeding season is likely to be most problematic; it could lead to pup mortality through disruption of the mum-pup bond immediately after birth and thus pup abandonment. Harbour Seals are sensitive to noise disturbance at sea, and prevalence of such disturbance events are predicted to increase due to tidal and marine energy developments, and increased shipping. Although, small in number, tidal turbine arrays are typically inshore and Harbour Seals appear to favour such high current areas (Hastie et al. 2016). This has led to concerns regarding collision risk and population level impacts in some areas, but evidence so far indicates the noise of operational tidal turbines results in some avoidance (Onoufriou et al. 2021). Similarly, pile driving for wind farms has the potential to cause short-term displacement (Russell et al. 2016). The ramifications of such avoidance likely depend on the availability of other suitable foraging opportunities. Pile driving can potentially impact auditory range (Whyte et al. 2020); the level of exposure is likely mediated by a trade-off between the importance of the area and sound level (Hastie et al. 2020). The ramifications of decreased auditory range are not well understood but could impact reproductive behaviour.

Natural threats include disease epidemics, biotoxins, predation (by Killer Whales, Polar Bears, and Grey Seals) and competition for prey. Harbour Seals are susceptible to disease outbreaks; mass morality events have been caused by Phocine Distemper Virus in 1988 (estimated mortality of  over 23,000) and 2002 (over 30,000; Härkönen et al. 2006), and Avian influenza in 2014 (over 2,000; Zohari et al. 2014, Bodewes et al. 2015). Recent evidence shows that, at least in some areas, many important prey species have considerably high levels of biotoxins year-round, potentially leading to chronic exposure in seals and reduction of demographic rates (Kershaw et al. 2021). Grey Seals, which compete with, and in some cases prey on (van Neer et al. 2015) Harbour Seals, are increasing both in abundance and distribution particularly in the southern North Sea. Historically Grey Seal colonies were, for the most part, restricted to a limited number of offshore islands that lacked land predators while Harbour Seals could exploit tidal sandbanks. However, the lack of such predators, and more recently the cessation of human exploitation, has facilitated an increase in the Grey Seal population. In southeast England, Grey Seals are increasing at a rate of  over 10% per annum while recent Harbour Seal counts indicate a c. 20% drop in what was a stable population (SCOS 2022). In addition, Grey Seals are much more wide-ranging than Harbour Seals (e.g. the northeast Atlantic is believed to be a single subpopulation), and could act as transmission vectors for infectious diseases. The increase in Grey Seal numbers in the southern North Sea could increase competition for prey, increase predation, and facilitate or enhance the spread of diseases such as the next PDV epidemic that is predicted to be imminent. The impacts of climate change are likely to be complex and multi-faceted, acting both directly and directly. Harbour Seal haul-out patterns are impacted by weather, with time hauled out lower at both very cold (Routti et al. 2014) and hot temperatures (Hansen et al. 1997). In many parts of their range sea level rise will likely lead to loss of current haul out sites. The availability of newly created haul-out sites will in many places depend on local mitigation efforts such as flood defences. The reduction of ice in the Arctic is likely to be associated with increased anthropogenic activity and thus potential for disturbance and pollution. Climate change will result in changes in prey composition throughout most of their range and could potentially result in an increased prevalence of harmful algal blooms (HAB; Gobler 2020) events and the spread of disease (Harvell et al. 1999).

Harbour Seals are hunted in Iceland, Norway, Sweden, and Denmark; ~625 were reported to be hunted in 2023 (ICES 2024, NAMMCO 2024). They are hunted for their meat and skin.

Recent changes have effectively ended deliberate killing of Harbour Seals throughout most of the area. A ban on hunting is quite recent in some areas and thus the impact on populations may not have been realised (e.g. since 2010 and 2019 in Greenland and Iceland, respectively). Hunting is currently only permitted in Norway, though a Management Plan was implemented in 2010 which aims to maintain the population size (at a count of 7,000 individuals during the moult). Some hunting, under licence is permitted in Sweden. Shooting seals to fisheries has recently been banned in many countries including Norway (2019), Canada (2020), and UK (2020). In the UK such shooting was already restricted (in numbers and/or location) and it was unlikely to have contributed to any regional declines). As well as country level protections, there are specific conservation strategies on haul out specific level (e.g. Special Areas of Conservation in the European Union and the UK) to multi-country levels (e.g OSPAR). Harbour Seals are included in The Bern Convention, EU Habitats Directive, Convention on the Conservation of Migratory Species of Wild Animals (CMS; Baltic and Wadden Sea populations only), The Convention on the Protection of the Marine Environment of the Baltic Sea Area (Helsinki Convention, HELCOM) and the Agreement on the Conservation of Seals in the Wadden Sea, and their abundance and distribution are biodiversity indicators under OSPAR.

There are additional conservation measures which act on a regional or haul-out site level including Coastal Reserves in Norway which exclude commercial fishing (these have been shown to reduce harbour seal mortality), Seal Conservation Areas (Scotland), and designated haul-out sites (at which it is illegal to intentionally or recklessly harass seals; Scotland).

Further research into the population trends and threats faced by the species is recommended.