Phoque commun

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).

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.