This term, as a rule, it seems, is applied to no particular kind of being or beings, but is rather a loose designation for sea-beings of any or all kinds. Some, however, appear to apply the term more particularly to mermen and selki-folk, who were believed to be fallen angels inhabiting the sea, while the real trolls or trows were fallen angels frequenting the land. Whales, etc., that went through nets and spoiled them were considered by some as sea-trolls, or agents of the Devil in whale-shape. People also spoke of "da mokkl sea-trow" ("the big sea-troll), a kind of evil spirit that haunted the deep, and apparently sometimes made storms and brought bad luck for fishermen.
Some spoke of a kind of sea-trow in the form of a woman, who uttered wailing cries which were heard on the sea by fishermen, both near and far from shore. To hear the wailing usually portended sorrow or distress to those hearing it (see also "sea-spirits").
- Teit, James A. (1918). "Water-beings in Shetlandic Folk-Lore, as remembered by Shetlanders in British Columbia." JAF 31:180-201, p. 197.
This article incorporates text from Water-beings in Shetlandic Folk-Lore, as remembered by Shetlanders in British Columbia (1918) by James Teit, which is in the public domain.