Immediately after this they have a period of feeding between late November and mid-January when they regain condition and it is only then that they undergo their annual moult.
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The Shetland population of the Common Seal was last counted in 1997 using the 'hi-tech' method of an aerial thermal image survey of the coastline. This is a minimum figure as only those animals hauled out on rocks are recorded in this way, although this is thought to represent between 60-80% of the total population.
The current UK population estimate of the species is 28,000 animals.
The old Shetland name for the Grey Seal is 'haaf fish' (deep sea fish), because it prefers more open sea conditions, while the Common Seal is known as 'tang fish' (seaweed fish) because of its preference for more sheltered shores and islands.
Contrary to expectations, there are fewer Common Seals than Grey Seals in Britain as a whole, with around 28,000 Common and 105,000 Grey.
In years gone by Shetlanders have hunted seals for their blubber, used for oil, their skins which were used to make clothes, and occasionally the flesh would be eaten.