Common seal

Wildlife Act 1976 / 2000
EU Directive 92/43 Annex V
Bern Convention Appendix III

The common seal, also known as the harbor seal is the smaller of Ireland’s two pinniped species. Males known as bulls will be larger and heavier than the female cows with mature bulls measuring up to 2m in length and weighing up to 150kg. Females grow to 1.8m in body length and generally weigh up to 130kg. The seal’s body shape consists of a rounded streamlined torpedo like design with an all over covering of short fur. The coat is moulted once a year in August and is variable in colour from an almost white to sliver grey tint covered in dark spots to a black or dark brown coat with white rings. The underbelly is less spotted than the back and flanks. A layer of blubber is located under the coat for insulation. The common seal has a rounded disc shaped face with a short snout that contains its distinctive V-shaped nostrils. The seal’s four limbs are small flippers with each containing five webbed digits. The fore flippers located on the sides have small claws at the ends of each digit. On land the common seal is not very maneuverable and moves in a flopping style but once in the water they are agile swimmers able to reach speeds of up to 25 knots. Common seals have good vision both on land and while submerged. The eyes are large and forward facing with several adaptations which allow for sight in dim underwater conditions, reduction of glare while on land and also protection from sand.

The common seal prefers sheltered coastal areas with a sandier coastline and calmer waters than the habitat of the grey seal. They will establish regularly visited resting sites on mudflats and sandbanks in an area within their range. They will also enter harbors, bays and estuaries if these areas provide an adequate source of fish. The common seal spend roughly half of their time on land at resting sites close to the water and the remainder hunting in the sea. In Ireland the common seal’s preferred habitats are located along the western seaboard in any area that can provide quick access to sandy resting sites. The common seal species does not form large colonies as other seals do, they are regarded as being less sociable and will spend most of their time alone or in small groups. An individual seal will spend most of the year within a few miles of their traditional breeding grounds. They will come ashore for longer periods in July and August to breed and to moult their coats and may form larger groups on sheltered shorelines at this time.

Common seals are carnivorous hunters who are also opportunistic feeders with a large and varied diet. They have the ability to dive for long periods into deep waters although in Ireland their hunting dives generally last under ten minutes and are usually conducted in shallow waters not more than seventy meters deep. During the dive the V-shaped nostrils will shut tightly with oxygen being stored in the muscles and blood vessels. Seals have twice the amount of blood found in terrestrial mammals of similar size, their muscles can also function with high levels of lactic acid being present. On deeper dives the common seal has the ability to reduce the blood supply to non-essential areas and can slow their heart rates down to fewer than fifteen beats per minute. All of the common seal’s prey is taken while underwater. In areas of good visibility they will hunt by sight alone but in deeper areas or where light levels are poor the seal depends on its underwater directional hearing or sensitive whiskers which pick up movement vibrations. While hunting the common seal will emit a series of clicking sounds which may have a function in the echolocation of prey similar to that used by dolphin species. The preferred prey items of common seals in Irish waters are fish species including herring, hake, sole and sculpin. They will also hunt for shrimp, octopus and squid in deeper waters while they will catch mollusks and crustaceans when the opportunity arises.

The mating season for common seals in Ireland starts in July and runs until August. Males become more aggressive at this time with fights for dominance occurring underwater. Males may lose a lot of their body weight at this time due to strenuous competing with neck wounds from bits being a common injury. Courtship and mating occurs underwater with females mating with only the strongest bulls. Males will use vocalizations to attract breeding females. Once pregnant the gestation period lasts for eleven months with a single pup born in June on land at the breeding ground. Pups are born well developed with fur coats and the ability to swim and dive within a few hours of birth. Pups weigh up to 6kg when born and are fed on their mother’s fatty nutrient rich milk for one month, by this time the pups can have doubled in weight. Pups will remain with their mothers for the duration of the weaning period as she alone will provide parental care for her offspring. After one month the pup may be able to fend for itself and the mother will begin mating again in the sea. There can be a high rate of mortality for pups in their first year from disease or if they do not learn to hunt for themselves within the short period they are with their mothers. Most common seals will not reach sexual maturity until they are 5 years old. Females of this species tend to live longer than males with a maximum lifespan of 32 years, males can reach up to 26 years old which may be due to the stresses caused by the mating season each year.

Like all seal species the common seal is believed to have originated from an otter like mammal between 15 and 20 million years ago. The species is now globally widespread with a total worldwide population estimated to be in the region of 500,000 individuals. The common seal species is concentrated in the northern hemisphere on land areas in the Atlantic and Pacific oceans. There are also populations in the North Sea and Baltic sea areas. The common seal is now the most widespread species of the Pinniped family which includes walruses, seals and sea lions. In the waters around the British coast there are estimated to be a population of over 100,000 common seals. In Ireland the common seal is mainly concentrated in counties along the western seaboard which can provide sheltered calm waters and have access to sandy rest areas. The current Irish population has been estimated to contain around 5,000 individuals.

The common seal has few natural predators in Irish waters. Man is still the main cause of seal deaths. They were for centuries hunted for their fur and meat which resulted in a huge decline in their numbers throughout their range by the 19th century. Seal hunting has been made illegal in most parts of the world but their increasing numbers can bring them into conflict with fishermen who can legally shoot seals which are found close to their nets. Disturbance of breeding and resting sites by the presence of humans can have a serious effect on a seal colony’s lifecycle. Common seals are less likely to haul out of the water if they detect human presence in an area. Pups and breeding grounds may be abandoned if they are continually disturbed. Natural causes of common seal deaths come from disease outbreaks in colonies and the losses of mature males during the breeding season. There is also a high natural mortality rate for pups in their first year of life.
Decreasing water quality of Irish seas may result in a future decline of common seal numbers as some chemicals like PCBs can accumulate in the environment and affect the population’s reproduction rate. The Irish common seal population should continue to grow since this species is now protected under international, European and Irish law.