- What is a Cetacean?
- Common Species
- Rare Species
- Other Species
Key FactsLength: Up to 1.9 metres
Range: Coastal waters of the North Atlantic and North Pacific Oceans
Threats: Marine litter, pollution, phocine distemper virus
Diet: Variety of fish, squid and octopus
Latin: Phoca vitulina
The common seal, also known as the harbour seal, is the smaller of the two species of seal seen in the Hebrides. It has a rotund yet streamlined body with a layer of blubber and a short coat to provide insulation against cold sea temperatures. When fully grown, adult males may be only slightly larger than females: males can grow to 1.9 metres long and weigh 170 kg whereas females can reach up to 1.7 metres in length and weigh around 130 kg. Common seals have a wide, pronounced snout, large eyes and no external ears; the common seal does not have the sloping forehead of the grey seal and is described as more dog-like in appearance. The forelimbs are quite short with obvious claws. Coat colouration varies with geographic region, and in the UK the colour is quite dark grey with a light mottled pattern. Pups are normally born in the inter-tidal zone having shed their furry ‘lanugo’ coat in utero. Life span is in the region of 25 to 35 years.
Habitat and Distribution
Common seals are found only in the northern hemisphere and their distribution is distinctly coastal. There are five sub-species currently recognised based on their geographic distribution. They can be found all around the UK coastline, preferentially hauling out at sandflat and estuarine habitats, but also using rocky shorelines in Scotland. In the Hebrides, the island of Lismore, the skerries and coastline of south-east Islay, and the rugged coastline of north-west Skye are particularly important sites. Compared with grey seals, common seal distribution is more coastal and they are often seen in estuaries, river mouths and may even venture upstream.
When they haul out on land, the common seal has a characteristic posture with the head and tail raised to form a curved shape. During the breeding season, usually June and July in the Hebrides, the female seals will haul out to give birth. Pups gain weight rapidly on the fat-rich milk produced by their mothers. The pups are born with an adult coat allowing them to swim very soon after birth; they can even be seen riding on their mothers’ backs to save energy in their first few weeks of life. Males arrive during this time, remaining in the water to compete with one another in order to mate with the females. The annual moult takes place shortly after the breeding season and, once completed, the seals leave once again to feed. Common seals are normally solitary when at sea. A commonly recorded behaviour in the Hebrides is that of ‘bottling’; the seal is vertical in the sea with just its head above the surface. They can be quite curious of humans and boats, and have been known to approach vessels and divers, but care must be taken when encountering a seal on land as they can be aggressive, especially during the breeding season. Common seals are the least vocal of the pinnipeds (the scientific term for all seals), but do use sound to communicate with each other (this is particularly true for mother and pup pairs), and to warn off predators.
Food and Foraging
Common seals feed on a varied diet of sandeel, cod, herring, sprat, flatfish, sprat, octopus and squid. They eat around three to five kilograms of food per seal per day, and normally forage within 31 miles of their haul out site. Common seals dive to depths between 10 and 150 metres for up to 31 minutes. They spend about 85% of their day diving, which includes active foraging, resting at the seafloor and drifting with currents and tidal movements.
Status and Conservation
The current UK population of common seals is estimated at between 50,000 to 60,000 individuals, which probably represents about one tenth of the global population. Around 85% of the UK’s common seals occur in Scotland. Under the Conservation of Seals Act (1970), fishermen with a special licence are permitted to shoot seals seen in the vicinity of fishing gear, although some illegal culling may also take place due to the perceived competition with fisheries for commercially valuable fish species. Acoustic Deterrent Devices (ADD’s) produce aversive sounds to deter seals from localised areas and are currently in-use in the Hebrides in an attempt to deter seals from fish farm areas; the effect they have on non-target species, such as cetaceans, currently being researched. In 1988 an epidemic of the phocine distemper virus (PDV) killed over 17,000 common seals in the North Sea and resulted in a 52% reduction of the population along the east coast of England. A second outbreak in 2002 caused further deaths. It has been suggested that accumulations of toxins, such as organochlorines (found in pesticides), may affect the immune system and reduce resistance to diseases including the phocine distemper virus (PDV); these toxins may also reduce reproductive success. Oil spills can devastate seal populations by affecting the waterproof quality of the coat, by causing respiratory and other health complications, and by reducing food availability. Seals are also at risk of entanglement in fishing nets and marine litter. Natural predators in the Hebrides are probably limited to killer whales. Common seals are protected in UK waters, principally by the Conservation of Seals Act (1970), Schedule 5 of the Wildlife and Countryside Act (1981), and the EU Habitats and Species Directive (1992).