The Scots have been settling in Shetland long before it became part of Scotland & about half the surnames are Scots, the Norse population used a patronymic until these became fixed in the late 18th century as Nicolson, Anderson etc' & what became of the Pictish inhabitants after the Norse settlement nobody knows. More Norse words are apparently used in Lewis Gaelic today than in Shetland Scots so no displacement of population there where Gaelic came to dominate, just as Scots replaced Norse in the Northern Isles. There is also the effect on Shetland of the Dutch fishing fleet & Hansa merchants who dominated the islands trade & economy for generations.
An interesting subject, I had a Shetland gt grannie, a Tulloch & Nicolson combo so Scots/Norse!
Skoosh.