The common seal Phoca vitulina is one of the most widely distributed of all seal species, occurring along the temperate, sub-arctic and arctic coasts of the North Pacific and North Atlantic. Five sub-species are generally recognised, based largely on geographical distribution. The UK population belongs to the European sub-species Phoca vitulina vitulina, which is confined to the eastern North Atlantic. It occurs regularly around the northern European coast up to Iceland and Spitzbergen and is occasionally found as far south as the coast of Portugal.
Common seals depend on the sea for their food but also have a need for safe areas of land to haul out to rest, give birth and moult. In west Scotland haul-out areas are characteristically found on the shores of small islands or isolated skerries. Individuals return to favoured haul-out sites and there are no known migratory movements. The common seal is the smaller of the two resident species in the UK, with length varying from 130 - 170 cm and with a maximum recorded weight of 150 kg (both measurements for adult males). Adult females bear a single pup in June or early July. Pups are weaned after about 4-5 weeks with mating occurring soon after weaning.
The Sea Mammal Research Unit (SMRU) is commissioned to survey seal numbers
in the UK. SMRU common seal research is conducted on a 5 year rolling
programme, with each common seal site being surveyed at least once in
a five year time period. Surveys of common seals are carried out during
the moult in August, as this is when the seals spend the largest proportion
of their time on land and are therefore most visible for counting. The
resulting counts are estimates of population numbers since there are
always a proportion of seals in the water that cannot be counted. It
is likely that the most recent count of the UK common seal population,
34,625, actually represents an estimated population of 50,000 – 60,000
seals. 87% of common seals in the UK are found in Scottish waters with
the major concentrations of the Scottish population found in the Northern
Isles and around the west coast and Inner Hebrides. The UK population
is thought to represent at least 40% of the European sub-species Phoca
vitulina vitulina and nearly 5% of the world population of common seals.