Were they only fishermen?
Résumé
There are many beliefs about islands and coastal areas in European minds. One of them is that if a settlement lies less than 3 miles from the coast the people living there are all sailors. Another one is that if a place has a "myth of creation" involving the sea, the people living there must all have inherited of their forefathers' ability to sail. The "myth of creation" in Shetland is, to some extent, the Viking invasions.